An excerpt from Dart: Up and Running

Chapter 3. A Tour of the Dart Libraries

This chapter shows you how to use the major features in Dart’s libraries. It’s just an overview, and by no means comprehensive. Whenever you need more details about a class, consult the Dart API reference.

dart:core - Numbers, Collections, Strings, and More

The Dart core library provides a small but critical set of built-in functionality. This library is automatically imported into every Dart program.

Numbers

The dart:core library defines the num, int, and double classes, which have some basic utilities for working with numbers.

You can convert a string into an integer or double with the parse() methods of int and double, respectively:

lang-dart
ch03/number-tests.dart
assert(int.parse('42') == 42);
assert(int.parse('0x42') == 66);
assert(double.parse('0.50') == 0.5);

Or use the parse() method of num, which creates an integer if possible and otherwise a double:

lang-dart
ch03/number-tests.dart
assert(num.parse('42') is int);
assert(num.parse('0x42') is int);
assert(num.parse('0.50') is double);

To specify the base of an integer, add a radix parameter:

lang-dart
ch03/number-tests.dart
assert(int.parse('42', radix: 16) == 66);

Use the toString() method (defined by Object) to convert an int or double to a string. To specify the number of digits to the right of the decimal, use toStringAsFixed() (defined by num). To specify the number of significant digits in the string, use toStringAsPrecision() (also in num):

lang-dart
ch03/number-tests.dart
// Convert an int to a string.
assert(42.toString() == '42');

// Convert a double to a string.
assert(123.456.toString() == '123.456');

// Specify the number of digits after the decimal.
assert(123.456.toStringAsFixed(2) == '123.46');

// Specify the number of significant figures.
assert(123.456.toStringAsPrecision(2) == '1.2e+2');
assert(double.parse('1.2e+2') == 120.0);

For more information, see the API documentation for int, double, and num. Also see the section called “dart:math - Math and Random”.

Strings and Regular Expressions

A string in Dart is an immutable sequence of UTF-16 code units. The language tour has more information about strings. You can use regular expressions (RegExp objects) to search within strings and to replace parts of strings.

The String class defines such methods as split(), contains(), startsWith(), endsWith(), and more.

Searching inside a string

You can find particular locations within a string, as well as check whether a string begins with or ends with a particular pattern. For example:

lang-dart
ch03/string-tests.dart
// Check whether a string contains another string.
assert('Never odd or even'.contains('odd'));

// Does a string start with another string?
assert('Never odd or even'.startsWith('Never'));

// Does a string end with another string?
assert('Never odd or even'.endsWith('even'));

// Find the location of a string inside a string.
assert('Never odd or even'.indexOf('odd') == 6);

Extracting data from a string

You can get the individual characters from a string as Strings or ints, respectively. To be precise, you actually get individual UTF-16 code units; high-numbered characters such as the treble clef symbol ('\u{1D11E}') are two code units apiece.

You can also extract a substring or split a string into a list of substrings:

lang-dart
ch03/string-tests.dart
// Grab a substring.
assert('Never odd or even'.substring(6, 9) == 'odd');

// Split a string using a string pattern.
var parts = 'structured web apps'.split(' ');
assert(parts.length == 3);
assert(parts[0] == 'structured');

// Get a UTF-16 code unit (as a string) by index.
assert('Never odd or even'[0] == 'N');

// Use split() with an empty string parameter to get a list of
// all code units (as Strings); good for iterating.
for (var char in 'hello'.split('')) {
  print(char);
}

// Get all the UTF-16 code units in the string.
var codeUnitList = 'Never odd or even'.codeUnits.toList();
assert(codeUnitList[0] == 78);

Converting to uppercase or lowercase

You can easily convert strings to their uppercase and lowercase variants:

lang-dart
ch03/string-tests.dart
// Convert to uppercase.
assert('structured web apps'.toUpperCase() == 'STRUCTURED WEB APPS');

// Convert to lowercase.
assert('STRUCTURED WEB APPS'.toLowerCase() == 'structured web apps');

Note

These methods don't work for every language. For example, the Turkish alphabet's dotless I is converted incorrectly.

Trimming and empty strings

Remove all leading and trailing white space with trim(). To check whether a string is empty (length is zero), use isEmpty.

lang-dart
ch03/string-tests.dart
// Trim a string.
assert('  hello  '.trim() == 'hello');

// Check whether a string is empty.
assert(''.isEmpty);

// Strings with only white space are not empty.
assert(!'  '.isEmpty);

Replacing part of a string

Strings are immutable objects, which means you can create them but you can’t change them. If you look closely at the String API docs, you’ll notice that none of the methods actually changes the state of a String. For example, the method replaceAll() returns a new String without changing the original String:

lang-dart
ch03/string-tests.dart
var greetingTemplate = 'Hello, NAME!';
var greeting = greetingTemplate.replaceAll(new RegExp('NAME'), 'Bob');

assert(greeting != greetingTemplate); // greetingTemplate didn't change.

Building a string

To programmatically generate a string, you can use StringBuffer. A StringBuffer doesn’t generate a new String object until toString() is called. The writeAll() method has an optional second parameter that lets you specify a separator—in this case, a space.

lang-dart
ch03/string-tests.dart
var sb = new StringBuffer();

sb..write('Use a StringBuffer ')
  ..writeAll(['for', 'efficient', 'string', 'creation'], ' ')
  ..write('.');

var fullString = sb.toString();

assert(fullString ==
    'Use a StringBuffer for efficient string creation.');

Regular expressions

The RegExp class provides the same capabilities as JavaScript regular expressions. Use regular expressions for efficient searching and pattern matching of strings.

lang-dart
ch03/string-tests.dart
// Here's a regular expression for one or more digits.
var numbers = new RegExp(r'\d+');

var allCharacters = 'llamas live fifteen to twenty years';
var someDigits = 'llamas live 15 to 20 years';

// contains() can use a regular expression.
assert(!allCharacters.contains(numbers));
assert(someDigits.contains(numbers));

// Replace every match with another string.
var exedOut = someDigits.replaceAll(numbers, 'XX');
assert(exedOut == 'llamas live XX to XX years');

You can work directly with the RegExp class, too. The Match class provides access to a regular expression match.

lang-dart
ch03/string-tests.dart
var numbers = new RegExp(r'\d+');
var someDigits = 'llamas live 15 to 20 years';

// Check whether the reg exp has a match in a string.
assert(numbers.hasMatch(someDigits));

// Loop through all matches.
for (var match in numbers.allMatches(someDigits)) {
  print(match.group(0)); // 15, then 20
}

More information

Refer to the String API docs for a full list of methods. Also see the API docs for StringBuffer, Pattern, RegExp, and Match.

Collections

Dart ships with a core collections API, which includes classes for lists, sets, and maps.

Lists

As the language tour shows, you can use literals to create and initialize lists. Alternatively, use one of the List constructors. The List class also defines several methods for adding items to and removing items from lists.

lang-dart
ch03/list-tests.dart
// Use a List constructor.
var vegetables = new List();

// Or simply use a list literal.
var fruits = ['apples', 'oranges'];

// Add to a list.
fruits.add('kiwis');

// Add multiple items to a list.
fruits.addAll(['grapes', 'bananas']);

// Get the list length.
assert(fruits.length == 5);

// Remove a single item.
var appleIndex = fruits.indexOf('apples');
fruits.removeAt(appleIndex);
assert(fruits.length == 4);

// Remove all elements from a list.
fruits.clear();
assert(fruits.length == 0);

Use indexOf() to find the index of an object in a list:

lang-dart
ch03/list-tests.dart
var fruits = ['apples', 'oranges'];

// Access a list item by index.
assert(fruits[0] == 'apples');

// Find an item in a list.
assert(fruits.indexOf('apples') == 0);

Sort a list using the sort() method. You can provide a sorting function that compares two objects. This sorting function must return < 0 for smaller, 0 for the same, and > 0 for bigger. The following example uses compareTo(), which is defined by Comparable and implemented by String.

lang-dart
ch03/list-tests.dart
var fruits = ['bananas', 'apples', 'oranges'];

// Sort a list.
fruits.sort((a, b) => a.compareTo(b));
assert(fruits[0] == 'apples');

Lists are parameterized types, so you can specify the type that a list should contain:

lang-dart
ch03/list-tests.dart
// This list should contain only strings.
var fruits = new List<String>();

fruits.add('apples');
var fruit = fruits[0];
assert(fruit is String);

// Generates static analysis warning, num is not a string.
fruits.add(5);  // BAD: Throws exception in checked mode.

Refer to the List API docs for a full list of methods.

Sets

A set in Dart is an unordered collection of unique items. Because a set is unordered, you can’t get a set’s items by index (position).

lang-dart
ch03/set-tests.dart
var ingredients = new Set();
ingredients.addAll(['gold', 'titanium', 'xenon']);
assert(ingredients.length == 3);

// Adding a duplicate item has no effect.
ingredients.add('gold');
assert(ingredients.length == 3);

// Remove an item from a set.
ingredients.remove('gold');
assert(ingredients.length == 2);

Use contains() and containsAll() to check whether one or more objects are in a set:

lang-dart
ch03/set-tests.dart
var ingredients = new Set();
ingredients.addAll(['gold', 'titanium', 'xenon']);

// Check whether an item is in the set.
assert(ingredients.contains('titanium'));

// Check whether all the items are in the set.
assert(ingredients.containsAll(['titanium', 'xenon']));

An intersection is a set whose items are in two other sets.

lang-dart
ch03/set-tests.dart
var ingredients = new Set();
ingredients.addAll(['gold', 'titanium', 'xenon']);

// Create the intersection of two sets.
var nobleGases = new Set.from(['xenon', 'argon']);
var intersection = ingredients.intersection(nobleGases);
assert(intersection.length == 1);
assert(intersection.contains('xenon'));

Refer to the Set API docs for a full list of methods.

Maps

A map, commonly known as a dictionary or hash, is an unordered collection of key-value pairs. Maps associate a key to some value for easy retrieval. Unlike in JavaScript, Dart objects are not maps.

You can declare a map using a terse literal syntax, or you can use a traditional constructor:

lang-dart
ch03/map-1.dart
// Maps often use strings as keys.
var hawaiianBeaches = {
  'oahu'       : ['waikiki', 'kailua', 'waimanalo'],
  'big island' : ['wailea bay', 'pololu beach'],
  'kauai'      : ['hanalei', 'poipu']
};

// Maps can be built from a constructor.
var searchTerms = new Map();

// Maps are parameterized types; you can specify what types
// the key and value should be.
var nobleGases = new Map<int, String>();

You add, get, and set map items using the bracket syntax. Use remove() to remove a key and its value from a map.

lang-dart
ch03/map-1.dart
var nobleGases = { 54: 'xenon' };

// Retrieve a value with a key.
assert(nobleGases[54] == 'xenon');

// Check whether a map contains a key.
assert(nobleGases.containsKey(54));

// Remove a key and its value.
nobleGases.remove(54);
assert(!nobleGases.containsKey(54));

You can retrieve all the values or all the keys from a map:

lang-dart
ch03/map-1.dart
var hawaiianBeaches = {
  'oahu' : ['waikiki', 'kailua', 'waimanalo'],
  'big island' : ['wailea bay', 'pololu beach'],
  'kauai' : ['hanalei', 'poipu']
};

// Get all the keys as an unordered collection (an Iterable).
var keys = hawaiianBeaches.keys;

assert(keys.length == 3);
assert(new Set.from(keys).contains('oahu'));

// Get all the values as an unordered collection (an Iterable of Lists).
var values = hawaiianBeaches.values;
assert(values.length == 3);
assert(values.any((v) => v.contains('waikiki')));

To check whether a map contains a key, use containsKey(). Because map values can be null, you cannot rely on simply getting the value for the key and checking for null to determine the existence of a key.

lang-dart
ch03/map-1.dart
var hawaiianBeaches = {
  'oahu' : ['waikiki', 'kailua', 'waimanalo'],
  'big island' : ['wailea bay', 'pololu beach'],
  'kauai' : ['hanalei', 'poipu']
};

assert(hawaiianBeaches.containsKey('oahu'));
assert(!hawaiianBeaches.containsKey('florida'));

Use the putIfAbsent() method when you want to assign a value to a key if and only if the key does not already exist in a map. You must provide a function that returns the value.

lang-dart
ch03/map-1.dart
var teamAssignments = {};
teamAssignments.putIfAbsent('Catcher', () => pickToughestKid());
assert(teamAssignments['Catcher'] != null);

Refer to the Map API docs for a full list of methods.

Common collection methods

List, Set, and Map share common functionality found in many collections. Some of this common functionality is defined by the Iterable class, which List and Set implement.

Note

Although Map doesn’t implement Iterable, you can get Iterables from it using the Map keys and values properties.

Use isEmpty to check whether a list, set, or map has no items:

lang-dart
ch03/collection-isEmpty.dart
var teas = ['green', 'black', 'chamomile', 'earl grey'];
assert(!teas.isEmpty);

To apply a function to each item in a list, set, or map, you can use forEach():

lang-dart
ch03/collection-apply-function.dart
var teas = ['green', 'black', 'chamomile', 'earl grey'];

teas.forEach((tea) => print('I drink $tea'));

When you invoke forEach() on a map, your function must take two arguments (the key and value):

lang-dart
ch03/map-1.dart
hawaiianBeaches.forEach((k, v) {
  print('I want to visit $k and swim at $v');
  // I want to visit oahu and swim at [waikiki, kailua, waimanalo], etc.
});

Iterables provide the map() method, which gives you all the results in a single object:

lang-dart
ch03/collection-apply-function.dart
var teas = ['green', 'black', 'chamomile', 'earl grey'];

var loudTeas = teas.map((tea) => tea.toUpperCase());
loudTeas.forEach(print);

Note

The object returned by map() is an Iterable that’s lazily evaluated: your function isn’t called until you ask for an item from the returned object.

To force your function to be called immediately on each item, use map().toList() or map().toSet():

lang-dart
ch03/collection-apply-function.dart
var loudTeaList = teas.map((tea) => tea.toUpperCase()).toList();

Use Iterable’s where() method to get all the items that match a condition. Use Iterable’s any() and every() methods to check whether some or all items match a condition. {PENDING: if time permits, change example as suggested by floitsch to have (maybe) cities instead of isDecaffeinated.}

lang-dart
ch03/collection-any-every.dart
var teas = ['green', 'black', 'chamomile', 'earl grey'];

// Chamomile is not caffeinated.
bool isDecaffeinated(String teaName) => teaName == 'chamomile';

// Use where() to find only the items that return true
// from the provided function.
var decaffeinatedTeas = teas.where((tea) => isDecaffeinated(tea));
// or teas.where(isDecaffeinated)

// Use any() to check whether at least one item in the collection
// satisfies a condition.
assert(teas.any(isDecaffeinated));

// Use every() to check whether all the items in a collection
// satisfy a condition.
assert(!teas.every(isDecaffeinated));

For a full list of methods, refer to the Iterable API docs, as well as those for List, Set, and Map.

URIs

The Uri class provides functions to encode and decode strings for use in URIs (which you might know as URLs). These functions handle characters that are special for URIs, such as & and =. The Uri class also parses and exposes the components of a URI—host, port, scheme, and so on. {PENDING: show constructors: Uri.http, Uri.https, Uri.file, per floitsch's suggestion}

Encoding and Decoding Fully Qualified URIs

To encode and decode characters except those with special meaning in a URI (such as /, :, &, #), use the encodeFull() and decodeFull() methods. These methods are good for encoding or decoding a fully qualified URI, leaving intact special URI characters.

lang-dart
code/ch03/encodeUri.dart
main() {
  var uri = 'http://example.org/api?foo=some message';
  var encoded = Uri.encodeFull(uri);
  assert(encoded == 'http://example.org/api?foo=some%20message');

  var decoded = Uri.decodeFull(encoded);
  assert(uri == decoded);
}

Notice how only the space between some and message was encoded.

Encoding and Decoding URI Components

To encode and decode all of a string’s characters that have special meaning in a URI, including (but not limited to) /, &, and :, use the encodeComponent() and decodeComponent() methods.

lang-dart
code/ch03/encodeUriComponents.dart
main() {
  var uri = 'http://example.org/api?foo=some message';
  var encoded = Uri.encodeComponent(uri);
  assert(encoded == 'http%3A%2F%2Fexample.org%2Fapi%3Ffoo%3Dsome%20message');

  var decoded = Uri.decodeComponent(encoded);
  assert(uri == decoded);
}

Notice how every special character is encoded. For example, / is encoded to %2F.

Parsing URIs

If you have a Uri object or a URI string, you can get its parts using Uri fields such as path. To create a Uri from a string, use the parse() static method:

lang-dart
code/ch03/parseUri.dart
main() {
  var uri = Uri.parse('http://example.org:8080/foo/bar#frag');

  assert(uri.scheme   == 'http');
  assert(uri.host     == 'example.org');
  assert(uri.path     == '/foo/bar');
  assert(uri.fragment == 'frag');
  assert(uri.origin   == 'http://example.org:8080');
}

See the Uri API docs for more URI components that you can get.

Building URIs

You can build up a URI from individual parts using the Uri() constructor:

lang-dart
code/ch03/uriFromComponents.dart
main() {
  var uri = new Uri(scheme: 'http', host: 'example.org',
                    path: '/foo/bar', fragment: 'frag');
  assert(uri.toString() == 'http://example.org/foo/bar#frag');
}

Dates and Times

A DateTime object is a point in time. The time zone is either UTC or the local time zone.

You can create DateTime objects using several constructors:

lang-dart
ch03/date.dart
// Get the current date and time.
var now = new DateTime.now();

// Create a new DateTime with the local time zone.
var y2k = new DateTime(2000);   // January 1, 2000

// Specify the month and day.
y2k = new DateTime(2000, 1, 2); // January 2, 2000

// Specify the date as a UTC time.
y2k = new DateTime.utc(2000);   // January 1, 2000, UTC

// Specify a date and time in milliseconds since the Unix epoch.
y2k = new DateTime.fromMillisecondsSinceEpoch(946684800000, isUtc: true);

// Parse an ISO 8601 date.
y2k = DateTime.parse('2000-01-01T00:00:00Z');

The millisecondsSinceEpoch property of a date returns the number of milliseconds since the “Unix epoch”—January 1, 1970, UTC:

lang-dart
ch03/date.dart
var y2k = new DateTime.utc(2000);           // 1/1/2000, UTC
assert(y2k.millisecondsSinceEpoch == 946684800000);
var unixEpoch = new DateTime.utc(1970); // 1/1/1970, UTC
assert(unixEpoch.millisecondsSinceEpoch == 0);

Use the Duration class to calculate the difference between two dates and to shift a date forward or backward:

lang-dart
ch03/date.dart
var y2k = new DateTime.utc(2000);

// Add one year.
var y2001 = y2k.add(const Duration(days: 366));
assert(y2001.year == 2001);

// Subtract 30 days.
var december2000 = y2001.subtract(const Duration(days: 30));
assert(december2000.year == 2000);
assert(december2000.month == 12);

// Calculate the difference between two dates.
// Returns a Duration object.
var duration = y2001.difference(y2k);
assert(duration.inDays == 366); // y2k was a leap year.

Warning

Using a Duration to shift a DateTime by days can be problematic, due to clock shifts (to daylight saving time, for example). Use UTC dates if you must shift days.

Refer to the API docs for DateTime and Duration for a full list of methods.

Utility Classes

The core library contains various utility classes, useful for sorting, mapping values, and iterating.

Comparing objects

Implement the Comparable interface to indicate that an object can be compared to another object, usually for sorting. The compareTo() method returns < 0 for smaller, 0 for the same, and > 0 for bigger.

lang-dart
ch03/comparable.dart
class Line implements Comparable {
  final length;
  const Line(this.length);
  int compareTo(Line other) => length - other.length;
}

main() {
  var short = const Line(1);
  var long = const Line(100);
  assert(short.compareTo(long) < 0);
}

Implementing map keys

Each object in Dart automatically provides an integer hash code, and thus can be used as a key in a map. However, you can override the hashCode getter to generate a custom hash code. If you do, you might also want to override the == operator. Objects that are equal (via ==) must have identical hash codes. A hash code doesn’t have to be unique, but it should be well distributed.

Note: There’s disagreement over whether to include identical() in the == implementation. It might improve speed, at least when you need to compare many fields. They don’t do identical() automatically because, by convention, NaN != NaN.

lang-dart
ch03/map-keys.dart
class Person {
  final String firstName, lastName;

  Person(this.firstName, this.lastName);

  // Override hashCode using strategy from Effective Java, Chapter 11.
  int get hashCode {
    int result = 17;
    result = 37 * result + firstName.hashCode;
    result = 37 * result + lastName.hashCode;
    return result;
  }

  // You should generally implement operator== if you override hashCode.
  bool operator==(other) {
    if (other is! Person) return false;
    Person person = other;
    return (person.firstName == firstName && person.lastName == lastName);
  }
}

main() {
  var p1 = new Person('bob', 'smith');
  var p2 = new Person('bob', 'smith');
  var p3 = 'not a person';
  assert(p1.hashCode == p2.hashCode);
  assert(p1 == p2);
  assert(p1 != p3);
}

Iteration

The Iterable and Iterator classes support for-in loops. Extend (if possible) or implement Iterable whenever you create a class that can provide Iterators for use in for-in loops. Implement Iterator to define the actual iteration ability.

lang-dart
ch03/iterator.dart
class Process {
  // Represents a process...
}

class ProcessIterator implements Iterator<Process> {
  Process current;
  bool moveNext() {
    return false;
  }
}

// A mythical class that lets you iterate through all processes.
// Extends a subclass of Iterable.
class Processes extends IterableBase<Process> {
  final Iterator<Process> iterator = new ProcessIterator();
}

main() {
  // Iterable objects can be used with for-in.
  for (var process in new Processes()) {
    // Do something with the process.
  }
}

Exceptions

The Dart core library defines many common exceptions and errors. Exceptions are considered conditions that you can plan ahead for and catch. Errors are conditions that you don’t expect or plan for.

A couple of the most common errors are:

NoSuchMethodError

Thrown when a receiving object (which might be null) does not implement a method.

ArgumentError

Can be thrown by a method that encounters an unexpected argument.

Throwing an application-specific exception is a common way to indicate that an error has occurred. You can define a custom exception by implementing the Exception interface:

lang-dart
ch03/exceptions.dart
class FooException implements Exception {
  final String msg;
  const FooException([this.msg]);
  String toString() => msg == null ? 'FooException' : msg;
}

For more information, see the section called “Exceptions” and the Exception API docs.

dart:async - Asynchronous Programming

Asynchronous programming often uses callback functions, but Dart provides alternatives: Future and Stream objects. A Future is like a promise for a result to be provided sometime in the future. A Stream is a way to get a sequence of values, such as events. Future, Stream, and more are in the dart:async library.

The dart:async library works in both web apps and command-line apps. To use it, import dart:async:

lang-dart
import 'dart:async';

Future

Future objects appear throughout the Dart libraries, often as the object returned by an asynchronous method. When a future completes, its value is ready to use.

Basic Usage

Use then() to schedule code that runs when the future completes. For example, HttpRequest.getString() returns a Future, since HTTP requests can take a while. Using then() lets you run some code when that Future has completed and the promised string value is available:

lang-dart
ch03/ch03_4_async/web/ch03_4_async.dart
HttpRequest.getString(url)
  .then((String result) {
    print(result); });
  // Should handle errors here

Use catchError() to handle any errors or exceptions that a Future object might throw.

lang-dart
ch03/ch03_4_async/web/ch03_4_async.dart
HttpRequest.getString(url)
  .then((String result) {  // callback function
    print(result); })
  .catchError((e) {
    // Handle or ignore the error.
  });

The then().catchError() pattern is the asynchronous version of try-catch.

Note

Be sure to invoke catchError() on the result of then()—not on the result of the original Future. Otherwise, the catchError() can handle errors only from the original Future's computation, but not from the handler registered by then().

Chaining Multiple Asynchronous Methods

The then() method returns a Future, providing a useful way to run multiple asynchronous functions in a certain order. If the callback registered with then() returns a Future, then() returns an equivalent Future. If the callback returns a value of any other type, then() creates a new Future that completes with the value.

lang-dart
ch03/futures.dart
Future result = costlyQuery();

return result.then((value) => expensiveWork())
             .then((value) => lengthyComputation())
             .then((value) => print('done!'))
             .catchError((exception) => print('DOH!'));

In the preceding example, the methods run in the following order:

  1. costlyQuery()

  2. expensiveWork()

  3. lengthyComputation()

Waiting for Multiple Futures

Sometimes your algorithm needs to invoke many asynchronous functions and wait for them all to complete before continuing. Use the Future.wait() static method to manage multiple Futures and wait for them to complete:

lang-dart
ch03/futures.dart
Future deleteDone = deleteLotsOfFiles();
Future copyDone = copyLotsOfFiles();
Future checksumDone = checksumLotsOfOtherFiles();

Future.wait([deleteDone, copyDone, checksumDone]).then((List values) {
  print('Done with all the long steps');
});

Stream

Stream objects appear throughout Dart APIs, representing sequences of data. For example, HTML events such as button clicks are delivered using streams. You can also read a file as a stream.

Listening for Stream Data

To get each value as it arrives, subscribe to the stream using the listen() method:

lang-dart
ch03_html/ch03_html.dart
// Find a button by ID and add an event handler.
querySelector('#submitInfo').onClick.listen((e) {
  // When the button is clicked, it runs this code.
  submitData();
});

In this example, the onClick property is a Stream object provided by the 'submitInfo' button.

If you care about only one event, you can get it using a property such as first, last, or single. To test the event before handling it, use a method such as firstWhere(), lastWhere(), or singleWhere(). {PENDING: example}

If you care about a subset of events, you can use methods such as skip(), skipWhile(), take(), takeWhile(), and where(). {PENDING: example}

Transforming Stream Data

Often, you need to change the format of a stream's data before you can use it. Use the transform() method to produce a stream with a different type of data:

lang-dart
ch03/readFile.dart
var config = new File('config.txt');
Stream<List<int>> inputStream = config.openRead();

inputStream
  .transform(UTF8.decoder)
  .transform(new LineSplitter())
  .listen(
    (String line) {...} ...);

This example uses two transformers. First it uses UTF8.decoder to transform the stream of integers into a stream of strings. Then it uses a LineSplitter to transform the stream of strings into a stream of separate lines. These transformers are from the dart:convert library (see the section called “dart:convert - Decoding and Encoding JSON, UTF-8, and more”). {PENDING: add onDone and onError. (See "Streaming file contents".)}

More Information

For some examples of using Future and Stream in command-line apps, see the section called “dart:io - I/O for Command-Line Apps”. Also see these articles and tutorials:

dart:math - Math and Random

The Math library provides common functionality such as sine and cosine, maximum and minimum, and constants such as pi and e. Most of the functionality in the Math library is implemented as top-level functions.

To use the Math library in your app, import dart:math. The following examples use the prefix math to make clear which top-level functions and constants are from the Math library:

lang-dart
ch03/math-tests.dart
import 'dart:math' as math;

Trigonometry

The Math library provides basic trigonometric functions:

lang-dart
ch03/math-tests.dart
// Cosine
assert(math.cos(math.PI) == -1.0);

// Sine
var degrees = 30;
var radians = degrees * (math.PI / 180);
// radians is now 0.52359.
var sinOf30degrees = math.sin(radians);

// Truncate the decimal places to 2.
assert(double.parse(sinOf30degrees.toStringAsPrecision(2)) == 0.5);

Note

These functions use radians, not degrees!

Maximum and Minimum

The Math library provides max() and min() methods:

lang-dart
ch03/math-tests.dart
assert(math.max(1, 1000) == 1000);
assert(math.min(1, -1000) == -1000);

Math Constants

Find your favorite constants—pi, e, and more—in the Math library:

lang-dart
ch03/math-tests.dart
// See the Math library for additional constants.
print(math.E);     // 2.718281828459045
print(math.PI);    // 3.141592653589793
print(math.SQRT2); // 1.4142135623730951

Random Numbers

Generate random numbers with the Random class. You can optionally provide a seed to the Random constructor.

lang-dart
ch03/math-tests.dart
var random = new math.Random();
random.nextDouble(); // Between 0.0 and 1.0: [0, 1)
random.nextInt(10);  // Between 0 and 9.

You can even generate random booleans:

lang-dart
ch03/math-tests.dart
var random = new math.Random();
random.nextBool();  // true or false

More Information

Refer to the Math API docs for a full list of methods. Also see the API docs for num, int, and double.

dart:html - Browser-Based Apps

Use the dart:html library to program the browser, manipulate objects and elements in the DOM, and access HTML5 APIs. DOM stands for Document Object Model, which describes the hierarchy of an HTML page.

Other common uses of dart:html are manipulating styles (CSS), getting data using HTTP requests, and exchanging data using WebSockets. HTML5 (and dart:html) has many additional APIs that this section doesn’t cover. Only web apps can use dart:html, not command-line apps.

Note

For higher level approaches to web app UIs, see Polymer.dart and AngularDart.

To use the HTML library in your web app, import dart:html:

lang-dart
ch03_html/ch03_html.dart
import 'dart:html';

Note

Parts of the dart:html library are experimental, as noted in the API documentation.

Manipulating the DOM

To use the DOM, you need to know about windows, documents, elements, and nodes.

A Window object represents the actual window of the web browser. Each Window has a Document object, which points to the document that's currently loaded. The Window object also has accessors to various APIs such as IndexedDB (for storing data), requestAnimationFrame (for animations), and more. In tabbed browsers, each tab has its own Window object.

With the Document object, you can create and manipulate Elements within the document. Note that the document itself is an element and can be manipulated.

The DOM models a tree of Nodes. These nodes are often elements, but they can also be attributes, text, comments, and other DOM types. Except for the root node, which has no parent, each node in the DOM has one parent and might have many children.

Finding elements

To manipulate an element, you first need an object that represents it. You can get this object using a query.

Find one or more elements using the top-level functions querySelector() and querySelectorAll(). You can query by ID, class, tag, name, or any combination of these. The CSS Selector Specification guide defines the formats of the selectors such as using a # prefix to specify IDs and a period (.) for classes.

The querySelector() function returns the first element that matches the selector, while querySelectorAll() returns a collection of elements that match the selector.

lang-dart
ch03_html/ch03_html.dart
Element elem1 = querySelector('#an-id');           // Find an element by id (an-id).
Element elem2 = querySelector('.a-class');         // Find an element by class (a-class).
List<Element> elems1 = querySelectorAll('div');    // Find all elements by tag (<div>).
List<Element> elems2 = querySelectorAll('input[type="text"]'); // Find all text inputs.

// Find all elements with the CSS class 'class' inside of a <p>
// that is inside an element with the ID 'id'.
List<Element> elems3 = querySelectorAll('#id p.class');

Manipulating elements

You can use properties to change the state of an element. Node and its subtype Element define the properties that all elements have. For example, all elements have classes, hidden, id, style, and title properties that you can use to set state. Subclasses of Element define additional properties, such as the href property of AnchorElement.

Consider this example of specifying an anchor element in HTML:

lang-dart
ch03_html/ch03_html.dart
<a id="example" href="http://example.com">link text</a>

This <a> tag specifies an element with an href attribute and a text node (accessible via a text property) that contains the string “linktext”. To change the URL that the link goes to, you can use AnchorElement’s href property:

lang-dart
ch03_html/ch03_html.dart
querySelector('#example').href = 'http://dartlang.org';

Often you need to set properties on multiple elements. For example, the following code sets the hidden property of all elements that have a class of “mac”, “win”, or “linux”. Setting the hidden property to true has the same effect as adding display:none to the CSS.

lang-dart
ch03_html/ch03_html.dart
<!-- In HTML: -->
<p>
  <span class="os linux">Words for Linux</span>
  <span class="os mac">Words for Mac</span>
  <span class="os win">Words for Windows</span>
</p>

// In Dart:
final osList = ['mac', 'win', 'linux'];

var userOs = 'linux'; // In real code you'd programmatically determine this.

for (var os in osList) {            // For each possible OS...
  bool shouldShow = (os == userOs); // Does this OS match the user's OS?
  for (var elem in querySelectorAll('.$os')) { // Find all elements for this OS.
    elem.hidden = !shouldShow;      // Show or hide each element.
  }
}

When the right property isn’t available or convenient, you can use Element’s attributes property. This property is a Map<String, String>, where the keys are attribute names. For a list of attribute names and their meanings, see the MDN Attributes page. Here’s an example of setting an attribute’s value:

lang-dart
ch03_html/ch03_html.dart
elem.attributes['someAttribute'] = 'someValue';

Creating elements

You can add to existing HTML pages by creating new elements and attaching them to the DOM. Here’s an example of creating a paragraph (<p>) element:

lang-dart
ch03_html/ch03_html.dart
var elem = new ParagraphElement();
elem.text = 'Creating is easy!';

You can also create an element by parsing HTML text. Any child elements are also parsed and created.

lang-dart
ch03_html/ch03_html.dart
var elem2 = new Element.html('<p>Creating <em>is</em> easy!</p>');

Note that elem2 is a ParagraphElement in the above example.

Attach the newly created element to the document by assigning a parent to the element. You can add an element to any existing element’s children. In the following example, body is an element, and its child elements are accessible (as a List<Element>) from the children property.

lang-dart
ch03_html/ch03_html.dart
document.body.children.add(elem2);

Adding, replacing, and removing nodes

Recall that elements are just a kind of node. You can find all the children of a node using the nodes property of Node, which returns a List<Node> (as opposed to children, which omits non-Element nodes). Once you have this list, you can use the usual List methods and operators to manipulate the children of the node.

To add a node as the last child of its parent, use the List add() method:

lang-dart
ch03_html/ch03_html.dart
// Find the parent by ID, and add elem as its last child.
querySelector('#inputs').nodes.add(elem);

To replace a node, use the Node replaceWith() method:

lang-dart
ch03_html/ch03_html.dart
// Find a node by ID, and replace it in the DOM.
querySelector('#status').replaceWith(elem);

To remove a node, use the Node remove() method:

lang-dart
ch03_html/ch03_html.dart
// Find a node by ID, and remove it from the DOM.
querySelector('#expendable').remove();

Manipulating CSS styles

CSS, or cascading style sheets, defines the presentation styles of DOM elements. You can change the appearance of an element by attaching ID and class attributes to it.

Each element has a classes field, which is a list. Add and remove CSS classes simply by adding and removing strings from this collection. For example, the following sample adds the warning class to an element:

lang-dart
ch03_html/ch03_html.dart
var element = querySelector('#message');
element.classes.add('warning');

It’s often very efficient to find an element by ID. You can dynamically set an element ID with the id property:

lang-dart
ch03_html/ch03_html.dart
var message = new DivElement();
message.id = 'message2';
message.text = 'Please subscribe to the Dart mailing list.';

You can reduce the redundant text in this example by using method cascades:

lang-dart
ch03_html/ch03_html.dart
var message = new DivElement()
    ..id = 'message2'
    ..text = 'Please subscribe to the Dart mailing list.';

While using IDs and classes to associate an element with a set of styles is best practice, sometimes you want to attach a specific style directly to the element:

lang-dart
ch03_html/ch03_html.dart
message.style
    ..fontWeight = 'bold'
    ..fontSize = '3em';

Handling events

To respond to external events such as clicks, changes of focus, and selections, add an event listener. You can add an event listener to any element on the page. Event dispatch and propagation is a complicated subject; research the details if you’re new to web programming.

Add an event handler using element.onEvent.listen(function), where Event is the event name and function is the event handler.

For example, here’s how you can handle clicks on a button:

lang-dart
ch03_html/ch03_html.dart
// Find a button by ID and add an event handler.
querySelector('#submitInfo').onClick.listen((e) {
  // When the button is clicked, it runs this code.
  submitData();
});

Events can propagate up and down through the DOM tree. To discover which element originally fired the event, use e.target:

lang-dart
ch03_html/ch03_html.dart
document.body.onClick.listen((e) {
  var clickedElem = e.target;
  print('You clicked the ${clickedElem.id} element.');
});

To see all the events for which you can register an event listener, look for "onEventType" properties in the API docs for Element and its subclasses. Some common events include:

  • change

  • blur

  • keyDown

  • keyUp

  • mouseDown

  • mouseUp

Using HTTP Resources with HttpRequest

Formerly known as XMLHttpRequest, the HttpRequest class gives you access to HTTP resources from within your browser-based app. Traditionally, AJAX-style apps make heavy use of HttpRequest. Use HttpRequest to dynamically load JSON data or any other resource from a web server. You can also dynamically send data to a web server.

The following examples assume all resources are served from the same web server that hosts the script itself. Due to security restrictions in the browser, the HttpRequest class can’t easily use resources that are hosted on an origin that is different from the origin of the app. If you need to access resources that live on a different web server, you need to either use a technique called JSONP or enable CORS headers on the remote resources.

Getting data from the server

The HttpRequest static method getString() is an easy way to get data from a web server. Use then() after getString() to specify the function that handles the string data.

lang-dart
ch03_html/ch03_2_html.dart
import 'dart:html';
import 'dart:async';

// A JSON-formatted file in the same location as this page.
var uri = 'data.json';

main() {
  // Read a JSON file.
  HttpRequest.getString(uri).then(processString);
}

processString(String jsonText) {
  parseText(jsonText);
}

The function you specify (in the example, processString()) runs when the data at the specified URI is successfully retrieved. In this case, we are dynamically loading a JSON file. Information about the JSON API is in the section called “Decoding and Encoding JSON”.

Add .catchError() after the call to .then() to specify an error handler:

lang-dart
ch03_html/ch03_2_html.dart
...
HttpRequest.getString(uri)
    .then(processString)
    .catchError(handleError);
...
handleError(error) {
  print('Uh oh, there was an error.');
  print(error.toString());
}

If you need access to the HttpRequest, not just the text data it retrieves, you can use the request() static method instead of getString(). Here’s an example of reading XML data:

lang-dart
ch03_html/ch03_2_html.dart
import 'dart:html';
import 'dart:async';

// An XML-formatted file in the same location as this page.
var xmlUri = 'data.xml';

main() {
  // Read an XML file.
  HttpRequest.request(xmlUri)
      .then(processRequest)
      .catchError(handleError);
}

processRequest(HttpRequest request) {
  var xmlDoc = request.responseXml;
  try {
    var license = xmlDoc.querySelector('license').text;
    print('License: $license');
  } catch(e) {
    print('$xmlUri doesn\'t have correct XML formatting.');
  }
}
...

You can also use the full API to handle more interesting cases. For example, you can set arbitrary headers.

The general flow for using the full API of HttpRequest is as follows:

  1. Create the HttpRequest object.

  2. Open the URL with either GET or POST.

  3. Attach event handlers.

  4. Send the request.

For example:

lang-dart
dart-tutorials-samples/web/portmanteaux/portmanteaux.dart
import 'dart:html';
...
var httpRequest = new HttpRequest()
    ..open('POST', dataUrl)
    ..onLoadEnd.listen((_) => requestComplete(httpRequest))
    ..send(encodedData);

Sending data to the server

HttpRequest can send data to the server using the HTTP method POST. For example, you might want to dynamically submit data to a form handler. Sending JSON data to a RESTful web service is another common example.

Submitting data to a form handler requires you to provide name-value pairs as URI-encoded strings. (Information about the URI class is in the section called “URIs”.) You must also set the Content-type header to application/x-www-form-urlencode if you wish to send data to a form handler.

lang-dart
ch03/ch03/ch03_3_html/json-send-to-server.dart
import 'dart:html';

String encodeMap(Map data) {
  return data.keys.map((k) {
    return '${Uri.encodeComponent(k)}=${Uri.encodeComponent(data[k])}';
  }).join('&');
}

loadEnd(HttpRequest request) {
  if (request.status != 200) {
    print('Uh oh, there was an error of ${request.status}');
  } else {
    print('Data has been posted');
  }
}

main() {
  var dataUrl = '/registrations/create';
  var data = {'dart': 'fun', 'editor': 'productive'};
  var encodedData = encodeMap(data);

  var httpRequest = new HttpRequest();
  httpRequest.open('POST', dataUrl);
  httpRequest.setRequestHeader('Content-type',
                               'application/x-www-form-urlencoded');
  httpRequest.onLoadEnd.listen((e) => loadEnd(httpRequest));
  httpRequest.send(encodedData);
}

Sending and Receiving Real-Time Data with WebSockets

A WebSocket allows your web app to exchange data with a server interactively—no polling necessary. A server creates the WebSocket and listens for requests on a URL that starts with ws://—for example, ws://127.0.0.1:1337/ws. The data transmitted over a WebSocket can be a string, a blob, or an ArrayBuffer. Often, the data is a JSON-formatted string.

To use a WebSocket in your web app, first create a WebSocket object, passing the WebSocket URL as an argument:

lang-dart
github.com/dart-lang/dart-samples/.../html5/web/websockets/basics/websocket_sample.dart
var ws = new WebSocket('ws://echo.websocket.org');

Sending data

To send string data on the WebSocket, use the send() method:

lang-dart
github.com/dart-lang/dart-samples/.../html5/web/websockets/basics/websocket_sample.dart
ws.send('Hello from Dart!');

Receiving data

To receive data on the WebSocket, register a listener for message events:

lang-dart
github.com/dart-lang/dart-samples/.../html5/web/websockets/basics/websocket_sample.dart
ws.onMessage.listen((MessageEvent e) {
  print('Received message: ${e.data}');
});

The message event handler receives a MessageEvent object. This object’s data field has the data from the server.

Handling WebSocket events

Your app can handle the following WebSocket events: open, close, error, and (as shown earlier) message. Here’s an example of a method that creates a WebSocket object and registers handlers for open, close, error, and message events:

lang-dart
github.com/dart-lang/dart-samples/.../html5/web/websockets/basics/websocket_sample.dart
void initWebSocket([int retrySeconds = 2]) {
  var reconnectScheduled = false;

  print("Connecting to websocket");
  ws = new WebSocket('ws://echo.websocket.org');

  void scheduleReconnect() {
    if (!reconnectScheduled) {
      new Timer(new Duration(milliseconds: 1000 * retrySeconds),
                () => initWebSocket(retrySeconds * 2));
    }
    reconnectScheduled = true;
  }

  ws.onOpen.listen((e) {
    print('Connected');
    ws.send('Hello from Dart!');
  });

  ws.onClose.listen((e) {
    print('Websocket closed, retrying in $retrySeconds seconds');
    scheduleReconnect();
  });

  ws.onError.listen((e) {
    print("Error connecting to ws");
    scheduleReconnect();
  });

  ws.onMessage.listen((MessageEvent e) {
    print('Received message: ${e.data}');
  });
}

For more information and examples of using WebSockets, see the Dart Code Samples.

More information

This section barely scratched the surface of using the dart:html library. For more information, see the documentation for dart:html and the code and explanations in the Dart Code Samples. Dart has additional libraries for more specialized web APIs, such as web audio, IndexedDB, and WebGL.

dart:io - I/O for Command-Line Apps

The dart:io library provides APIs to deal with files, directories, processes, sockets, WebSockets, and HTTP clients and servers. Only command-line apps can use dart:io—not web apps.

In general, the dart:io library implements and promotes an asynchronous API. Synchronous methods can easily block an application, making it difficult to scale. Therefore, most operations return results via Future or Stream objects, a pattern common with modern server platforms such as Node.js.

The few synchronous methods in the dart:io library are clearly marked with a Sync suffix on the method name. We don’t cover them here.

Note

Only command-line apps can import and use dart:io.

Files and Directories

The I/O library enables command-line apps to read and write files and browse directories. You have two choices for reading the contents of a file: all at once, or streaming. Reading a file all at once requires enough memory to store all the contents of the file. If the file is very large or you want to process it while reading it, you should use a Stream, as described in the section called “Streaming file contents”.

Reading a file as text

When reading a text file encoded using UTF-8, you can read the entire file contents with readAsString(). When the individual lines are important, you can use readAsLines(). In both cases, a Future object is returned that provides the contents of the file as one or more strings.

lang-dart
ch03/textRead.dart
import 'dart:io';

main() {
  var config = new File('config.txt');

  // Put the whole file in a single string.
  config.readAsString().then((String contents) {
    print('The entire file is ${contents.length} characters long');
  });

  // Put each line of the file into its own string.
  config.readAsLines().then((List<String> lines) {
    print('The entire file is ${lines.length} lines long');
  });
}

Reading a file as binary

The following code reads an entire file as bytes into a list of ints. The call to readAsBytes() returns a Future, which provides the result when it’s available.

lang-dart
ch03/binaryRead.dart
import 'dart:io';

main() {
  var config = new File('config.txt');

  config.readAsBytes().then((List<int> contents) {
    print('The entire file is ${contents.length} bytes long');
  });
}

Handling errors

To capture errors so they don't result in uncaught exceptions, you can register a catchError handler:

lang-dart
ch03/fileErrors.dart
import 'dart:io';

main() {
  var config = new File('config.txt');
  config.readAsString().then((String contents) {
    print(contents);
  }).catchError((e) {
    print(e);
  });
}

Streaming file contents

Use a Stream to read a file, a little at a time. The listen() method specifies a handler to be called when data is available. When the Stream is finished reading the file, the onDone callback executes.

lang-dart
ch03/readFile.dart
import 'dart:io';
import 'dart:convert';
import 'dart:async';

main() {
  var config = new File('config.txt');
  Stream<List<int>> inputStream = config.openRead();

  inputStream
    .transform(UTF8.decoder)
    .transform(new LineSplitter())
    .listen(
      (String line) {
        print('Got ${line.length} characters from stream');
      },
      onDone: () { print('file is now closed'); },
      onError: (e) { print(e.toString()); });
}

Writing file contents

You can use an IOSink to write data to a file. Use the File openWrite() method to get an IOSink that you can write to. The default mode, FileMode.WRITE, completely overwrites existing data in the file.

lang-dart
ch03/writeFile.dart
var logFile = new File('log.txt');
var sink = logFile.openWrite();
sink.write('FILE ACCESSED ${new DateTime.now()}\n');
sink.close();

To add to the end of the file, use the optional mode parameter to specify FileMode.APPEND:

lang-dart
ch03/writeFile.dart
var sink = logFile.openWrite(mode: FileMode.APPEND); 

To write binary data, use add(List<int> data).

Listing files in a directory

Finding all files and subdirectories for a directory is an asynchronous operation. The list() method returns a Stream on which you can register handlers (using listen()) to be notified when a file or directory is encountered.

lang-dart
ch03/listFiles.dart
import 'dart:io';
import 'dart:async';

main() {
  var dir = new Directory('/tmp');
  var contentsStream = dir.list(recursive:true);
  contentsStream.listen(
    (FileSystemEntity f) {
      if (f is File) {
        print('Found file ${f.path}');
      } else if (f is Directory) {
        print('Found dir ${f.path}');
      }
    },
    onError: (e) { print(e.toString()); }
  );
}

Other common functionality

The File and Directory classes contain other functionality, including but not limited to:

  • Creating a file or directory: create() in File and Directory

  • Deleting a file or directory: delete() in File and Directory

  • Getting the length of a file: length() in File

  • Getting random access to a file: open() in File

Refer to the API docs for File and Directory for a full list of methods.

HTTP Clients and Servers

The dart:io library provides classes that command-line apps can use for accessing HTTP resources, as well as running HTTP servers.

HTTP server

The HttpServer class provides the low-level functionality for building web servers. You can match request handlers, set headers, stream data, and more.

The following sample web server can return only simple text information. This server listens on port 8888 and address 127.0.0.1 (localhost), responding to requests for the path /languages/dart. All other requests are handled by the default request handler, which returns a response code of 404 (not found).

lang-dart
ch03/httpServer.dart
import 'dart:io';

main() {
  dartHandler(HttpRequest request) {
    request.response.headers.contentType = new ContentType('text', 'plain');
    request.response.write('Dart is optionally typed');
    request.response.close();
  };

  HttpServer.bind('127.0.0.1', 8888).then((HttpServer server) {
    server.listen((request) {
      print('Got request for ${request.uri.path}');
      if (request.uri.path == '/languages/dart') {
        dartHandler(request);
      } else {
        request.response.write('Not found');
        request.response.close();
      }
    });
  });
}

For a more comprehensive HTTP server, see Chapter 5, Walkthrough: Dartiverse Search. It features the Dartiverse Search sample, which implements a web server using dart:io and packages such as http_server and route.

HTTP client

The HttpClient class helps you connect to HTTP resources from your Dart command-line or server-side application. You can set headers, use HTTP methods, and read and write data. The HttpClient class does not work in browser-based apps. When programming in the browser, use the HttpRequest class. Here’s an example of using HttpClient:

lang-dart
ch03/httpClient.dart
import 'dart:io';
import 'dart:convert';

main() {
  var url = Uri.parse('http://127.0.0.1:8888/languages/dart');
  var httpClient = new HttpClient();
  httpClient.getUrl(url)
    .then((HttpClientRequest request) {
      print('have request');
      return request.close();
    })
    .then((HttpClientResponse response) {
      print('have response');
      response.transform(UTF8.decoder).toList().then((data) {
        var body = data.join('');
        print(body);
        httpClient.close();
      });
    });
}

More Information

Besides the APIs discussed in this section, the dart:io library also provides APIs for processes, sockets, and web sockets.

For more examples of using dart:io, see the Dart Code Samples.

dart:convert - Decoding and Encoding JSON, UTF-8, and more

The dart:convert library has converters for JSON and UTF-8, as well as support for creating additional converters. JSON is a simple text format for representing structured objects and collections. UTF-8 is a common variable-width encoding that can represent every character in the Unicode character set.

The dart:convert library works in both web apps and command-line apps. To use it, import dart:convert.

Decoding and Encoding JSON

Decode a JSON-encoded string into a Dart object with JSON.decode():

lang-dart
ch03/jsonParse.dart
import 'dart:convert' show JSON;

main() {
  // NOTE: Be sure to use double quotes ("), not single quotes ('),
  // inside the JSON string. This string is JSON, not Dart.
  var jsonString = '''
  [
    {"score": 40},
    {"score": 80}
  ]
  ''';

  var scores = JSON.decode(jsonString);
  assert(scores is List);

  var firstScore = scores[0];
  assert(firstScore is Map);
  assert(firstScore['score'] == 40);
}

Encode a supported Dart object into a JSON-formatted string with JSON.encode():

lang-dart
ch03/jsonStringify.dart
import 'dart:convert' show JSON;

main() {
  var scores = [
    {'score': 40},
    {'score': 80},
    {'score': 100, 'overtime': true, 'special_guest': null}
  ];

  var jsonText = JSON.encode(scores);
  assert(jsonText == '[{"score":40},{"score":80},'
                     '{"score":100,"overtime":true,'
                     '"special_guest":null}]');
}

Only objects of type int, double, String, bool, null, List, or Map (with string keys) are directly encodable into JSON. List and Map objects are encoded recursively.

You have two options for encoding objects that aren't directly encodable. The first is to invoke encode() with a second argument: a function that returns an object that is directly encodable. Your second option is to omit the second argument, in which case the encoder calls the object's toJson() method.

Decoding and Encoding UTF-8 Characters

Use UTF8.decode() to decode UTF8-encoded bytes to a Dart string:

lang-dart
ch03/decodeUtf8.dart
import 'dart:convert' show UTF8;

main() {
  var string = UTF8.decode([0xc3, 0x8e, 0xc3, 0xb1, 0xc5, 0xa3, 0xc3, 0xa9,
                            0x72, 0xc3, 0xb1, 0xc3, 0xa5, 0xc5, 0xa3, 0xc3,
                            0xae, 0xc3, 0xb6, 0xc3, 0xb1, 0xc3, 0xa5, 0xc4,
                            0xbc, 0xc3, 0xae, 0xc5, 0xbe, 0xc3, 0xa5, 0xc5,
                            0xa3, 0xc3, 0xae, 0xe1, 0xbb, 0x9d, 0xc3, 0xb1]);
  print(string); // 'Îñţérñåţîöñåļîžåţîờñ'
}

To convert a stream of UTF-8 characters into a Dart string, specify UTF8.decoder to the Stream transform() method:

lang-dart
ch03/readFile.dart
inputStream
  .transform(UTF8.decoder)
  .transform(new LineSplitter())
  .listen(
    (String line) {
      print('Read ${line.length} bytes from stream');
    });

Use UTF8.encode() to encode a Dart string as a list of UTF8-encoded bytes:

lang-dart
ch03/encodeUtf8.dart
import 'dart:convert' show UTF8;

main() {
  List<int> expected = [0xc3, 0x8e, 0xc3, 0xb1, 0xc5, 0xa3, 0xc3, 0xa9, 0x72,
                        0xc3, 0xb1, 0xc3, 0xa5, 0xc5, 0xa3, 0xc3, 0xae, 0xc3,
                        0xb6, 0xc3, 0xb1, 0xc3, 0xa5, 0xc4, 0xbc, 0xc3, 0xae,
                        0xc5, 0xbe, 0xc3, 0xa5, 0xc5, 0xa3, 0xc3, 0xae, 0xe1,
                        0xbb, 0x9d, 0xc3, 0xb1];

  List<int> encoded = UTF8.encode('Îñţérñåţîöñåļîžåţîờñ');

  assert(() {
    if (encoded.length != expected.length) return false;
    for (int i = 0; i < encoded.length; i++) {
      if (encoded[i] != expected[i]) return false;
    }
    return true;
  });
}

Other Functionality

The dart:convert library also has converters for ASCII and ISO-8859-1 (Latin1). For details, see the API docs for the dart:convert library.

dart:mirrors - Reflection

The dart:mirrors library provides basic reflection abilities to Dart. Use mirrors to query the structure of your program and to dynamically invoke functions or methods at runtime.

The dart:mirrors library works in both web apps and command-line apps. To use it, import dart:mirrors.

Warning

Using dart:mirrors can cause dart2js to generate very large JavaScript files.

The current workaround is to add a @MirrorsUsed annotation before the import of dart:mirrors. For details, see the MirrorsUsed API documentation. This workaround is very likely to change, as the dart:mirrors library is still under development.

Symbols

The mirror system represents the names of Dart declarations (classes, fields, and so on) by instances of the class Symbol. Symbols work even in code where names have changed due to minification.

When you know the name of the symbol ahead of time, use a symbol literal. This way, repeated uses of the same symbol can use the same canonicalized instance. If the name of the symbol is determined dynamically at runtime, use the Symbol constructor.

lang-dart
ch03/mirrors.dart
import 'dart:mirrors';

// If the symbol name is known at compile time.
const className = #MyClass;

// If the symbol name is dynamically determined.
var userInput = askUserForNameOfFunction();
var functionName = new Symbol(userInput);

During minification, a compiler might replace a symbol name with a different (often smaller) name. To convert from a symbol back to a string, use MirrorSystem.getName(). This function returns the correct name, even if the code was minified.

lang-dart
ch03/mirrors.dart
import 'dart:mirrors';

const className = #MyClass;
assert('MyClass' == MirrorSystem.getName(className));

Introspection

Use mirrors to introspect the running program's structure. You can inspect classes, libraries, instances, and more.

The examples in this section use the following Person class:

lang-dart
ch03/mirrors.dart
class Person {
  String firstName;
  String lastName;
  int age;

  Person(this.firstName, this.lastName, this.age);

  String get fullName => '$firstName $lastName';

  void greet(String other) {
    print('Hello there, $other!');
  }
}

To begin, you need to reflect on a class or object to get its mirror.

Class Mirrors

Reflect on a Type to get its ClassMirror.

lang-dart
ch03/mirrors.dart
ClassMirror mirror = reflectClass(Person);

assert('Person' == MirrorSystem.getName(mirror.simpleName));

You can also call runtimeType to get a Type from an instance.

lang-dart
ch03/mirrors.dart
var person = new Person('Bob', 'Smith', 33);
ClassMirror mirror = reflectClass(person.runtimeType);

assert('Person' == MirrorSystem.getName(mirror.simpleName));

Once you have a ClassMirror, you can get a class's constructors, fields, and more. Here is an example of listing the constructors of a class.

lang-dart
ch03/mirrors.dart
showConstructors(ClassMirror mirror) {
  var constructors = mirror.declarations.values
      .where((m) => m is MethodMirror && m.isConstructor);

  constructors.forEach((m) {
    print('The constructor ${m.simpleName} has '
          '${m.parameters.length} parameters.');
  });
}

Here is an example of listing all of the fields declared by a class.

lang-dart
ch03/mirrors.dart
showFields(ClassMirror mirror) {
  var fields = mirror.declarations.values.where((m) => m is VariableMirror);

  fields.forEach((VariableMirror m) {
    var finalStatus = m.isFinal ? 'final' : 'not final';
    var privateStatus = m.isPrivate ? 'private' : 'not private';
    var typeAnnotation = m.type.simpleName;

    print('The field ${m.simpleName} is $privateStatus and $finalStatus '
          'and is annotated as $typeAnnotation.');
  });
}

For a full list of methods, consult the API docs for ClassMirror.

Instance Mirrors

Reflect on an object to get an InstanceMirror.

lang-dart
ch03/mirrors.dart
var p = new Person('Bob', 'Smith', 42);
InstanceMirror mirror = reflect(p);

If you have an InstanceMirror and you want to get the object that it reflects, use reflectee.

lang-dart
ch03/mirrors.dart
var person = mirror.reflectee;
assert(identical(p, person));

Invocation

Once you have an InstanceMirror, you can invoke methods and call getters and setters. For a full list of methods, consult the API docs for InstanceMirror.

Invoke Methods

Use InstanceMirror's invoke() method to invoke a method on an object. The first parameter specifies the method to be invoked, and the second is a list of positional arguments to the method. An optional third parameter lets you specify named arguments.

lang-dart
ch03/mirrors.dart
var p = new Person('Bob', 'Smith', 42);
InstanceMirror mirror = reflect(p);

mirror.invoke(#greet, ['Shailen']);

Invoke Getters and Setters

Use InstanceMirror's getField() and setField() methods to get and set properties of an object.

lang-dart
ch03/mirrors.dart
var p = new Person('Bob', 'Smith', 42);
InstanceMirror mirror = reflect(p);

// Get the value of a property.
var fullName = mirror.getField(#fullName).reflectee;
assert(fullName == 'Bob Smith');

// Set the value of a property.
mirror.setField(#firstName, 'Mary');
assert(p.firstName == 'Mary');

More Information

The article Reflection in Dart with Mirrors has more information and examples. Also see the API docs for dart:mirror, especially MirrorsUsed, ClassMirror, and InstanceMirror.

Summary

This chapter introduced you to the most commonly used functionality in many of Dart’s built-in libraries. It didn’t cover all the built-in libraries, however. Others that you might want to look into include dart:collection, dart:isolate, dart:js, and dart:typed_data. You can get yet more libraries by using the pub tool, discussed in the next chapter. The args, logging, polymer, and unittest libraries are just a sampling of what you can install using pub.