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Effective Dart: Usage

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This is the most “blue-collar” guide in Effective Dart. You’ll apply the guidelines here every day in the bodies of your Dart code. Users of your library may not be able to tell that you’ve internalized the ideas here, but maintainers of it sure will.

Strings

Here are some best practices to keep in mind when composing strings in Dart.

DO use adjacent strings to concatenate string literals.

If you have two string literals—not values, but the actual quoted literal form—you do not need to use + to concatenate them. Just like in C and C++, simply placing them next to each other does it. This is a good way to make a single long string that doesn’t fit on one line.

raiseAlarm(
    'ERROR: Parts of the spaceship are on fire. Other '
    'parts are overrun by martians. Unclear which are which.');
raiseAlarm(
    'ERROR: Parts of the spaceship are on fire. Other ' +
    'parts are overrun by martians. Unclear which are which.');

PREFER using interpolation to compose strings and values.

If you’re coming from other languages, you’re used to using long chains of + to build a string out of literals and other values. That does work in Dart, but it’s almost always cleaner and shorter to use interpolation:

'Hello, $name! You are ${year - birth} years old.';
'Hello, ' + name + '! You are ' + (year - birth) + ' years old.';

AVOID using curly braces in interpolation when not needed.

If you’re interpolating a simple identifier not immediately followed by more alphanumeric text, the {} should be omitted.

'Hi, $name!'
"Wear your wildest $decade's outfit."
'Wear your wildest ${decade}s outfit.'
'Hi, ${name}!'
"Wear your wildest ${decade}'s outfit."

Collections

Out of the box, Dart supports four collection types: lists, maps, queues, and sets. The following best practices apply to collections.

DO use collection literals when possible.

There are two ways to make an empty growable list: [] and new List(). Likewise, there are three ways to make an empty linked hash map: {}, new Map(), and new LinkedHashMap().

If you want to create a non-growable list, or some other custom collection type then, by all means, use a constructor. Otherwise, use the nice literal syntax. The core library exposes those constructors to ease adoption, but idiomatic Dart code does not use them.

var points = [];
var addresses = {};
var points = new List();
var addresses = new Map();

You can even provide a type argument for them if that matters.

var points = <Point>[];
var addresses = <String, Address>{};
var points = new List<Point>();
var addresses = new Map<String, Address>();

Note that this doesn’t apply to the named constructors for those classes. List.from(), Map.fromIterable(), and friends all have their uses. Likewise, if you’re passing a size to new List() to create a non-growable one, then it makes sense to use that.

DON’T use .length to see if a collection is empty.

The Iterable contract does not require that a collection know its length or be able to provide it in constant time. Calling .length just to see if the collection contains anything can be painfully slow.

Instead, there are faster and more readable getters: .isEmpty and .isNotEmpty. Use the one that doesn’t require you to negate the result.

if (lunchBox.isEmpty) return 'so hungry...';
if (words.isNotEmpty) return words.join(' ');
if (lunchBox.length == 0) return 'so hungry...';
if (!words.isEmpty) return words.join(' ');

CONSIDER using higher-order methods to transform a sequence.

If you have a collection and want to produce a new modified collection from it, it’s often shorter and more declarative to use .map(), .where(), and the other handy methods on Iterable.

Using those instead of an imperative for loop makes it clear that your intent is to produce a new sequence and not to produce side effects.

var aquaticNames = animals
    .where((animal) => animal.isAquatic)
    .map((animal) => animal.name);

At the same time, this can be taken too far. If you are chaining or nesting many higher-order methods, it may be clearer to write a chunk of imperative code.

AVOID using Iterable.forEach() with a function literal.

forEach() functions are widely used in JavaScript because the built in for-in loop doesn’t do what you usually want. In Dart, if you want to iterate over a sequence, the idiomatic way to do that is using a loop.

for (var person in people) {
  ...
}
people.forEach((person) {
  ...
});

The exception is if all you want to do is invoke some already existing function on each element. In that case, forEach() is handy.

people.forEach(print);

Functions

In Dart, even functions are objects. Here are some best practices involving functions.

DO use a function declaration to bind a function to a name.

Modern languages have realized how useful local nested functions and closures are. It’s common to have a function defined inside another one. In many cases, this function is used as a callback immediately and doesn’t need a name. A function expression is great for that.

But, if you do need to give it a name, use a function declaration statement instead of binding a lambda to a variable.

void main() {
  localFunction() {
    ...
  }
}
void main() {
  var localFunction = () {
    ...
  };
}

DON’T create a lambda when a tear-off will do.

If you refer to a method on an object but omit the parentheses, Dart gives you a “tear-off”—a closure that takes the same parameters as the method and invokes it when you call it.

If you have a function that invokes a method with the same arguments as are passed to it, you don’t need to manually wrap the call in a lambda.

names.forEach(print);
names.forEach((name) {
  print(name);
});

Variables

The following best practices describe how to best use variables in Dart.

DON’T explicitly initialize variables to null.

In Dart, a variable or field that is not explicitly initialized automatically gets initialized to null. This is reliably specified by the language. There’s no concept of “uninitialized memory” in Dart. Adding = null is redundant and unneeded.

int _nextId;

class LazyId {
  int _id;

  int get id {
    if (_nextId == null) _nextId = 0;
    if (_id == null) _id = _nextId++;

    return _id;
  }
}
int _nextId = null;

class LazyId {
  int _id = null;

  int get id {
    if (_nextId == null) _nextId = 0;
    if (_id == null) _id = _nextId++;

    return _id;
  }
}

AVOID storing what you can calculate.

When designing a class, you often want to expose multiple views into the same underlying state. Often you see code that calculates all of those views in the constructor and then stores them:

class Circle {
  num radius;
  num area;
  num circumference;

  Circle(num radius)
      : radius = radius,
        area = math.PI * radius * radius,
        circumference = math.PI * 2.0 * radius;
}

This code has two things wrong with it. First, it’s likely wasting memory. The area and circumference, strictly speaking, are caches. They are stored calculations that we could recalculate from other data we already have. They are trading increased memory for reduced CPU usage. Do we know we have a performance problem that merits that trade-off?

Worse, the code is wrong. The problem with caches is invalidation—how do you know when the cache is out of date and needs to be recalculated? Here, we never do, even though radius is mutable. You can assign a different value and the area and circumference will retain their previous, now incorrect values.

To correctly handle cache invalidation, we need to do this:

class Circle {
  num _radius;
  num get radius => _radius;
  set radius(num value) {
    _radius = value;
    _recalculate();
  }

  num _area;
  num get area => _area;

  num _circumference;
  num get circumference => _circumference;

  Circle(this._radius) {
    _recalculate();
  }

  void _recalculate() {
    _area = math.PI * _radius * _radius,
    _circumference = math.PI * 2.0 * _radius;
  }
}

That’s an awful lot of code to write, maintain, debug, and read. Instead, your first implementation should be:

class Circle {
  num radius;

  num get area => math.PI * radius * radius;
  num get circumference => math.PI * 2.0 * radius;

  Circle(this.radius);
}

This code is shorter, uses less memory, and is less error-prone. It stores the minimal amount of data needed to represent the circle. There are no fields to get out of sync because there is only a single source of truth.

In some cases, you may need to cache the result of a slow calculation, but only do that after you know you have a performance problem, do it carefully, and leave a comment explaining the optimization.

CONSIDER omitting the types for local variables.

Method bodies in modern code tend to be short, and the types of local variables are almost always trivially inferrable from the initializing expression, so explicit type annotations are usually just visual noise.

Dart comes with powerful static analysis tools that will infer the type of local variables and still provide the auto-complete and tooling support you expect.

Map<int, List<Person>> groupByZip(Iterable<Person> people) {
  var peopleByZip = <int, List<Person>>{};
  for (var person in people) {
    peopleByZip.putIfAbsent(person.zip, () => <Person>[]);
    peopleByZip[person.zip].add(person);
  }
  return peopleByZip;
}
Map<int, List<Person>> groupByZip(Iterable<Person> people) {
  Map<int, List<Person>> peopleByZip = <int, List<Person>>{};
  for (Person person in people) {
    peopleByZip.putIfAbsent(person.zip, () => <Person>[]);
    peopleByZip[person.zip].add(person);
  }
  return peopleByZip;
}

Members

In Dart, objects have members which can be functions (methods) or data (instance variables). The following best practices apply to an object’s members.

DON’T wrap a field in a getter and setter unnecessarily.

In Java and C#, it’s common to hide all fields behind getters and setters (or properties in C#), even if the implementation just forwards to the field. That way, if you ever need to do more work in those members, you can without needing to touch the callsites. This is because calling a getter method is different than accessing a field in Java, and accessing a property isn’t binary-compatible with accessing a raw field in C#.

Dart doesn’t have this limitation. Fields and getters/setters are completely indistinguishable. You can expose a field in a class and later wrap it in a getter and setter without having to touch any code that uses that field.

class Box {
  var contents;
}
class Box {
  var _contents;
  get contents => _contents;
  set contents(value) {
    _contents = value;
  }
}

PREFER using a final field to make a read-only property.

If you have a field that outside code should be able to see but not assign to, a simple solution that works in many cases is to simply mark it final.

class Box {
  final contents = [];
}
class Box {
  var _contents;
  get contents => _contents;
}

Of course, if you need to internally assign to the field outside of the constructor, you may need to do the “private field, public getter” pattern, but don’t reach for that until you need to.

CONSIDER using => for short members whose body is a single return statement.

In addition to using => for function expressions, Dart also lets you define members with them. They are a good fit for simple members that just calculate and return a value.

get width => right - left;
bool ready(num time) => minTime == null || minTime <= time;
containsValue(String value) => getValues().contains(value);

Members that don’t fit on one line can still use =>, but if you find yourself cramming a single expression into several continued lines, it is probably cleaner to just use a curly body with an explicit return.

It’s not a good idea to use this for void members. Readers expect => to mean “returns a useful value”, so even though it can be terse to use => for a member that doesn’t return anything, it’s clearer to use { ... }.

DON’T use this. when not needed to avoid shadowing.

JavaScript requires an explicit this. to refer to members on the object whose method is currently being executed, but Dart—like C++, Java, and C#—doesn’t have that limitation.

The only time you need to use this. is when a local variable with the same name shadows the member you want to access.

class Box {
  var value;

  void clear() {
    this.update(null);
  }

  void update(value) {
    this.value = value;
  }
}
class Box {
  var value;

  void clear() {
    update(null);
  }

  void update(value) {
    this.value = value;
  }
}

Note that constructor parameters never shadow fields in constructor initialization lists:

class Box extends BaseBox {
  var value;

  Box(value)
      : value = value,
        super(value)
      {}
}

This looks surprising, but works like you want. Fortunately, code like this is relatively rare thanks to initializing formals.

DO initialize fields at their declaration when possible.

If a field doesn’t depend on any constructor parameters, it can and should be initialized at its declaration. It takes less code and makes sure you won’t forget to initialize it if the class has multiple constructors.

class Folder {
  final String name;
  final List<Document> contents;

  Folder(this.name) : contents = [];
  Folder.temp() : name = 'temporary'; // Oops! Forgot contents.
}
class Folder {
  final String name;
  final List<Document> contents = [];

  Folder(this.name);
  Folder.temp() : name = 'temporary';
}

Of course, if a field depends on constructor parameters, or is initialized differently by different constructors, then this guideline does not apply.

Constructors

The following best practices apply to declaring constructors for a class.

DO use initializing formals when possible.

Many fields are initialized directly from a constructor parameter, like:

class Point {
  num x, y;
  Point(num x, num y) {
    this.x = x;
    this.y = y;
  }
}

We’ve got to type x four times here define a field. Lame. We can do better:

class Point {
  num x, y;
  Point(this.x, this.y);
}

This this. syntax before a constructor parameter is called an “initializing formal”. You can’t always take advantage of it. In particular, using it means the parameter is not visible in the initialization list. But, when you can, you should.

DON’T type annotate initializing formals.

If a constructor parameter is using this. to initialize a field, then the type of the parameter is understood to be the same type as the field.

class Point {
  int x, y;
  Point(this.x, this.y);
}
class Point {
  int x, y;
  Point(int this.x, int this.y);
}

DO use ; instead of {} for empty constructor bodies.

In Dart, a constructor with an empty body can be terminated with just a semicolon. (In fact, it’s required for const constructors.)

class Point {
  int x, y;
  Point(this.x, this.y);
}
class Point {
  int x, y;
  Point(this.x, this.y) {}
}

DO place the super() call last in a constructor initialization list.

Field initializers are evaluated in the order that they appear in the constructor initialization list. If you place a super() call in the middle of an initializer list, the superclass’s initializers will be evaluated right then before evaluating the rest of the subclass’s initializers.

What it doesn’t mean is that the superclass’s constructor body is executed then. That always happens after all initializers are run regardless of where super() appears. Placing the super() elsewhere is confusing and almost never useful. In fact, DDC requires that it appear last.

View(Style style, List children)
    : _children = children,
      super(style) {
View(Style style, List children)
    : super(style),
      _children = children {

Error handling

Dart uses exceptions when an error occurs in your program. The following best practices apply to catching and throwing exceptions.

AVOID catches without on clauses.

A catch clause with no on qualifier catches anything thrown by the code in the try block. Pokémon exception handling is very likely not what you want. Does your code correctly handle StackOverflowError or OutOfMemoryError? If you incorrectly pass the wrong argument to a method in that try block do you want to have your debugger point you to the mistake or would you rather that helpful ArgumentError get swallowed? Do you want any assert() statements inside that code to effectively vanish since you’re catching the thrown AssertionErrors?

The answer is probably “no”, in which case you should filter the types you catch. In most cases, you should have an on clause that limits you to the kinds of runtime failures you are aware of and are correctly handling.

In rare cases, you may wish to catch any runtime error. This is usually in framework or low-level code that tries to insulate arbitrary application code from causing problems. Even here, it is usually better to catch Exception than to catch all types. Exception is the base class for all runtime errors and excludes errors that indicate programmatic bugs in the code.

DON’T discard errors from catches without on clauses.

If you really do feel you need to catch everything that can be thrown from a region of code, do something with what you catch. Log it, display it to the user or rethrow it, but do not silently discard it.

DO throw objects that implement Error only for programmatic errors.

The Error class is the base class for programmatic errors. When an object of that type or one of its subinterfaces like ArgumentError is thrown, it means there is a bug in your code. When your API wants to report to a caller that it is being used incorrectly throwing an Error sends that signal clearly.

Conversely, if the exception is some kind of runtime failure that doesn’t indicate a bug in the code, then throwing an Error is misleading. Instead, throw one of the core Exception classes or some other type.

DON’T explicitly catch Error or types that implement it.

This follows from the above. Since an Error indicates a bug in your code, it should unwind the entire callstack, halt the program, and print a stack trace so you can locate and fix the bug.

Catching errors of these types breaks that process and masks the bug. Instead of adding error-handling code to deal with this exception after the fact, go back and fix the code that is causing it to be thrown in the first place.

DO use rethrow to rethrow a caught exception.

If you decide to rethrow an exception, prefer using the rethrow statement instead of throwing the same exception object using throw. rethrow preserves the original stack trace of the exception. throw on the other hand resets the stack trace to the last thrown position.

try {
  somethingRisky();
} catch(e) {
  if (!canHandle(e)) throw e;
  handle(e);
}
try {
  somethingRisky();
} catch(e) {
  if (!canHandle(e)) rethrow;
  handle(e);
}

Asynchrony

Dart has several language features to support asynchronous programming. The following best practices apply to asynchronous coding.

PREFER async/await over using raw futures.

Explicit asynchronous code is notoriously hard to read and debug, even when using a nice abstraction like futures. This is why we added async/await to the language. They make a huge improvement in readability code and let you use all of the built-in control flow structures of the language within your asynchronous code.

Future<bool> doAsyncComputation() async {
  try {
    var result = await longRunningCalculation();
    return verifyResult(result.summary);
  } catch(e) {
    log.error(e);
    return false;
  }
}
Future<bool> doAsyncComputation() {
  return longRunningCalculation().then((result) {
    return verifyResult(result.summary);
  }).catchError((e) {
    log.error(e);
    return new Future.value(false);
  });
}

DON’T use async when it has no useful effect.

It’s easy to get in the habit of using async on any function that does anything related to asynchrony. But in some cases, it’s extraneous. If you can omit the async without changing the behavior of the function, do so.

Future afterTwoThings(Future first, second) {
  return Future.wait([first, second]);
}
Future afterTwoThings(Future first, second) async {
  return Future.wait([first, second]);
}

Cases where async is useful include:

  • You are using await. (This is the obvious one.)

  • You are returning an error asynchronously. async and then throw is shorter than return new Future.error(...).

  • You are returning a value and you want it implicitly wrapped in a future. async is shorter than new Future.value(...).

  • You don’t want any of the code to execute until after the event loop has taken a turn.

Future usesAwait(Future later) async {
  print(await later);
}

Future asyncError() async {
  throw 'Error!';
}

Future asyncValue() async {
  return 'value';
}

CONSIDER using higher-order methods to transform a stream.

This parallels the above suggestion on iterables. Streams support many of the same methods and also handle things like transmitting errors, closing, etc. correctly.

AVOID using Completer directly.

Many people new to asynchronous programming want to write code that produces a future. The constructors in Future don’t seem to fit their need so they eventually find the Completer class and use that.

Future<bool> fileContainsBear(String path) {
  var completer = new Completer<bool>();

  new File(path).readAsString().then((contents) {
    completer.complete(contents.contains('bear'));
  });

  return completer.future;
}

Completer is needed for two kinds of low-level code: new asynchronous primitives, and interfacing with asynchronous code that doesn’t use futures. Most other code should use async/await or Future.then(), because they’re clearer and make error handling easier.

Future<bool> fileContainsBear(String path) {
  return new File(path).readAsString().then((contents) {
    return contents.contains('bear');
  });
}
Future<bool> fileContainsBear(String path) async {
  var contents = await new File(path).readAsString();
  return contents.contains('bear');
}