Asynchronous Programming: Futures

What's the point?

  • Dart is single-threaded.
  • Synchronous code can make your program freeze.
  • Use Future objects (futures) to perform asynchronous operations.
  • Use await in an async function to suspend execution until a future completes.
  • Or use the then() method.
  • Use try-catch expressions in async functions to catch errors.
  • Or use the catchError() method.
  • You can chain futures to run asynchronous functions in order.

Dart is a single-threaded programming language. If any code blocks the thread of execution (for example, by performing a long-running calculation or blocking on I/O) the program effectively freezes. Asynchronous operations let your program complete other work while waiting for an operation to complete. Dart uses Future objects to represent asynchronous operations.

Introduction

Let’s look at some code that could possibly cause a program to freeze:

// Synchronous code
void printDailyNewsDigest() {
  var newsDigest = gatherNewsReports(); // Can take a while.
  print(newsDigest);
}

main() {
  printDailyNewsDigest();
  printWinningLotteryNumbers();
  printWeatherForecast();
  printBaseballScore();
}

Our program gathers the news of the day, prints it, and then prints a bunch of other items of interest to the user:

<gathered news goes here>
Winning lotto numbers: [23, 63, 87, 26, 2]
Tomorrow's forecast: 70F, sunny.
Baseball score: Red Sox 10, Yankees 0

Our code is problematic: since gatherNewsReports() blocks, the remaining code runs only after gatherNewsReports() returns with the contents of the file, however long that takes. If reading the file takes a long time, the user has to wait, wondering if they won the lottery, what tomorrow’s weather will be, and who won today’s game.

To help keep the application responsive, Dart library authors use an asynchronous model when defining functions that do potentially expensive work. Such functions return their value using a future.

What is a future?

A future is a Future<T> object, which represents an asynchronous operation that produces a result of type T. If the result isn’t a usable value, then the future’s type is Future<void>. When a function that returns a future is invoked, two things happen:

  1. The function queues up work to be done and returns an uncompleted Future object.
  2. Later, when the operation is finished, the Future object completes with a value or with an error.

When writing code that depends on a future, you have two options:

  • Use async and await
  • Use the Future API

Async and await

The async and await keywords are part of the Dart language’s asynchrony support. They allow you to write asynchronous code that looks like synchronous code and doesn’t use the Future API. An async function is one that has the async keyword before its body. The await keyword works only in async functions.

The following app simulates reading the news by using async and await to read the contents of a file on www.dartlang.org. Click run to start the app. Or open a DartPad window containing the app, run the app, and click CONSOLE to see the app’s output.

Notice that printDailyNewsDigest() is the first function called, but the news is the last thing to print, even though the file contains only a single line. This is because the code that reads and prints the file is running asynchronously.

In this example, the printDailyNewsDigest() function calls gatherNewsReports(), which is non-blocking. Calling gatherNewsReports() queues up the work to be done but doesn’t stop the rest of the code from executing. The program prints the lottery numbers, the forecast, and the baseball score; when gatherNewsReports() finishes gathering news, the program prints. If gatherNewsReports() takes a little while to complete its work, no great harm is done: the user gets to read other things before the daily news digest is printed.

Note the return types. The return type of gatherNewsReports() is Future<String>, which means that it returns a future that completes with a string value. The printDailyNewsDigest() function, which doesn’t return a value, has the return type Future<void>.

The following diagram shows the flow of execution through the code. Each number corresponds to a step below.

diagram showing flow of control through the main() and printDailyNewsDigest functions

  1. The app begins executing.
  2. The main() function calls the async function printDailyNewsDigest(), which begins executing synchronously.
  3. printDailyNewsDigest() uses await to call the function gatherNewsReports(), which begins executing.
  4. The gatherNewsReports() function returns an uncompleted future (an instance of Future<String>).
  5. Because printDailyNewsDigest() is an async function and is awaiting a value, it pauses its execution and returns an uncompleted future (in this case, an instance of Future<void>) to its caller (main()).
  6. The remaining print functions execute. Because they’re synchronous, each function executes fully before moving on to the next print function. For example, the winning lottery numbers are all printed before the weather forecast is printed.
  7. When main() has finished executing, the asynchronous functions can resume execution. First, the future returned by gatherNewsReports() completes. Then printDailyNewsDigest() continues executing, printing the news.
  8. When the printDailyNewsDigest() function body finishes executing, the future that it originally returned completes, and the app exits.

Note that an async function starts executing right away (synchronously). The function suspends execution and returns an uncompleted future when it reaches the first occurrence of any of the following:

  • The function’s first await expression (after the function gets the uncompleted future from that expression).
  • Any return statement in the function.
  • The end of the function body.

Handling errors

If a Future-returning function completes with an error, you probably want to capture that error. Async functions can handle errors using try-catch:

Future<void> printDailyNewsDigest() async {
  try {
    var newsDigest = await gatherNewsReports();
    print(newsDigest);
  } catch (e) {
    // Handle error...
  }
}

The try-catch code behaves in the same way with asynchronous code as it does with synchronous code: if the code within the try block throws an exception, the code inside the catch clause executes.

Sequential processing

You can use multiple await expressions to ensure that each statement completes before executing the next statement:

// Sequential processing using async and await.
main() async {
  await expensiveA();
  await expensiveB();
  doSomethingWith(await expensiveC());
}

The expensiveB() function doesn’t execute until expensiveA() has finished, and so on.


The Future API

Before async and await were added in Dart 1.9, you had to use the Future API. You might still see the Future API used in older code and in code that needs more functionality than async-await offers.

To write asynchronous code using the Future API, you use the then() method to register a callback. This callback fires when the Future completes.

The following app simulates reading the news by using the Future API to read the contents of a file on www.dartlang.org. Click run to start the app. Or open a DartPad window containing the app, run the app, and click CONSOLE to see the app’s output.

Notice that printDailyNewsDigest() is the first function called, but the news is the last thing to print, even though the file contains only a single line. This is because the code that reads the file is running asynchronously.

This app executes as follows:

  1. The app begins executing.
  2. The main function calls the printDailyNewsDigest() function, which does not return immediately, but calls gatherNewsReports().
  3. gatherNewsReports() starts gathering news and returns a Future.
  4. printDailyNewsDigest() uses then() to specify a response to the Future. Calling then() returns a new Future that will complete with the value returned by then()’s callback.
  5. The remaining print functions execute. Because they’re synchronous, each function executes fully before moving on to the next print function. For example, the winning lottery numbers are all printed before the weather forecast is printed.
  6. When all of the news has arrived, the Future returned by gatherNewsReports() completes with a string containing the gathered news.
  7. The code specified by then() in printDailyNewsDigest() runs, printing the news.
  8. The app exits.

Alternatively, the code inside then() can use curly braces:

Future<void> printDailyNewsDigest() {
  final future = gatherNewsReports();
  return future.then((newsDigest) {
    print(newsDigest);
    // Do something else...
  });
}

You need to provide an argument to then()’s callback, even if the Future is of type Future<void>. By convention, an unused argument is named _ (underscore).

final future = printDailyNewsDigest();
return future.then((_) {
  // Code that doesn't use the `_` parameter...
  print('All reports printed.');
});

Handling errors

With the Future API, you can capture an error using catchError():

Future<void> printDailyNewsDigest() =>
    gatherNewsReports().then(print).catchError(handleError);

If the news stream isn’t available for reading, the code above executes as follows:

  1. The future returned by gatherNewsReports() completes with an error.
  2. The future returned by then() completes with an error; print() isn’t called.
  3. The callback for catchError() (handleError()) handles the error, the future returned by catchError() completes normally, and the error does not propagate.

Like then(), catchError() returns a new Future that completes with the return value of its callback.

For more details and examples, read Futures and Error Handling.

Calling multiple functions that return futures

Consider three functions, expensiveA(), expensiveB(), and expensiveC(), that return Future objects. You can invoke them sequentially (one function starts when a previous one completes), or you can kick off all of them at the same time and do something once all the values return. The Future interface is fluid enough to deal with both use cases.

Chaining function calls using then()

When Future-returning functions need to run in order, use chained then() calls:

expensiveA()
    .then((aValue) => expensiveB())
    .then((bValue) => expensiveC())
    .then((cValue) => doSomethingWith(cValue));

Nested callbacks also work, but they’re harder to read and not as Dart-y.

Waiting on multiple futures to complete using Future.wait()

If the order of execution of the functions is not important, you can use Future.wait().

When you pass Future.wait() a list of futures, it immediately returns a Future. That future doesn’t complete until all of the given futures have completed. Then it completes with a list containing the values produced by each future in the original list.

Future.wait([expensiveA(), expensiveB(), expensiveC()])
    .then((List responses) => chooseBestResponse(responses, moreInfo))
    .catchError(handleError);

If any of the invoked functions completes with an error, the Future returned by Future.wait() also completes with an error. Use catchError() to handle the error.

Other resources

Read the following documentation for more details on using futures and asynchronous programming in Dart:

What next?