Asynchronous Programming: Futures

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What’s the point?

  • Dart is single-threaded.
  • Synchronous code can make your program freeze.
  • Use Futures to perform asynchronous operations.
  • Use await in an async function to pause execution until a Future completes.
  • Or use Future’s then() method.
  • Use try-catch expressions in async functions to catch errors.
  • Or use Future’s 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 waiting for a time-consuming operation or blocking on I/O) the program effectively freezes. Asynchronous operations let your program run without getting blocked. 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
printDailyNewsDigest() {
  String news = gatherNewsReports(); // Can take a while.
  print(news);
}

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 when gatherNewsReports() returns with the contents of the file, however long that takes. And if reading the file takes a long time, the user waits passively, wondering if she won the lottery, what tomorrow’s weather will be like, and who won today’s game. Not good.

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 represents a means for getting a value sometime in the future. 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 a value is available, the Future object completes with that value (or with an error; we’ll discuss that later).

To get the value that the Future represents, 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.

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 ( red-run.png ) to start the app.

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.

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 printDailyNewsDigest(), which (because it’s marked async), immediately returns a Future, before any code is executed.
  3. 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.
  4. The body of the printDailyNewsDigest() function starts executing.
  5. After reaching the await expression (await gatherNewsReports()) and calling gatherNewsReports(), the program pauses, waiting for the Future returned by gatherNewsReports() to complete.
  6. Once that Future completes, execution of printDailyNewsDigest() continues, printing the news.
  7. When the printDailyNewsDigest() function body has completed executing, the Future that it originally returned completes, and the app exits.

Handling errors

If a Future-returning function completes with an error, you probably want to capture that error. Async functions can use try-catch to capture the error.

The try-catch code behaves in the same way with asynchronous code that it does for 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 will not 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 ( red-run.png ) to start the app.

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.

In the printDailyNewsDigest() function, the code inside then() could be written in a couple different ways.

  • Using curly braces. This is useful if you want to perform more than one operation. Try it! Replace the printDailyNewsDigest() method with the following:

    printDailyNewsDigest() {
      var future = gatherNewsReports();
      future.then((content) {
        print(content);
        // ... do something else ...
      });
    }
    
  • Passing the print function directly, since it takes a single argument—the completed value of the Future. Try this version of printDailyNewsDigest():

    printDailyNewsDigest() {
      var future = gatherNewsReports();
      future.then(print);
    }
    

Handling errors

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

If dailyNewsDigest.txt doesn’t exist or isn’t available for reading, the code above executes as follows:

  1. gatherNewsReports()’s Future completes with an error.
  2. then()’s Future completes with an error.
  3. catchError()’s callback handles the error, catchError()’s Future 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 Futures. 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().

The functions get triggered in quick succession; when all of them complete with a value, Future.wait() returns a new Future. This Future completes with a list containing the values produced by each function.

// Parallel processing using the Future API
Future.wait([expensiveA(), expensiveB(), expensiveC()])
      .then((List responses) => chooseBestResponse(responses))
      .catchError((e) => handleError(e));

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:

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