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site</a></li></ul></li></ul></div></div><main id="page-content"><div id="site-toc--side" class="site-toc"><header class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry nav-item"><a class="nav-link" href="#future">Future</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#using-await">Using await</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#basic-usage">Basic usage</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#chaining-multiple-asynchronous-methods">Chaining multiple asynchronous methods</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#waiting-for-multiple-futures">Waiting for multiple futures</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#handling-errors-for-multiple-futures">Handling errors for multiple futures</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#stream">Stream</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#using-an-asynchronous-for-loop">Using an asynchronous for loop</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#listening-for-stream-data">Listening for stream data</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#transforming-stream-data">Transforming stream data</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#handling-errors-and-completion">Handling errors and completion</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#more-information">More information</a></li></ul></div><article><div class="content"><nav id="subnav"><ul><li class="previous"><a href="/libraries/dart-core">&lang;  dart:core</a></li><li class="next"><a href="/libraries/dart-math">dart:math  &rang;</a></li></ul></nav><div id="site-content-title"><h1>dart:async</h1></div><div id="site-toc--inline" class="site-toc toc-collapsible toc-collapsed"><header class="site-toc__title">Contents keyboard_arrow_down keyboard_arrow_up</header><ul class="section-nav"><li class="toc-entry"><a href="#future">Future</a><ul><li class="toc-entry"><a href="#using-await">Using await</a></li><li class="toc-entry"><a href="#basic-usage">Basic usage</a></li><li class="toc-entry"><a href="#chaining-multiple-asynchronous-methods">Chaining multiple asynchronous methods</a></li><li class="toc-entry"><a href="#waiting-for-multiple-futures">Waiting for multiple futures</a></li><li class="toc-entry"><a href="#handling-errors-for-multiple-futures">Handling errors for multiple futures</a></li></ul></li><li class="toc-entry"><a href="#stream">Stream</a><ul><li class="toc-entry"><a href="#using-an-asynchronous-for-loop">Using an asynchronous for loop</a></li><li class="toc-entry"><a href="#listening-for-stream-data">Listening for stream data</a></li><li class="toc-entry"><a href="#transforming-stream-data">Transforming stream data</a></li><li class="toc-entry"><a href="#handling-errors-and-completion">Handling errors and completion</a></li></ul></li><li class="toc-entry"><a href="#more-information">More information</a></li></ul>more_horiz</div> <?code-excerpt replace="/ *\/\/\s+ignore_for_file:[^\n]+\n//g; /(^|\n) *\/\/\s+ignore:[^\n]+\n/$1/g; /(\n[^\n]+) *\/\/\s+ignore:[^\n]+\n/$1\n/g"?> <?code-excerpt plaster="none"?> Asynchronous programming often uses callback functions, but Dart provides alternatives: <a href="https://api.dart.dev/dart-async/Future-class.html">Future</a> and <a href="https://api.dart.dev/dart-async/Stream-class.html">Stream</a> 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 (<a href="https://api.dart.dev/dart-async/dart-async-library.html">API reference</a>).<aside class="alert alert-info"><div class="alert-header">info Note</div><div class="alert-content">You don't always need to use the Future or Stream APIs directly. The Dart language supports asynchronous coding using keywords such as <code>async</code> and <code>await</code>. Check out the <a href="/libraries/async/async-await">asynchronous programming tutorial</a> for details.</div></aside>The dart:async library works in both web apps and command-line apps. To use it, import dart:async: <?code-excerpt "misc/lib/library_tour/async/future.dart (import)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>import 'dart:async';</code></pre></div></div><aside class="alert alert-success"><div class="alert-header">lightbulb Tip</div><div class="alert-content">You don't need to import dart:async to use the Future and Stream APIs, because dart:core exports those classes.</div></aside><div class="header-wrapper"><h2 id="future">Future</h2><a class="heading-link" href="#future" aria-label="Link to 'Future' section">#</a></div>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.<div class="header-wrapper"><h3 id="using-await">Using await</h3><a class="heading-link" href="#using-await" aria-label="Link to 'Using await' section">#</a></div>Before you directly use the Future API, consider using <code>await</code> instead. Code that uses <code>await</code> expressions can be easier to understand than code that uses the Future API.Consider the following function. It uses Future's <code>then()</code> method to execute three asynchronous functions in a row, waiting for each one to complete before executing the next one. <?code-excerpt "misc/lib/library_tour/async/future.dart (run-using-future)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>void runUsingFuture() { // ... findEntryPoint().then((entryPoint) { return runExecutable(entryPoint, args); }).then(flushThenExit); }</code></pre></div></div>The equivalent code with await expressions looks more like synchronous code: <?code-excerpt "misc/lib/library_tour/async/future.dart (run-using-async-await)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>Future<void> runUsingAsyncAwait() async { // ... var entryPoint = await findEntryPoint(); var exitCode = await runExecutable(entryPoint, args); await flushThenExit(exitCode); }</code></pre></div></div>An <code>async</code> function can catch exceptions from Futures. For example: <?code-excerpt "misc/lib/library_tour/async/future.dart (catch)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>var entryPoint = await findEntryPoint(); try { var exitCode = await runExecutable(entryPoint, args); await flushThenExit(exitCode); } catch (e) { // Handle the error... }</code></pre></div></div><aside class="alert alert-warning"><div class="alert-header">error Important</div><div class="alert-content">Async functions return Futures. If you don't want your function to return a future, then use a different solution. For example, you might call an <code>async</code> function from your function.</div></aside>For more information on using <code>await</code> and related Dart language features, see the <a href="/libraries/async/async-await">asynchronous programming tutorial</a>.<div class="header-wrapper"><h3 id="basic-usage">Basic usage</h3><a class="heading-link" href="#basic-usage" aria-label="Link to 'Basic usage' section">#</a></div>You can use <code>then()</code> to schedule code that runs when the future completes. For example, <a href="https://pub.dev/documentation/http/latest/http/Client/read.html"><code>Client.read()</code></a> returns a Future, since HTTP requests can take a while. Using <code>then()</code> lets you run some code when that Future has completed and the promised string value is available: <?code-excerpt "misc/lib/library_tour/async/basic.dart (then)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>httpClient.read(url).then((String result) { print(result); });</code></pre></div></div>Use <code>catchError()</code> to handle any errors or exceptions that a Future object might throw. <?code-excerpt "misc/lib/library_tour/async/basic.dart (catch-error)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>httpClient.read(url).then((String result) { print(result); }).catchError((e) { // Handle or ignore the error. });</code></pre></div></div>The <code>then().catchError()</code> pattern is the asynchronous version of <code>try</code>-<code>catch</code>.<aside class="alert alert-warning"><div class="alert-header">error Important</div><div class="alert-content">Be sure to invoke <code>catchError()</code> on the result of <code>then()</code>鈥攏ot on the result of the original <code>Future</code>. Otherwise, the <code>catchError()</code> can handle errors only from the original Future's computation, but not from the handler registered by <code>then()</code>.</div></aside><div class="header-wrapper"><h3 id="chaining-multiple-asynchronous-methods">Chaining multiple asynchronous methods</h3><a class="heading-link" href="#chaining-multiple-asynchronous-methods" aria-label="Link to 'Chaining multiple asynchronous methods' section">#</a></div>The <code>then()</code> method returns a Future, providing a useful way to run multiple asynchronous functions in a certain order. If the callback registered with <code>then()</code> returns a Future, <code>then()</code> returns a Future that will complete with the same result as the Future returned from the callback. If the callback returns a value of any other type, <code>then()</code> creates a new Future that completes with the value. <?code-excerpt "misc/lib/library_tour/async/future.dart (then-chain)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>Future result = costlyQuery(url); result .then((value) => expensiveWork(value)) .then((_) => lengthyComputation()) .then((_) => print('Done!')) .catchError((exception) { /* Handle exception... */ });</code></pre></div></div>In the preceding example, the methods run in the following order:<ol><li><code>costlyQuery()</code></li><li><code>expensiveWork()</code></li><li><code>lengthyComputation()</code></li></ol>Here is the same code written using await: <?code-excerpt "misc/lib/library_tour/async/future.dart (then-chain-as-await)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>try { final value = await costlyQuery(url); await expensiveWork(value); await lengthyComputation(); print('Done!'); } catch (e) { /* Handle exception... */ }</code></pre></div></div><div class="header-wrapper"><h3 id="waiting-for-multiple-futures">Waiting for multiple futures</h3><a class="heading-link" href="#waiting-for-multiple-futures" aria-label="Link to 'Waiting for multiple futures' section">#</a></div>Sometimes your algorithm needs to invoke many asynchronous functions and wait for them all to complete before continuing. Use the <a href="https://api.dart.dev/dart-async/Future/wait.html">Future.wait()</a> static method to manage multiple Futures and wait for them to complete: <?code-excerpt "misc/lib/library_tour/async/future.dart (wait)" replace="/elideBody;/\/* ... *\//g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>Future<void> deleteLotsOfFiles() async => ... Future<void> copyLotsOfFiles() async => ... Future<void> checksumLotsOfOtherFiles() async => ... await Future.wait([ deleteLotsOfFiles(), copyLotsOfFiles(), checksumLotsOfOtherFiles(), ]); print('Done with all the long steps!');</code></pre></div></div><code>Future.wait()</code> returns a future which completes once all the provided futures have completed. It completes either with their results, or with an error if any of the provided futures fail.<div class="header-wrapper"><h3 id="handling-errors-for-multiple-futures">Handling errors for multiple futures</h3><a class="heading-link" href="#handling-errors-for-multiple-futures" aria-label="Link to 'Handling errors for multiple futures' section">#</a></div>You can also wait for parallel operations on an <a href="https://api.dart.dev/dart-async/FutureIterable/wait.html">iterable</a> or <a href="https://api.dart.dev/dart-async/FutureRecord2/wait.html">record</a> of futures.These extensions return a future with the resulting values of all provided futures. Unlike <code>Future.wait</code>, they also let you handle errors.If any future in the collection completes with an error, <code>wait</code> completes with a <a href="https://api.dart.dev/dart-async/ParallelWaitError-class.html"><code>ParallelWaitError</code></a>. This allows the caller to handle individual errors and dispose successful results if necessary.When you don't need the result values from each individual future, use <code>wait</code> on an iterable of futures:<div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>void main() async { Future<void> delete() async => ... Future<void> copy() async => ... Future<void> errorResult() async => ... try { // Wait for each future in a list, returns a list of futures: var results = await [delete(), copy(), errorResult()].wait; } on ParallelWaitError<List<bool?>, List<AsyncError?>> catch (e) { print(e.values[0]); // Prints successful future print(e.values[1]); // Prints successful future print(e.values[2]); // Prints null when the result is an error print(e.errors[0]); // Prints null when the result is successful print(e.errors[1]); // Prints null when the result is successful print(e.errors[2]); // Prints error } }</code></pre></div></div>When you do need the individual result values from each future, use <code>wait</code> on a record of futures. This provides the additional benefit that the futures can be of different types:<div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>void main() async { Future<int> delete() async => ... Future<String> copy() async => ... Future<bool> errorResult() async => ... try { // Wait for each future in a record, returns a record of futures: (int, String, bool) result = await (delete(), copy(), errorResult()).wait; } on ParallelWaitError<(int?, String?, bool?), (AsyncError?, AsyncError?, AsyncError?)> catch (e) { // ... } // Do something with the results: var deleteInt = result.$1; var copyString = result.$2; var errorBool = result.$3; }</code></pre></div></div><div class="header-wrapper"><h2 id="stream">Stream</h2><a class="heading-link" href="#stream" aria-label="Link to 'Stream' section">#</a></div>Stream objects appear throughout Dart APIs, representing sequences of data. For example, HTML events such as button clicks are delivered using streams. You can also read a file as a stream.<div class="header-wrapper"><h3 id="using-an-asynchronous-for-loop">Using an asynchronous for loop</h3><a class="heading-link" href="#using-an-asynchronous-for-loop" aria-label="Link to 'Using an asynchronous for loop' section">#</a></div>Sometimes you can use an asynchronous for loop (<code>await for</code>) instead of using the Stream API.Consider the following function. It uses Stream's <code>listen()</code> method to subscribe to a list of files, passing in a function literal that searches each file or directory. <?code-excerpt "misc/lib/library_tour/async/stream.dart (listen)" replace="/listen/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>void main(List<String> arguments) { // ... FileSystemEntity.isDirectory(searchPath).then((isDir) { if (isDir) { final startingDir = Directory(searchPath); startingDir.list().listen((entity) { if (entity is File) { searchFile(entity, searchTerms); } }); } else { searchFile(File(searchPath), searchTerms); } }); }</code></pre></div></div>The equivalent code with await expressions, including an asynchronous for loop (<code>await for</code>), looks more like synchronous code: <?code-excerpt "misc/lib/library_tour/async/stream.dart (await-for)" replace="/await for/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>void main(List<String> arguments) async { // ... if (await FileSystemEntity.isDirectory(searchPath)) { final startingDir = Directory(searchPath); await for (final entity in startingDir.list()) { if (entity is File) { searchFile(entity, searchTerms); } } } else { searchFile(File(searchPath), searchTerms); } }</code></pre></div></div><aside class="alert alert-warning"><div class="alert-header">error Important</div><div class="alert-content">Before using <code>await for</code>, make sure that it makes the code clearer and that you really do want to wait for all of the stream's results. For example, you usually should not use <code>await for</code> for DOM event listeners, because the DOM sends endless streams of events. If you use <code>await for</code> to register two DOM event listeners in a row, then the second kind of event is never handled.</div></aside>For more information on using <code>await</code> and related Dart language features, see the <a href="/libraries/async/async-await">asynchronous programming tutorial</a>.<div class="header-wrapper"><h3 id="listening-for-stream-data">Listening for stream data</h3><a class="heading-link" href="#listening-for-stream-data" aria-label="Link to 'Listening for stream data' section">#</a></div>To get each value as it arrives, either use <code>await for</code> or subscribe to the stream using the <code>listen()</code> method: <?code-excerpt "misc/lib/library_tour/async/stream_web.dart (listen)" replace="/listen/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>// Add an event handler to a button. submitButton.onClick.listen((e) { // When the button is clicked, it runs this code. submitData(); });</code></pre></div></div>In this example, the <code>onClick</code> property is a <code>Stream</code> object provided by the submit button.If you care about only one event, you can get it using a property such as <code>first</code>, <code>last</code>, or <code>single</code>. To test the event before handling it, use a method such as <code>firstWhere()</code>, <code>lastWhere()</code>, or <code>singleWhere()</code>.If you care about a subset of events, you can use methods such as <code>skip()</code>, <code>skipWhile()</code>, <code>take()</code>, <code>takeWhile()</code>, and <code>where()</code>.<div class="header-wrapper"><h3 id="transforming-stream-data">Transforming stream data</h3><a class="heading-link" href="#transforming-stream-data" aria-label="Link to 'Transforming stream data' section">#</a></div>Often, you need to change the format of a stream's data before you can use it. Use the <code>transform()</code> method to produce a stream with a different type of data: <?code-excerpt "misc/lib/library_tour/async/stream.dart (transform)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>var lines = inputStream.transform(utf8.decoder).transform(const LineSplitter());</code></pre></div></div>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 <a href="/libraries/dart-convert">dart:convert section</a>).<div class="header-wrapper"><h3 id="handling-errors-and-completion">Handling errors and completion</h3><a class="heading-link" href="#handling-errors-and-completion" aria-label="Link to 'Handling errors and completion' section">#</a></div>How you specify error and completion handling code depends on whether you use an asynchronous for loop (<code>await for</code>) or the Stream API.If you use an asynchronous for loop, then use try-catch to handle errors. Code that executes after the stream is closed goes after the asynchronous for loop. <?code-excerpt "misc/lib/library_tour/async/stream.dart (read-file-await-for)" replace="/try|catch/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>Future<void> readFileAwaitFor() async { var config = File('config.txt'); Stream<List<int>> inputStream = config.openRead(); var lines = inputStream.transform(utf8.decoder).transform(const LineSplitter()); try { await for (final line in lines) { print('Got ${line.length} characters from stream'); } print('file is now closed'); } catch (e) { print(e); } }</code></pre></div></div>If you use the Stream API, then handle errors by registering an <code>onError</code> listener. Run code after the stream is closed by registering an <code>onDone</code> listener. <?code-excerpt "misc/lib/library_tour/async/stream.dart (on-done)" replace="/onDone|onError/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>var config = File('config.txt'); Stream<List<int>> inputStream = config.openRead(); inputStream.transform(utf8.decoder).transform(const LineSplitter()).listen( (String line) { print('Got ${line.length} characters from stream'); }, onDone: () { print('file is now closed'); }, onError: (e) { print(e); });</code></pre></div></div><div class="header-wrapper"><h2 id="more-information">More information</h2><a class="heading-link" href="#more-information" aria-label="Link to 'More information' section">#</a></div>For some examples of using Future and Stream in command-line apps, check out the <a href="/libraries/dart-io">dart:io documentation</a>. Also see these articles and tutorials:<ul><li><a href="/libraries/async/async-await">Asynchronous programming: futures, async, await</a></li><li><a href="/libraries/async/futures-error-handling">Futures and error handling</a></li><li><a href="/libraries/async/using-streams">Asynchronous programming: streams</a></li><li><a href="/libraries/async/creating-streams">Creating streams in Dart</a></li><li><a href="/language/concurrency">Dart asynchronous programming: Isolates and event loops</a></li></ul><nav id="subnav"><ul><li class="previous"><a href="/libraries/dart-core">&lang;  dart:core</a></li><li class="next"><a href="/libraries/dart-math">dart:math  &rang;</a></li></ul></nav>Unless stated otherwise, the documentation on this site reflects Dart 3.5.4. 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