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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="#handling-futures">Handling Futures</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#declaring-async-functions">Declaring async functions</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#handling-streams">Handling Streams</a></li></ul></div><article><div class="content"><nav id="subnav"><ul><li class="previous"><a href="/language/concurrency">&lang;  Concurrency</a></li><li class="next"><a href="/language/isolates">Isolates  &rang;</a></li></ul></nav><div id="site-content-title"><h1>Asynchrony support</h1></div><div id="site-toc--inline" class="site-toc"><header class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry"><a href="#handling-futures">Handling Futures</a></li><li class="toc-entry"><a href="#declaring-async-functions">Declaring async functions</a></li><li class="toc-entry"><a href="#handling-streams">Handling Streams</a></li></ul></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; / *\/\/\s+ignore:[^\n]+//g; /([A-Z]\w*)\d\b/$1/g"?> <p>Dart libraries are full of functions that return <a href="https://api.dart.dev/dart-async/Future-class.html"><code>Future</code></a> or <a href="https://api.dart.dev/dart-async/Stream-class.html"><code>Stream</code></a> objects. These functions are <em>asynchronous</em>: they return after setting up a possibly time-consuming operation (such as I/O), without waiting for that operation to complete.</p><p>The <code>async</code> and <code>await</code> keywords support asynchronous programming, letting you write asynchronous code that looks similar to synchronous code.</p><div class="header-wrapper"><h2 id="handling-futures">Handling Futures</h2><a class="heading-link" href="#handling-futures" aria-label="Link to 'Handling Futures' section">#</a></div><p>When you need the result of a completed Future, you have two options:</p><ul><li>Use <code>async</code> and <code>await</code>, as described here and in the <a href="/libraries/async/async-await">asynchronous programming tutorial</a>.</li><li>Use the Future API, as described in the <a href="/libraries/dart-async#future"><code>dart:async</code> documentation</a>.</li></ul><p>Code that uses <code>async</code> and <code>await</code> is asynchronous, but it looks a lot like synchronous code. For example, here's some code that uses <code>await</code> to wait for the result of an asynchronous function:</p> <?code-excerpt "misc/lib/language_tour/async.dart (await-look-up-version)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">await</span><span style="color:#6200EE"> lookUpVersion</span><span style="color:#222222">();</span></span></code></pre></div></div><p>To use <code>await</code>, code must be in an <code>async</code> function鈥攁 function marked as <code>async</code>:</p> <?code-excerpt "misc/lib/language_tour/async.dart (checkVersion)" replace="/async|await/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#0468D7">Future</span><span style="color:#222222"><</span><span style="color:#D43324">void</span><span style="color:#222222">> </span><span style="color:#6200EE">checkVersion</span><span style="color:#222222">() </span><mark class="highlight"><span style="color:#D43324">async</span></mark><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#D43324"> var</span><span style="color:#222222"> version = </span><mark class="highlight"><span style="color:#D43324">await</span></mark><span style="color:#6200EE"> lookUpVersion</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#6E6E70"> // Do something with version</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><aside class="alert alert-info"><div class="alert-header"><i class="material-symbols" aria-hidden="true">info</i> <span>Note</span></div><div class="alert-content"><p>Although an <code>async</code> function might perform time-consuming operations, it doesn't wait for those operations. Instead, the <code>async</code> function executes only until it encounters its first <code>await</code> expression. Then it returns a <code>Future</code> object, resuming execution only after the <code>await</code> expression completes.</p></div></aside><p>Use <code>try</code>, <code>catch</code>, and <code>finally</code> to handle errors and cleanup in code that uses <code>await</code>:</p> <?code-excerpt "misc/lib/language_tour/async.dart (try-catch)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">try</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#222222"> version = </span><span style="color:#D43324">await</span><span style="color:#6200EE"> lookUpVersion</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">} </span><span style="color:#D43324">catch</span><span style="color:#222222"> (e) {</span></span> <span class="line"><span style="color:#6E6E70"> // React to inability to look up the version</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>You can use <code>await</code> multiple times in an <code>async</code> function. For example, the following code waits three times for the results of functions:</p> <?code-excerpt "misc/lib/language_tour/async.dart (repeated-await)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">var</span><span style="color:#222222"> entrypoint = </span><span style="color:#D43324">await</span><span style="color:#6200EE"> findEntryPoint</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#D43324">var</span><span style="color:#222222"> exitCode = </span><span style="color:#D43324">await</span><span style="color:#6200EE"> runExecutable</span><span style="color:#222222">(entrypoint, args);</span></span> <span class="line"><span style="color:#D43324">await</span><span style="color:#6200EE"> flushThenExit</span><span style="color:#222222">(exitCode);</span></span></code></pre></div></div><p>In <code>await <em>expression</em></code>, the value of <code><em>expression</em></code> is usually a Future; if it isn't, then the value is automatically wrapped in a Future. This Future object indicates a promise to return an object. The value of <code>await <em>expression</em></code> is that returned object. The await expression makes execution pause until that object is available.</p><p><strong>If you get a compile-time error when using <code>await</code>, make sure <code>await</code> is in an <code>async</code> function.</strong> For example, to use <code>await</code> in your app's <code>main()</code> function, the body of <code>main()</code> must be marked as <code>async</code>:</p> <?code-excerpt "misc/lib/language_tour/async.dart (main)" replace="/async|await/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">void</span><span style="color:#6200EE"> main</span><span style="color:#222222">() </span><mark class="highlight"><span style="color:#D43324">async</span></mark><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6200EE"> checkVersion</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#6200EE"> print</span><span style="color:#222222">(</span><span style="color:#11796D">'In main: version is </span><span style="color:#11796D">${</span><mark class="highlight"><span style="color:#222222">await</span></mark><span style="color:#6200EE"> lookUpVersion</span><span style="color:#11796D">()}</span><span style="color:#11796D">'</span><span style="color:#222222">);</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><aside class="alert alert-info"><div class="alert-header"><i class="material-symbols" aria-hidden="true">info</i> <span>Note</span></div><div class="alert-content"><p>The preceding example uses an <code>async</code> function (<code>checkVersion()</code>) without waiting for a result鈥攁 practice that can cause problems if the code assumes that the function has finished executing. To avoid this problem, use the <a href="/tools/linter-rules/unawaited_futures">unawaited_futures linter rule</a>.</p></div></aside><p>For an interactive introduction to using futures, <code>async</code>, and <code>await</code>, see the <a href="/libraries/async/async-await">asynchronous programming tutorial</a>.</p><div class="header-wrapper"><h2 id="declaring-async-functions">Declaring async functions</h2><a class="heading-link" href="#declaring-async-functions" aria-label="Link to 'Declaring async functions' section">#</a></div><p>An <code>async</code> function is a function whose body is marked with the <code>async</code> modifier.</p><p>Adding the <code>async</code> keyword to a function makes it return a Future. For example, consider this synchronous function, which returns a String:</p> <?code-excerpt "misc/lib/language_tour/async.dart (sync-look-up-version)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#0468D7">String</span><span style="color:#6200EE"> lookUpVersion</span><span style="color:#222222">() => </span><span style="color:#11796D">'1.0.0'</span><span style="color:#222222">;</span></span></code></pre></div></div><p>If you change it to be an <code>async</code> function鈥攆or example, because a future implementation will be time consuming鈥攖he returned value is a Future:</p> <?code-excerpt "misc/lib/language_tour/async.dart (async-look-up-version)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#0468D7">Future</span><span style="color:#222222"><</span><span style="color:#0468D7">String</span><span style="color:#222222">> </span><span style="color:#6200EE">lookUpVersion</span><span style="color:#222222">() </span><span style="color:#D43324">async</span><span style="color:#222222"> => </span><span style="color:#11796D">'1.0.0'</span><span style="color:#222222">;</span></span></code></pre></div></div><p>Note that the function's body doesn't need to use the Future API. Dart creates the Future object if necessary. If your function doesn't return a useful value, make its return type <code>Future<void></code>.</p><p>For an interactive introduction to using futures, <code>async</code>, and <code>await</code>, see the <a href="/libraries/async/async-await">asynchronous programming tutorial</a>.</p><div class="header-wrapper"><h2 id="handling-streams">Handling Streams</h2><a class="heading-link" href="#handling-streams" aria-label="Link to 'Handling Streams' section">#</a></div><p>When you need to get values from a Stream, you have two options:</p><ul><li>Use <code>async</code> and an <em>asynchronous for loop</em> (<code>await for</code>).</li><li>Use the Stream API, as described in the <a href="/libraries/dart-async#stream"><code>dart:async</code> documentation</a>.</li></ul><aside class="alert alert-info"><div class="alert-header"><i class="material-symbols" aria-hidden="true">info</i> <span>Note</span></div><div class="alert-content"><p>Before using <code>await for</code>, be 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 <strong>not</strong> use <code>await for</code> for UI event listeners, because UI frameworks send endless streams of events.</p></div></aside><p>An asynchronous for loop has the following form:</p> <?code-excerpt "misc/lib/language_tour/async.dart (await-for)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">await</span><span style="color:#D43324"> for</span><span style="color:#222222"> (varOrType identifier </span><span style="color:#D43324">in</span><span style="color:#222222"> expression) {</span></span> <span class="line"><span style="color:#6E6E70"> // Executes each time the stream emits a value.</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>The value of <code><em>expression</em></code> must have type Stream. Execution proceeds as follows:</p><ol><li>Wait until the stream emits a value.</li><li>Execute the body of the for loop, with the variable set to that emitted value.</li><li>Repeat 1 and 2 until the stream is closed.</li></ol><p>To stop listening to the stream, you can use a <code>break</code> or <code>return</code> statement, which breaks out of the for loop and unsubscribes from the stream.</p><p><strong>If you get a compile-time error when implementing an asynchronous for loop, make sure the <code>await for</code> is in an <code>async</code> function.</strong> For example, to use an asynchronous for loop in your app's <code>main()</code> function, the body of <code>main()</code> must be marked as <code>async</code>:</p> <?code-excerpt "misc/lib/language_tour/async.dart (number-thinker)" replace="/async|await for/[!$&!]/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body"><span class="code-block-language" title="Language dart">dart</span><pre class="shiki dash-light" tabindex="0"><code><span class="line"><span style="color:#D43324">void</span><span style="color:#6200EE"> main</span><span style="color:#222222">() </span><mark class="highlight"><span style="color:#D43324">async</span></mark><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#6E6E70"> // ...</span></span> <span class="line"><span style="color:#D43324"> </span><mark class="highlight"><span style="color:#D43324">await</span><span style="color:#D43324"> for</span></mark><span style="color:#222222"> (</span><span style="color:#D43324">final</span><span style="color:#222222"> request </span><span style="color:#D43324">in</span><span style="color:#222222"> requestServer) {</span></span> <span class="line"><span style="color:#6200EE"> handleRequest</span><span style="color:#222222">(request);</span></span> <span class="line"><span style="color:#222222"> }</span></span> <span class="line"><span style="color:#6E6E70"> // ...</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>For more information about Dart's asynchronous programming support, check out the <a href="/libraries/dart-async"><code>dart:async</code></a> library documentation.</p><nav id="subnav"><ul><li class="previous"><a href="/language/concurrency">&lang;  Concurrency</a></li><li class="next"><a href="/language/isolates">Isolates  &rang;</a></li></ul></nav><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.5.4. Page last updated on 2024-11-17.</span> <a href="https://github.com/dart-lang/site-www/tree/main/src/content/language/async.md" target="_blank" rel="noopener">View source</a> <span>or </span><a href="https://github.com/dart-lang/site-www/issues/new?template=1_page_issue.yml&page-url=https://dart.dev/language/async/&page-source=https://github.com/dart-lang/site-www/tree/main/src/content/language/async.md" title="Report an issue with this page" target="_blank" rel="noopener">report an issue</a>.</p></div></article></main><footer id="page-footer"><div class="footer-section footer-main"><a href="/" class="brand" title="Dart"><img src="/assets/img/logo/logo-white-text.svg" alt="Dart" width="164"></a><div class="footer-social-links"><a href="https://medium.com/dartlang" target="_blank" rel="noopener" title="Medium blog"><svg><use href="/assets/img/social/medium.svg#medium"></use></svg> </a><a href="https://github.com/dart-lang" target="_blank" rel="noopener" title="GitHub"><svg><use href="/assets/img/social/github.svg#github"></use></svg> </a><a href="https://twitter.com/dart_lang" target="_blank" rel="noopener" title="X (Twitter)"><svg><use href="/assets/img/social/x.svg#x"></use></svg></a></div></div><div class="footer-section footer-tray"><div class="footer-licenses">Except as otherwise noted, this site is licensed under a <a href="https://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 International License</a>, and code samples are licensed under the <a href="https://opensource.org/licenses/BSD-3-Clause">3-Clause BSD License</a>.</div><div class="footer-utility-links"><ul><li><a href="/terms" title="Terms of use">Terms</a></li><li><a href="https://policies.google.com/privacy" target="_blank" rel="noopener" title="Privacy policy">Privacy</a></li><li><a href="/security" title="Security philosophy and practices">Security</a></li></ul></div></div></footer></body></html>