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href="https://pub.dev" target="_blank" rel="noopener">Package 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="#using-libraries">Using libraries</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#specifying-a-library-prefix">Specifying a library prefix</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#importing-only-part-of-a-library">Importing only part of a library</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#library-directive">The library directive</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#implementing-libraries">Implementing libraries</a></li></ul></div><article><div class="content"><nav id="subnav"><ul><li class="previous"><a href="/language/metadata">&lang;  Metadata</a></li><li class="next"><a href="/language/keywords">Keywords  &rang;</a></li></ul></nav><div id="site-content-title"><h1>Libraries & imports</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="#using-libraries">Using libraries</a><ul><li class="toc-entry"><a href="#specifying-a-library-prefix">Specifying a library prefix</a></li><li class="toc-entry"><a href="#importing-only-part-of-a-library">Importing only part of a library</a></li><li class="toc-entry"><a href="#library-directive">The library directive</a></li></ul></li><li class="toc-entry"><a href="#implementing-libraries">Implementing libraries</a></li></ul></div><p>The <code>import</code> and <code>library</code> directives can help you create a modular and shareable code base. Libraries not only provide APIs, but are a unit of privacy: identifiers that start with an underscore (<code>_</code>) are visible only inside the library. <em>Every Dart file (plus its parts) is a <a href="/tools/pub/glossary#library">library</a></em>, even if it doesn't use a <a href="#library-directive"><code>library</code></a> directive.</p><p>Libraries can be distributed using <a href="/tools/pub/packages">packages</a>.</p><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>To learn why Dart uses underscores instead of access modifier keywords like <code>public</code> or <code>private</code>, consult <a href="https://github.com/dart-lang/sdk/issues/33383">SDK issue 33383</a>.</p></div></aside><div class="header-wrapper"><h2 id="using-libraries">Using libraries</h2><a class="heading-link" href="#using-libraries" aria-label="Link to 'Using libraries' section">#</a></div><p>Use <code>import</code> to specify how a namespace from one library is used in the scope of another library.</p><p>For example, Dart web apps generally use the <a href="https://api.dart.dev/dart-html">dart:html</a> library, which they can import like this:</p> <?code-excerpt "misc/test/language_tour/browser_test.dart (dart-html-import)"?> <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">import</span><span style="color:#11796D"> 'dart:html'</span><span style="color:#222222">;</span></span></code></pre></div></div><p>The only required argument to <code>import</code> is a URI specifying the library. For built-in libraries, the URI has the special <code>dart:</code> scheme. For other libraries, you can use a file system path or the <code>package:</code> scheme. The <code>package:</code> scheme specifies libraries provided by a package manager such as the pub tool. For example:</p> <?code-excerpt "misc/test/language_tour/browser_test.dart (package-import)"?> <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">import</span><span style="color:#11796D"> 'package:test/test.dart'</span><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><em>URI</em> stands for uniform resource identifier. <em>URLs</em> (uniform resource locators) are a common kind of URI.</p></div></aside><div class="header-wrapper"><h3 id="specifying-a-library-prefix">Specifying a library prefix</h3><a class="heading-link" href="#specifying-a-library-prefix" aria-label="Link to 'Specifying a library prefix' section">#</a></div><p>If you import two libraries that have conflicting identifiers, then you can specify a prefix for one or both libraries. For example, if library1 and library2 both have an Element class, then you might have code like this:</p> <?code-excerpt "misc/lib/language_tour/libraries/import_as.dart" replace="/(lib\d)\.dart/package:$1\/$&/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">import</span><span style="color:#11796D"> 'package:lib1/lib1.dart'</span><span style="color:#222222">;</span></span> <span class="line"><span style="color:#D43324">import</span><span style="color:#11796D"> 'package:lib2/lib2.dart'</span><span style="color:#D43324"> as</span><span style="color:#222222"> lib2;</span></span> <span class="line"></span> <span class="line"><span style="color:#6E6E70">// Uses Element from lib1.</span></span> <span class="line"><span style="color:#0468D7">Element</span><span style="color:#222222"> element1 = </span><span style="color:#0468D7">Element</span><span style="color:#222222">();</span></span> <span class="line"></span> <span class="line"><span style="color:#6E6E70">// Uses Element from lib2.</span></span> <span class="line"><span style="color:#222222">lib2.</span><span style="color:#0468D7">Element</span><span style="color:#222222"> element2 = lib2.</span><span style="color:#0468D7">Element</span><span style="color:#222222">();</span></span></code></pre></div></div><div class="header-wrapper"><h3 id="importing-only-part-of-a-library">Importing only part of a library</h3><a class="heading-link" href="#importing-only-part-of-a-library" aria-label="Link to 'Importing only part of a library' section">#</a></div><p>If you want to use only part of a library, you can selectively import the library. For example:</p> <?code-excerpt "misc/lib/language_tour/libraries/show_hide.dart (imports)" replace="/(lib\d)\.dart/package:$1\/$&/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:#6E6E70">// Import only foo.</span></span> <span class="line"><span style="color:#D43324">import</span><span style="color:#11796D"> 'package:lib1/lib1.dart'</span><span style="color:#D43324"> show</span><span style="color:#222222"> foo;</span></span> <span class="line"></span> <span class="line"><span style="color:#6E6E70">// Import all names EXCEPT foo.</span></span> <span class="line"><span style="color:#D43324">import</span><span style="color:#11796D"> 'package:lib2/lib2.dart'</span><span style="color:#D43324"> hide</span><span style="color:#222222"> foo;</span></span></code></pre></div></div><div class="header-wrapper"><h4 id="lazily-loading-a-library">Lazily loading a library</h4><a class="heading-link" href="#lazily-loading-a-library" aria-label="Link to 'Lazily loading a library' section">#</a></div><p><em>Deferred loading</em> (also called <em>lazy loading</em>) allows a web app to load a library on demand, if and when the library is needed. Use deferred loading when you want to meet one or more of the following needs.</p><ul><li>Reduce a web app's initial startup time.</li><li>Perform A/B testing鈥攖rying out alternative implementations of an algorithm, for example.</li><li>Load rarely used functionality, such as optional screens and dialogs.</li></ul><p>That doesn't mean Dart loads all the deferred components at start time. The web app can download deferred components via the web when needed.</p><p>The <code>dart</code> tool doesn't support deferred loading for targets other than web. If you're building a Flutter app, consult its implementation of deferred loading in the Flutter guide on <a href="https://docs.flutter.dev/perf/deferred-components">deferred components</a>.</p><p>To lazily load a library, first import it using <code>deferred as</code>.</p> <?code-excerpt "misc/lib/language_tour/libraries/greeter.dart (import)" replace="/hello\.dart/package:greetings\/$&/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">import</span><span style="color:#11796D"> 'package:greetings/hello.dart'</span><span style="color:#222222"> deferred </span><span style="color:#D43324">as</span><span style="color:#222222"> hello;</span></span></code></pre></div></div><p>When you need the library, invoke <code>loadLibrary()</code> using the library's identifier.</p> <?code-excerpt "misc/lib/language_tour/libraries/greeter.dart (load-library)"?> <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">greet</span><span style="color:#222222">() </span><span style="color:#D43324">async</span><span style="color:#222222"> {</span></span> <span class="line"><span style="color:#D43324"> await</span><span style="color:#222222"> hello.</span><span style="color:#6200EE">loadLibrary</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222"> hello.</span><span style="color:#6200EE">printGreeting</span><span style="color:#222222">();</span></span> <span class="line"><span style="color:#222222">}</span></span></code></pre></div></div><p>In the preceding code, the <code>await</code> keyword pauses execution until the library is loaded. For more information about <code>async</code> and <code>await</code>, see <a href="/language/async">asynchrony support</a>.</p><p>You can invoke <code>loadLibrary()</code> multiple times on a library without problems. The library is loaded only once.</p><p>Keep in mind the following when you use deferred loading:</p><ul><li>A deferred library's constants aren't constants in the importing file. Remember, these constants don't exist until the deferred library is loaded.</li><li>You can't use types from a deferred library in the importing file. Instead, consider moving interface types to a library imported by both the deferred library and the importing file.</li><li>Dart implicitly inserts <code>loadLibrary()</code> into the namespace that you define using <code>deferred as <em>namespace</em></code>. The <code>loadLibrary()</code> function returns a <a href="/libraries/dart-async#future"><code>Future</code></a>.</li></ul><div class="header-wrapper"><h3 id="library-directive">The <code>library</code> directive</h3><a class="heading-link" href="#library-directive" aria-label="Link to 'The library directive' section">#</a></div><p>To specify library-level <a href="/effective-dart/documentation#consider-writing-a-library-level-doc-comment">doc comments</a> or <a href="/language/metadata">metadata annotations</a>, attach them to a <code>library</code> declaration at the start of the file.</p> <?code-excerpt "misc/lib/effective_dart/docs_good.dart (library-doc)"?> <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:#6E6E70">/// A really great test library.</span></span> <span class="line"><span style="color:#D43324">@TestOn</span><span style="color:#222222">(</span><span style="color:#11796D">'browser'</span><span style="color:#222222">)</span></span> <span class="line"><span style="color:#D43324">library</span><span style="color:#222222">;</span></span></code></pre></div></div><div class="header-wrapper"><h2 id="implementing-libraries">Implementing libraries</h2><a class="heading-link" href="#implementing-libraries" aria-label="Link to 'Implementing libraries' section">#</a></div><p>See <a href="/guides/libraries/create-packages">Create Packages</a> for advice on how to implement a package, including:</p><ul><li>How to organize library source code.</li><li>How to use the <code>export</code> directive.</li><li>When to use the <code>part</code> directive.</li><li>How to use conditional imports and exports to implement a library that supports multiple platforms.</li></ul><nav id="subnav"><ul><li class="previous"><a href="/language/metadata">&lang;  Metadata</a></li><li class="next"><a href="/language/keywords">Keywords  &rang;</a></li></ul></nav><p id="page-github-links"><span>Unless stated otherwise, the documentation on this site reflects Dart 3.5.4. 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