Improve app performance with Kotlin coroutines

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analytics-category="Site-Wide Custom Events" analytics-action-show="Callout Profile displayed" analytics-action-close="Callout Profile dismissed" analytics-label="Create Collection Callout" class="devsite-page-bookmark-tooltip nocontent" dismiss-button="true" id="devsite-collections-dropdown" dismiss-button-text="Dismiss" close-button-text="Got it"> <devsite-bookmark></devsite-bookmark> <span slot="popout-heading"> Stay organized with collections </span> <span slot="popout-contents"> Save and categorize content based on your preferences. </span> </devsite-feature-tooltip> <div class="devsite-page-title-meta"><devsite-view-release-notes></devsite-view-release-notes></div> <devsite-toc class="devsite-nav" depth="2" devsite-toc-embedded > </devsite-toc> <devsite-recommendations-dropdown class="nocontent"></devsite-recommendations-dropdown> <div class="devsite-article-body clearfix "> <p><a href="https://kotlinlang.org/docs/reference/coroutines/coroutines-guide.html" class="external">Kotlin coroutines</a> enable you to write clean, simplified asynchronous code that keeps your app responsive while managing long-running tasks such as network calls or disk operations.</p> <p>This topic provides a detailed look at coroutines on Android. If you're unfamiliar with coroutines, be sure to read <a href="/kotlin/coroutines">Kotlin coroutines on Android</a> before reading this topic.</p> <h2 id="manage" data-text="Manage long-running tasks" tabindex="-1">Manage long-running tasks</h2> <p>Coroutines build upon regular functions by adding two operations to handle long-running tasks. In addition to <code translate="no" dir="ltr">invoke</code> (or <code translate="no" dir="ltr">call</code>) and <code translate="no" dir="ltr">return</code>, coroutines add <code translate="no" dir="ltr">suspend</code> and <code translate="no" dir="ltr">resume</code>:</p> <ul> <li><code translate="no" dir="ltr">suspend</code> pauses the execution of the current coroutine, saving all local variables.</li> <li><code translate="no" dir="ltr">resume</code> continues execution of a suspended coroutine from the place where it was suspended.</li> </ul> <p>You can call <code translate="no" dir="ltr">suspend</code> functions only from other <code translate="no" dir="ltr">suspend</code> functions or by using a coroutine builder such as <code translate="no" dir="ltr">launch</code> to start a new coroutine.</p> <p>The following example shows a simple coroutine implementation for a hypothetical long-running task:</p> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"><code translate="no" dir="ltr">suspend fun fetchDocs() { // Dispatchers.Main val result = get("https://developer.android.com") // Dispatchers.IO for `get` show(result) // Dispatchers.Main } suspend fun get(url: String) = withContext(Dispatchers.IO) { /* ... */ } </code></pre> <p>In this example, <code translate="no" dir="ltr">get()</code> still runs on the main thread, but it suspends the coroutine before it starts the network request. When the network request completes, <code translate="no" dir="ltr">get</code> resumes the suspended coroutine instead of using a callback to notify the main thread.</p> <p>Kotlin uses a <em>stack frame</em> to manage which function is running along with any local variables. When suspending a coroutine, the current stack frame is copied and saved for later. When resuming, the stack frame is copied back from where it was saved, and the function starts running again. Even though the code might look like an ordinary sequential blocking request, the coroutine ensures that the network request avoids blocking the main thread.</p> <h2 id="main-safety" data-text="Use coroutines for main-safety" tabindex="-1">Use coroutines for main-safety</h2> <p>Kotlin coroutines use <em>dispatchers</em> to determine which threads are used for coroutine execution. To run code outside of the main thread, you can tell Kotlin coroutines to perform work on either the <em>Default</em> or <em>IO</em> dispatcher. In Kotlin, all coroutines must run in a dispatcher, even when they're running on the main thread. Coroutines can suspend themselves, and the dispatcher is responsible for resuming them.</p> <p>To specify where the coroutines should run, Kotlin provides three dispatchers that you can use:</p> <ul> <li><strong>Dispatchers.Main</strong> - Use this dispatcher to run a coroutine on the main Android thread. This should be used only for interacting with the UI and performing quick work. Examples include calling <code translate="no" dir="ltr">suspend</code> functions, running Android UI framework operations, and updating <a href="/topic/libraries/architecture/livedata"><code translate="no" dir="ltr">LiveData</code></a> objects.</li> <li><strong>Dispatchers.IO</strong> - This dispatcher is optimized to perform disk or network I/O outside of the main thread. Examples include using the <a href="/topic/libraries/architecture/room">Room component</a>, reading from or writing to files, and running any network operations.</li> <li><strong>Dispatchers.Default</strong> - This dispatcher is optimized to perform CPU-intensive work outside of the main thread. Example use cases include sorting a list and parsing JSON.</li> </ul> <p>Continuing the previous example, you can use the dispatchers to re-define the <code translate="no" dir="ltr">get</code> function. Inside the body of <code translate="no" dir="ltr">get</code>, call <code translate="no" dir="ltr">withContext(Dispatchers.IO)</code> to create a block that runs on the IO thread pool. Any code you put inside that block always executes via the <code translate="no" dir="ltr">IO</code> dispatcher. Since <code translate="no" dir="ltr">withContext</code> is itself a suspend function, the function <code translate="no" dir="ltr">get</code> is also a suspend function.</p> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"><code translate="no" dir="ltr">suspend fun fetchDocs() { // Dispatchers.Main val result = get("developer.android.com") // Dispatchers.Main show(result) // Dispatchers.Main } suspend fun get(url: String) = // Dispatchers.Main withContext(Dispatchers.IO) { // Dispatchers.IO (main-safety block) /* perform network IO here */ // Dispatchers.IO (main-safety block) } // Dispatchers.Main } </code></pre> <p>With coroutines, you can dispatch threads with fine-grained control. Because <code translate="no" dir="ltr">withContext()</code> lets you control the thread pool of any line of code without introducing callbacks, you can apply it to very small functions like reading from a database or performing a network request. A good practice is to use <code translate="no" dir="ltr">withContext()</code> to make sure every function is <em>main-safe</em>, which means that you can call the function from the main thread. This way, the caller never needs to think about which thread should be used to execute the function.</p> <p>In the previous example, <code translate="no" dir="ltr">fetchDocs()</code> executes on the main thread; however, it can safely call <code translate="no" dir="ltr">get</code>, which performs a network request in the background. Because coroutines support <code translate="no" dir="ltr">suspend</code> and <code translate="no" dir="ltr">resume</code>, the coroutine on the main thread is resumed with the <code translate="no" dir="ltr">get</code> result as soon as the <code translate="no" dir="ltr">withContext</code> block is done.</p> <aside class="special"><strong>Important:</strong><span> Using <code translate="no" dir="ltr">suspend</code> doesn't tell Kotlin to run a function on a background thread. It's normal for <code translate="no" dir="ltr">suspend</code> functions to operate on the main thread. It's also common to launch coroutines on the main thread. You should always use <code translate="no" dir="ltr">withContext()</code> inside a <code translate="no" dir="ltr">suspend</code> function when you need main-safety, such as when reading from or writing to disk, performing network operations, or running CPU-intensive operations.</span></aside> <h3 id="perf" data-text="Performance of withContext()" tabindex="-1">Performance of withContext()</h3> <p><a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/with-context.html" class="external"><code translate="no" dir="ltr">withContext()</code></a> does not add extra overhead compared to an equivalent callback-based implementation. Furthermore, it's possible to optimize <code translate="no" dir="ltr">withContext()</code> calls beyond an equivalent callback-based implementation in some situations. For example, if a function makes ten calls to a network, you can tell Kotlin to switch threads only once by using an outer <code translate="no" dir="ltr">withContext()</code>. Then, even though the network library uses <code translate="no" dir="ltr">withContext()</code> multiple times, it stays on the same dispatcher and avoids switching threads. In addition, Kotlin optimizes switching between <code translate="no" dir="ltr">Dispatchers.Default</code> and <code translate="no" dir="ltr">Dispatchers.IO</code> to avoid thread switches whenever possible.</p> <aside class="special"><strong>Important:</strong><span> Using a dispatcher that uses a thread pool like <code translate="no" dir="ltr">Dispatchers.IO</code> or <code translate="no" dir="ltr">Dispatchers.Default</code> does not guarantee that the block executes on the same thread from top to bottom. In some situations, Kotlin coroutines might move execution to another thread after a <code translate="no" dir="ltr">suspend</code>-and-<code translate="no" dir="ltr">resume</code>. This means thread-local variables might not point to the same value for the entire <code translate="no" dir="ltr">withContext()</code> block.</span></aside> <h2 id="start" data-text="Start a coroutine" tabindex="-1">Start a coroutine</h2> <p>You can start coroutines in one of two ways:</p> <ul> <li><a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/launch.html" class="external"><code translate="no" dir="ltr">launch</code></a> starts a new coroutine and doesn't return the result to the caller. Any work that is considered "fire and forget" can be started using <code translate="no" dir="ltr">launch</code>.</li> <li><a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/async.html" class="external"><code translate="no" dir="ltr">async</code></a> starts a new coroutine and allows you to return a result with a suspend function called <code translate="no" dir="ltr">await</code>.</li> </ul> <p>Typically, you should <code translate="no" dir="ltr">launch</code> a new coroutine from a regular function, as a regular function cannot call <code translate="no" dir="ltr">await</code>. Use <code translate="no" dir="ltr">async</code> only when inside another coroutine or when inside a suspend function and performing parallel decomposition.</p> <aside class="warning"><strong>Warning:</strong><span> <code translate="no" dir="ltr">launch</code> and <code translate="no" dir="ltr">async</code> handle exceptions differently. Since <code translate="no" dir="ltr">async</code> expects an eventual call to <code translate="no" dir="ltr">await</code>, it holds exceptions and rethrows them as part of the <code translate="no" dir="ltr">await</code> call. This means if you use <code translate="no" dir="ltr">async</code> to start a new coroutine from a regular function, you might silently drop an exception. These dropped exceptions won't appear in your crash metrics or be noted in logcat. For more information, see <a href="https://medium.com/androiddevelopers/cancellation-in-coroutines-aa6b90163629" class="external">Cancellation and Exceptions in Coroutines</a>.</span></aside> <h3 id="parallel" data-text="Parallel decomposition" tabindex="-1">Parallel decomposition</h3> <p>All coroutines that are started inside a <code translate="no" dir="ltr">suspend</code> function must be stopped when that function returns, so you likely need to guarantee that those coroutines finish before returning. With <em>structured concurrency</em> in Kotlin, you can define a <code translate="no" dir="ltr">coroutineScope</code> that starts one or more coroutines. Then, using <code translate="no" dir="ltr">await()</code> (for a single coroutine) or <code translate="no" dir="ltr">awaitAll()</code> (for multiple coroutines), you can guarantee that these coroutines finish before returning from the function.</p> <p>As an example, let's define a <code translate="no" dir="ltr">coroutineScope</code> that fetches two documents asynchronously. By calling <code translate="no" dir="ltr">await()</code> on each deferred reference, we guarantee that both <code translate="no" dir="ltr">async</code> operations finish before returning a value:</p> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"><code translate="no" dir="ltr">suspend fun fetchTwoDocs() = coroutineScope { val deferredOne = async { fetchDoc(1) } val deferredTwo = async { fetchDoc(2) } deferredOne.await() deferredTwo.await() } </code></pre> <p>You can also use <code translate="no" dir="ltr">awaitAll()</code> on collections, as shown in the following example:</p> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"><code translate="no" dir="ltr">suspend fun fetchTwoDocs() = // called on any Dispatcher (any thread, possibly Main) coroutineScope { val deferreds = listOf( // fetch two docs at the same time async { fetchDoc(1) }, // async returns a result for the first doc async { fetchDoc(2) } // async returns a result for the second doc ) deferreds.awaitAll() // use awaitAll to wait for both network requests } </code></pre> <p>Even though <code translate="no" dir="ltr">fetchTwoDocs()</code> launches new coroutines with <code translate="no" dir="ltr">async</code>, the function uses <code translate="no" dir="ltr">awaitAll()</code> to wait for those launched coroutines to finish before returning. Note, however, that even if we had not called <code translate="no" dir="ltr">awaitAll()</code>, the <code translate="no" dir="ltr">coroutineScope</code> builder does not resume the coroutine that called <code translate="no" dir="ltr">fetchTwoDocs</code> until after all of the new coroutines completed.</p> <p>In addition, <code translate="no" dir="ltr">coroutineScope</code> catches any exceptions that the coroutines throw and routes them back to the caller.</p> <p>For more information on parallel decomposition, see <a href="https://kotlinlang.org/docs/reference/coroutines/composing-suspending-functions.html" class="external">Composing suspending functions</a>.</p> <h2 id="concepts" data-text="Coroutines concepts" tabindex="-1">Coroutines concepts</h2> <h3 id="coroutinescope" data-text="CoroutineScope" tabindex="-1">CoroutineScope</h3> <p>A <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-scope/" class="external"><code translate="no" dir="ltr">CoroutineScope</code></a> keeps track of any coroutine it creates using <code translate="no" dir="ltr">launch</code> or <code translate="no" dir="ltr">async</code>. The ongoing work (i.e. the running coroutines) can be cancelled by calling <code translate="no" dir="ltr">scope.cancel()</code> at any point in time. In Android, some KTX libraries provide their own <code translate="no" dir="ltr">CoroutineScope</code> for certain lifecycle classes. For example, <code translate="no" dir="ltr">ViewModel</code> has a <a href="/reference/kotlin/androidx/lifecycle/package-summary#(androidx.lifecycle.ViewModel).viewModelScope:kotlinx.coroutines.CoroutineScope"><code translate="no" dir="ltr">viewModelScope</code></a>, and <code translate="no" dir="ltr">Lifecycle</code> has <a href="/reference/kotlin/androidx/lifecycle/package-summary#lifecyclescope"><code translate="no" dir="ltr">lifecycleScope</code></a>. Unlike a dispatcher, however, a <code translate="no" dir="ltr">CoroutineScope</code> doesn't run the coroutines.</p> <aside class="note"><strong>Note:</strong><span> For more information about <code translate="no" dir="ltr">viewModelScope</code>, see <a href="https://medium.com/androiddevelopers/easy-coroutines-in-android-viewmodelscope-25bffb605471" class="external">Easy Coroutines in Android: viewModelScope</a>.</span></aside> <p><code translate="no" dir="ltr">viewModelScope</code> is also used in the examples found in <a href="/kotlin/coroutines">Background threading on Android with Coroutines</a>. However, if you need to create your own <code translate="no" dir="ltr">CoroutineScope</code> to control the lifecycle of coroutines in a particular layer of your app, you can create one as follows:</p> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"><code translate="no" dir="ltr">class ExampleClass { // Job and Dispatcher are combined into a CoroutineContext which // will be discussed shortly val scope = CoroutineScope(Job() + Dispatchers.Main) fun exampleMethod() { // Starts a new coroutine within the scope scope.launch { // New coroutine that can call suspend functions fetchDocs() } } fun cleanUp() { // Cancel the scope to cancel ongoing coroutines work scope.cancel() } } </code></pre> <p>A cancelled scope cannot create more coroutines. Therefore, you should call <code translate="no" dir="ltr">scope.cancel()</code> only when the class that controls its lifecycle is being destroyed. When using <code translate="no" dir="ltr">viewModelScope</code>, the <a href="/topic/libraries/architecture/viewmodel"><code translate="no" dir="ltr">ViewModel</code></a> class cancels the scope automatically for you in the ViewModel's <code translate="no" dir="ltr">onCleared()</code> method.</p> <h3 id="job" data-text="Job" tabindex="-1">Job</h3> <p>A <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/index.html" class="external"><code translate="no" dir="ltr">Job</code></a> is a handle to a coroutine. Each coroutine that you create with <code translate="no" dir="ltr">launch</code> or <code translate="no" dir="ltr">async</code> returns a <code translate="no" dir="ltr">Job</code> instance that uniquely identifies the coroutine and manages its lifecycle. You can also pass a <code translate="no" dir="ltr">Job</code> to a <code translate="no" dir="ltr">CoroutineScope</code> to further manage its lifecycle, as shown in the following example:</p> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"><code translate="no" dir="ltr">class ExampleClass { ... fun exampleMethod() { // Handle to the coroutine, you can control its lifecycle val job = scope.launch { // New coroutine } if (...) { // Cancel the coroutine started above, this doesn't affect the scope // this coroutine was launched in job.cancel() } } } </code></pre> <h3 id="coroutinecontext" data-text="CoroutineContext" tabindex="-1">CoroutineContext</h3> <p>A <a href="https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.coroutines/-coroutine-context/index.html"><code translate="no" dir="ltr">CoroutineContext</code></a> defines the behavior of a coroutine using the following set of elements:</p> <ul> <li><a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-job/index.html" class="external"><code translate="no" dir="ltr">Job</code></a>: Controls the lifecycle of the coroutine.</li> <li><a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-dispatcher/index.html" class="external"><code translate="no" dir="ltr">CoroutineDispatcher</code></a>: Dispatches work to the appropriate thread.</li> <li><a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-name/index.html" class="external"><code translate="no" dir="ltr">CoroutineName</code></a>: The name of the coroutine, useful for debugging.</li> <li><a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-exception-handler/index.html" class="external"><code translate="no" dir="ltr">CoroutineExceptionHandler</code></a>: Handles uncaught exceptions.</li> </ul> <p>For new coroutines created within a scope, a new <code translate="no" dir="ltr">Job</code> instance is assigned to the new coroutine, and the other <code translate="no" dir="ltr">CoroutineContext</code> elements are inherited from the containing scope. You can override the inherited elements by passing a new <code translate="no" dir="ltr">CoroutineContext</code> to the <code translate="no" dir="ltr">launch</code> or <code translate="no" dir="ltr">async</code> function. Note that passing a <code translate="no" dir="ltr">Job</code> to <code translate="no" dir="ltr">launch</code> or <code translate="no" dir="ltr">async</code> has no effect, as a new instance of <code translate="no" dir="ltr">Job</code> is always assigned to a new coroutine.</p> <pre class="prettyprint lang-kotlin" translate="no" dir="ltr"><code translate="no" dir="ltr">class ExampleClass { val scope = CoroutineScope(Job() + Dispatchers.Main) fun exampleMethod() { // Starts a new coroutine on Dispatchers.Main as it's the scope's default val job1 = scope.launch { // New coroutine with CoroutineName = "coroutine" (default) } // Starts a new coroutine on Dispatchers.Default val job2 = scope.launch(Dispatchers.Default + CoroutineName("BackgroundCoroutine")) { // New coroutine with CoroutineName = "BackgroundCoroutine" (overridden) } } } </code></pre><aside class="note"><strong>Note:</strong><span> For more information about <a href="https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-coroutine-exception-handler/index.html" class="external"><code translate="no" dir="ltr">CoroutineExceptionHandler</code></a>, see <a href="https://medium.com/androiddevelopers/exceptions-in-coroutines-ce8da1ec060c" class="external">exceptions in coroutines blog post</a>.</span></aside> <h2 id="more" data-text="Additional coroutines resources" tabindex="-1">Additional coroutines resources</h2> <p>For more coroutines resources, see the following links:</p> <ul> <li><a href="/kotlin/coroutines">Kotlin coroutines on Android</a></li> <li><a href="/kotlin/coroutines/additional-resources">Additional resources for Kotlin coroutines and flow</a></li> </ul> <devsite-hats-survey class="nocontent" hats-id="VxqvKSur40kxBYCLVTd0SSGykbno" listnr-id="5207477"></devsite-hats-survey> </div> <devsite-recommendations display="in-page" hidden yield> </devsite-recommendations> <devsite-thumb-rating position="footer"> </devsite-thumb-rating> <devsite-recommendations id="recommendations-link" yield></devsite-recommendations> <div class="devsite-floating-action-buttons"> </div> </article> <devsite-content-footer class="nocontent"> <p>Content and code samples on this page are subject to the licenses described in the <a href="/license">Content License</a>. 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