The `Task` API is the standard way to handle asynchronous operations in Google Play services. It provides a powerful and flexible way to manage asynchronous calls, replacing the older `PendingResult` pattern. With `Task`, you can chain multiple calls, handle complex flows, and write clear success and failure handlers. ## Handle task results Many APIs in Google Play services and Firebase return a `Task` object to represent asynchronous operations. For example, [`FirebaseAuth.signInAnonymously()`](https://developers.google.com/android/reference/com/google/firebase/auth/FirebaseAuth) returns a `Task` which represents the result of the sign-in operation. The `Task` indicates that when the task completes successfully, it will return an `AuthResult` object. You can handle the result of a `Task` by attaching listeners that respond to successful completion, failure, or both: ``` Task task = FirebaseAuth.getInstance().signInAnonymously(); ``` To handle a successful task completion, attach an `OnSuccessListener`: ``` task.addOnSuccessListener(new OnSuccessListener() { @Override public void onSuccess(AuthResult authResult) { // Task completed successfully // ... } }); ``` To handle a failed task, attach an `OnFailureListener`: ``` task.addOnFailureListener(new OnFailureListener() { @Override public void onFailure(@NonNull Exception e) { // Task failed with an exception // ... } }); ``` To handle both success and failure in the same listener, attach an `OnCompleteListener`: ``` task.addOnCompleteListener(new OnCompleteListener() { @Override public void onComplete(@NonNull Task task) { if (task.isSuccessful()) { // Task completed successfully AuthResult result = task.getResult(); } else { // Task failed with an exception Exception exception = task.getException(); } } }); ``` ## Manage threads By default, listeners attached to a `Task` are run on the application main (UI) thread. This means that you should avoid doing long-running operations in listeners. If you need to perform a long-running operation, you can specify an `Executor` that is used to schedule listeners on a background thread. ``` // Create a new ThreadPoolExecutor with 2 threads for each processor on the // device and a 60 second keep-alive time. int numCores = Runtime.getRuntime().availableProcessors(); ThreadPoolExecutor executor = new ThreadPoolExecutor(numCores * 2, numCores *2, 60L, TimeUnit.SECONDS, new LinkedBlockingQueue()); task.addOnCompleteListener(executor, new OnCompleteListener() { @Override public void onComplete(@NonNull Task task) { // ... } }); ``` ## Use activity-scoped listeners When you need to handle task results within an `Activity`, it's important to manage the listeners' lifecycle to prevent them from being called when the `Activity` is no longer visible. To do this, you can use activity-scoped listeners. These listeners are automatically removed when the `onStop` method of your `Activity` is called, so that they won't be executed after the `Activity` is stopped. ``` Activity activity = MainActivity.this; task.addOnCompleteListener(activity, new OnCompleteListener() { @Override public void onComplete(@NonNull Task task) { // ... } }); ``` ## Chain tasks If you are using a set of APIs that return `Task` objects in a complex function, you can chain them together using continuations. This helps you avoid deeply nested callbacks and consolidates error handling for multiple chained tasks. For example, consider a scenario where you have a method `doSomething` that returns a `Task`, but it requires an `AuthResult` as a parameter. You can obtain this `AuthResult` asynchronously from another `Task`: ``` public Task doSomething(AuthResult authResult) { // ... } ``` Using the `Task.continueWithTask` method, you can chain these two tasks: ``` Task signInTask = FirebaseAuth.getInstance().signInAnonymously(); signInTask.continueWithTask(new Continuation>() { @Override public Task then(@NonNull Task task) throws Exception { // Take the result from the first task and start the second one AuthResult result = task.getResult(); return doSomething(result); } }).addOnSuccessListener(new OnSuccessListener() { @Override public void onSuccess(String s) { // Chain of tasks completed successfully, got result from last task. // ... } }).addOnFailureListener(new OnFailureListener() { @Override public void onFailure(@NonNull Exception e) { // One of the tasks in the chain failed with an exception. // ... } }); ``` ## Block a task If your program is already executing in a background thread, you can block the current thread and wait for the task to complete, instead of using a callback: ``` try { // Block on a task and get the result synchronously. This is generally done // when executing a task inside a separately managed background thread. Doing this // on the main (UI) thread can cause your application to become unresponsive. AuthResult authResult = Tasks.await(task); } catch (ExecutionException e) { // The Task failed, this is the same exception you'd get in a non-blocking // failure handler. // ... } catch (InterruptedException e) { // An interrupt occurred while waiting for the task to complete. // ... } ``` You can also specify a timeout when blocking a task to prevent your application from getting stuck indefinitely if the task takes too long to complete: ``` try { // Block on the task for a maximum of 500 milliseconds, otherwise time out. AuthResult authResult = Tasks.await(task, 500, TimeUnit.MILLISECONDS); } catch (ExecutionException e) { // ... } catch (InterruptedException e) { // ... } catch (TimeoutException e) { // Task timed out before it could complete. // ... } ``` ## Interoperability `Task` is designed to work well with other common Android asynchronous programming patterns. It can be converted to and from other primitives like `ListenableFuture` and Kotlin coroutines, which are [recommended by AndroidX](https://developer.android.com/jetpack/androidx/releases/concurrent), allowing you to use the approach that best fits your needs. Here's an example using a `Task`: ``` // ... simpleTask.addOnCompleteListener(this) { completedTask -> textView.text = completedTask.result } ``` ### Kotlin coroutine To use Kotlin coroutines with `Task`, add the following dependency to your project and then use the code snippet to convert from a `Task`. ##### Gradle (module-level `build.gradle`, usually `app/build.gradle`) ``` // Source: https://github.com/Kotlin/kotlinx.coroutines/tree/master/integration/kotlinx-coroutines-play-services implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-play-services:1.7.3' ``` ##### Snippet ``` import kotlinx.coroutines.tasks.await // ... textView.text = simpleTask.await() } ``` ### Guava `ListenableFuture` To use Guava `ListenableFuture` with `Task`, add the following dependency to your project and then use the code snippet to convert from a `Task`. ##### Gradle (module-level `build.gradle`, usually `app/build.gradle`) ``` implementation "androidx.concurrent:concurrent-futures:1.2.0" ``` ##### Snippet ``` import com.google.common.util.concurrent.ListenableFuture // ... /** Convert Task to ListenableFuture. */ fun taskToListenableFuture(task: Task): ListenableFuture { return CallbackToFutureAdapter.getFuture { completer -> task.addOnCompleteListener { completedTask -> if (completedTask.isCanceled) { completer.setCancelled() } else if (completedTask.isSuccessful) { completer.set(completedTask.result) } else { val e = completedTask.exception if (e != null) { completer.setException(e) } else { throw IllegalStateException() } } } } } // ... this.listenableFuture = taskToListenableFuture(simpleTask) this.listenableFuture?.addListener( Runnable { textView.text = listenableFuture?.get() }, ContextCompat.getMainExecutor(this) ) ``` ### RxJava2 `Observable` Add the following dependency, in addition to the relative async library of choice, to your project and then use the code snippet to convert from a `Task`. ##### Gradle (module-level `build.gradle`, usually `app/build.gradle`) ``` // Source: https://github.com/ashdavies/rx-tasks implementation 'io.ashdavies.rx.rxtasks:rx-tasks:2.2.0' ``` ##### Snippet ``` import io.ashdavies.rx.rxtasks.toSingle import java.util.concurrent.TimeUnit // ... simpleTask.toSingle(this).subscribe { result -> textView.text = result } ``` ## Next steps - [`Task` reference](https://developers.google.com/android/reference/com/google/android/gms/tasks/Task.md.txt) - [`Tasks` reference](https://developers.google.com/android/reference/com/google/android/gms/tasks/Tasks.md.txt)