从 Google Play 服务 9.0.0 版开始,您可以使用 Task
API 和
返回 Task
或其子类的方法数量。Task
是一个
表示异步方法调用,类似于前面的 PendingResult
多个版本的 Google Play 服务
处理任务结果
返回 Task
的常见方法是 FirebaseAuth.signInAnonymously()
。
它返回一个 Task<AuthResult>
,这意味着任务将返回
在成功时返回 AuthResult
对象:
Task<AuthResult> task = FirebaseAuth.getInstance().signInAnonymously();
如需在任务成功时收到通知,请附加 OnSuccessListener
:
task.addOnSuccessListener(new OnSuccessListener<AuthResult>() {
@Override
public void onSuccess(AuthResult authResult) {
// Task completed successfully
// ...
}
});
如需在任务失败时收到通知,请附加 OnFailureListener
:
task.addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
// Task failed with an exception
// ...
}
});
要在同一监听器中处理成功和失败,请将一个
OnCompleteListener
:
task.addOnCompleteListener(new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull Task<AuthResult> task) {
if (task.isSuccessful()) {
// Task completed successfully
AuthResult result = task.getResult();
} else {
// Task failed with an exception
Exception exception = task.getException();
}
}
});
线程处理
附加到线程的监听器在应用主 (UI) 线程上运行
默认情况。附加监听器时,您还可以指定一个 Executor
,即
来调度监听器
// 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<Runnable>());
task.addOnCompleteListener(executor, new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull Task<AuthResult> task) {
// ...
}
});
activity 作用域监听器
如果您正在监听 Activity
中的任务结果,则可能需要将
activity 作用域监听器。这些监听器会在
您的 Activity 的 onStop
方法,这样系统便不会调用监听器
当 activity 不再可见时触发。
Activity activity = MainActivity.this;
task.addOnCompleteListener(activity, new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull Task<AuthResult> task) {
// ...
}
});
链
如果您使用多个返回 Task
的 API,可以将它们链接在一起
使用接续。这有助于避免深度嵌套的回调并实现整合
任务链的错误处理。
例如,方法 doSomething
会返回 Task<String>
,但需要
AuthResult
,我们将从任务异步获取:
public Task<String> doSomething(AuthResult authResult) {
// ...
}
使用 Task.continueWithTask
方法,我们可以将这两个任务串联起来:
Task<AuthResult> signInTask = FirebaseAuth.getInstance().signInAnonymously();
signInTask.continueWithTask(new Continuation<AuthResult, Task<String>>() {
@Override
public Task<String> then(@NonNull Task<AuthResult> 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<String>() {
@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.
// ...
}
});
阻止
如果您的程序已在后台线程中执行,您可以阻止 同步获取结果并避免回调的任务:
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.
// ...
}
您还可以在阻止任务时指定超时,这样您的应用 不挂起:
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.
// ...
}
互操作性
Task
在概念上与几种常用的 Android 管理方法保持一致
异步代码,也可以直接将 Task
转换为其他
包括 ListenableFuture
和 Kotlin 协程在内的基元,
AndroidX 推荐。
下面是一个使用 Task
的示例:
// ...
simpleTask.addOnCompleteListener(this) {
completedTask -> textView.text = completedTask.result
}
Kotlin 协程
用法
将以下依赖项添加到您的项目中,并使用以下代码转换
(来自 Task
)。
Gradle(模块级 build.gradle
,通常为 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
将以下依赖项添加到您的项目中,并使用以下代码转换
(来自 Task
)。
Gradle(模块级 build.gradle
,通常为 app/build.gradle
)
implementation "androidx.concurrent:concurrent-futures:1.2.0"
Snippet
import com.google.common.util.concurrent.ListenableFuture
// ...
/** Convert Task to ListenableFuture. */
fun <T> taskToListenableFuture(task: Task<T>): ListenableFuture<T> {
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 可观察
除了
选项添加到您的项目中,然后使用以下代码从 Task
进行转换。
Gradle(模块级 build.gradle
,通常为 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 }