Android WorkManager 中的异步 Worker

Question

Google 最近宣布了新的 WorkManager 架构组件。通过在 Worker class 中实现 doWork() 可以轻松安排同步工作，但是如果我想在后台做一些异步工作？例如，我想使用 Retrofit 进行网络服务调用。我知道我可以发出同步网络请求，但它会阻塞线程并且感觉不对。有什么解决办法吗？还是目前不支持？

Answer 1

根据 WorkManager docs:

By default, WorkManager runs its operations on a background thread. If you are already running on a background thread and have need for synchronous (blocking) calls to WorkManager, use synchronous() to access such methods.

因此，如果您不使用 synchronous()，您可以安全地从 doWork() 执行同步网络调用。从设计的角度来看，这也是一种更好的方法，因为回调很混乱。

也就是说，如果你真的想从 doWork() 触发异步作业，你需要暂停执行线程并在异步作业完成后使用 wait/notify 机制（或其他一些线程管理机制，例如 Semaphore)。在大多数情况下我不会推荐。

附带说明一下，WorkManager 处于非常早期的 alpha 阶段。

Answer 2

我使用了一个倒计时锁存器并等待它达到 0，这只会在异步回调更新它后发生。请参阅此代码：

public WorkerResult doWork() {

        final WorkerResult[] result = {WorkerResult.RETRY};
        CountDownLatch countDownLatch = new CountDownLatch(1);
        FirebaseFirestore db = FirebaseFirestore.getInstance();

        db.collection("collection").whereEqualTo("this","that").get().addOnCompleteListener(task -> {
            if(task.isSuccessful()) {
                task.getResult().getDocuments().get(0).getReference().update("field", "value")
                        .addOnCompleteListener(task2 -> {
                            if (task2.isSuccessful()) {
                                result[0] = WorkerResult.SUCCESS;
                            } else {
                                result[0] = WorkerResult.RETRY;
                            }
                            countDownLatch.countDown();
                        });
            } else {
                result[0] = WorkerResult.RETRY;
                countDownLatch.countDown();
            }
        });

        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        return result[0];

    }

Answer 3

如果您正在谈论异步作业，您可以将您的工作转移到 RxJava Observables / Singles 中。

有一组运算符，例如 .blockingGet() 或 .blockingFirst() 将 Observable<T> 转换为阻塞 T

Worker 在后台线程上执行，所以不用担心 NetworkOnMainThreadException。

Answer 4

我用过BlockingQueue，它简化了线程同步和线程间的结果传递，你只需要一个对象

private var disposable = Disposables.disposed()

private val completable = Completable.fromAction { 
        //do some heavy computation
    }.subscribeOn(Schedulers.computation()) // you will do the work on background thread

override fun doWork(): Result {
    val result = LinkedBlockingQueue<Result>()

    disposable = completable.subscribe(
            { result.put(Result.SUCCESS) },
            { result.put(Result.RETRY) }
    )

    return try {
        result.take() //need to block this thread untill completable has finished
    } catch (e: InterruptedException) {
        Result.RETRY
    }
}

此外，如果您的 Worker 已停止，请不要忘记释放资源，这是相对于 .blockingGet() 的主要优势，因为现在您可以正确地免费取消您的 Rx 任务。

override fun onStopped(cancelled: Boolean) {
    disposable.dispose()
}

Answer 5

仅供参考，现在有 ListenableWorker，它被设计为异步的。

编辑：以下是示例用法的一些片段。我删除了大块我认为不具有说明性的代码，所以这里很可能会出现一两个小错误。

这适用于获取字符串 photoKey、从服务器检索元数据、进行一些压缩工作，然后上传压缩照片的任务。这发生在主线程之外。以下是我们发送工作请求的方式：

private void compressAndUploadFile(final String photoKey) {
    Data inputData = new Data.Builder()
            .putString(UploadWorker.ARG_PHOTO_KEY, photoKey)
            .build();
    Constraints constraints = new Constraints.Builder()
            .setRequiredNetworkType(NetworkType.CONNECTED)
            .build();
    OneTimeWorkRequest request = new OneTimeWorkRequest.Builder(UploadWorker.class)
            .setInputData(inputData)
            .setConstraints(constraints)
            .build();
    WorkManager.getInstance().enqueue(request);
}

并且在 UploadWorker 中：

public class UploadWorker extends ListenableWorker {
    private static final String TAG = "UploadWorker";
    public static final String ARG_PHOTO_KEY = "photo-key";

    private String mPhotoKey;

    /**
     * @param appContext   The application {@link Context}
     * @param workerParams Parameters to setup the internal state of this worker
     */
    public UploadWorker(@NonNull Context appContext, @NonNull WorkerParameters workerParams) {
        super(appContext, workerParams);
        mPhotoKey = workerParams.getInputData().getString(ARG_PHOTO_KEY);
    }

    @NonNull
    @Override
    public ListenableFuture<Payload> onStartWork() {
        SettableFuture<Payload> future = SettableFuture.create();
        Photo photo = getPhotoMetadataFromServer(mPhotoKey).addOnCompleteListener(task -> {
            if (!task.isSuccessful()) {
                Log.e(TAG, "Failed to retrieve photo metadata", task.getException());
                future.setException(task.getException());
                return;
            }
            MyPhotoType photo = task.getResult();
            File file = photo.getFile();
            Log.d(TAG, "Compressing " + photo);
            MyImageUtil.compressImage(file, MyConstants.photoUploadConfig).addOnCompleteListener(compressionTask -> {
                if (!compressionTask.isSuccessful()) {
                    Log.e(TAG, "Could not parse " + photo + " as an image.", compressionTask.getException());
                    future.set(new Payload(Result.FAILURE));
                    return;
                }
                byte[] imageData = compressionTask.getResult();
                Log.d(TAG, "Done compressing " + photo);
                UploadUtil.uploadToServer(photo, imageData);
                future.set(new Payload(Result.SUCCESS));
            });
        });
        return future;
    }
}

编辑

根据您在应用程序中使用的东西，您还可以扩展 RxWorker（如果您使用的是 RxJava）或 CoroutineWorker （如果您使用协程）。它们都来自 ListenableWorker.

Answer 6

借助协程的强大功能，您可以 'synchronise' 像这样 doWork()：

暂停获取位置的方法（异步）：

private suspend fun getLocation(): Location = suspendCoroutine { continuation ->
    val mFusedLocationClient = LocationServices.getFusedLocationProviderClient(appContext)
    mFusedLocationClient.lastLocation.addOnSuccessListener {
        continuation.resume(it)
    }.addOnFailureListener {
        continuation.resumeWithException(it)
    }
}

doWork()中的调用示例：

override fun doWork(): Result {
    val loc = runBlocking {
        getLocation()
    }
    val latitude = loc.latitude
}

2021 年更新：您现在可以使用 CoroutineWorker，它具有暂停 doWork() 方法。

class MySuspendWorker(private val appContext: Context, workerParams: WorkerParameters) : CoroutineWorker(appContext, workerParams) {
    override suspend fun doWork(): Result {
        //do your async work
    }
}

Answer 7

我也更喜欢@TomH 推荐的方法。不过，我将它与 Firebase Storage 一起使用。将 WorkManager 与 CountDownlatch did the trick for me. Here a code snippet. Logs are done with Timber.

一起使用

它 returns 在任务完成之后但在工作人员 returns 成功之前 returns 来自 Firebase 的 downloadUrl 作为字符串。

@NonNull
@Override
public Result doWork() {
    mFirebaseStorage = mFirebaseStorage.getInstance();
    mTriviaImageStorageReference = mFirebaseStorage.getReference().child("images");

    CountDownLatch countDown = new CountDownLatch(2);
    Uri imageUri = Uri.parse(getInputData().getString(KEY_IMAGE_URI));

    try {

    // get the image reference
    final StorageReference imageRef = mTriviaImageStorageReference.child(imageUri.getLastPathSegment());

    // upload the image to Firebase
    imageRef.putFile(imageUri).continueWithTask(new Continuation<UploadTask.TaskSnapshot, Task<Uri>>() {
        @Override
        public Task<Uri> then(@NonNull Task<UploadTask.TaskSnapshot> task) throws Exception {
            if (!task.isSuccessful()) {
                throw task.getException();
            }
            countDown.countDown();
            return imageRef.getDownloadUrl();
        }
    }).addOnCompleteListener(new OnCompleteListener<Uri>() {
        @Override
        public void onComplete(@NonNull Task<Uri> task) {
            if (task.isSuccessful()) {
                Timber.d("Image was successfully uploaded to Firebase");
                Uri downloadUri = task.getResult();
                String imageUrl = downloadUri.toString();

                Timber.d(("URl of the image is: " + imageUrl));

                mOutputData = new Data.Builder()
                        .putString(KEY_FIREBASE_IMAGE_URL, imageUrl)
                        .build();
                countDown.countDown();
            } else {
                Toast.makeText(getApplicationContext(), "upload failed", Toast.LENGTH_SHORT).show();
                countDown.countDown();
            }
        }
    });
    countDown.await();
    return Result.success(mOutputData);

    } catch (Throwable throwable) {
        Timber.e(throwable, "Error uploading image");
        return Result.failure();
    }
}

Answer 8

来晚了，但这可能会对其他人有所帮助，

您可以使用 CoroutineWorker 并在 doWork() 内部使用名为 suspendCancellableCoroutine 的东西，它专门为此目的而设计。

下面是代码片段：

class FileDownloader(private val appContext: Context, params: WorkerParameters) :
CoroutineWorker(appContext, params) {

   override suspend fun doWork(): Result {

       try {

          suspendCancellableCoroutine<Int> { cancellableContinuation ->

              // Here you can call your asynchronous callback based network

                override fun onComplete() {
                        cancellableContinuation.resumeWith(
                            kotlin.Result.success(100))
                }

                override fun onError(error: Error?) {

                        cancellableContinuation.resumeWithException(
                            error?.connectionException ?: Throwable()
                        )
                   
               }
               
     }

     }catch (e: Exception) {
           return Result.failure()
      }

  return Result.success()
}
}

在这里，协程将停止，直到您调用 cancellableContinuation.resumeWith。

Android WorkManager 中的异步 Worker

Asynchronous Worker in Android WorkManager

android

android-architecture-components

android-workmanager

编辑