最近,Google 宣布了新的WorkManager
架构组件。它通过在 Worker
类中实现 doWork()
方法,使同步工作调度变得容易,但是如果我想在后台执行一些异步工作怎么办?例如,我想使用 Retrofit 进行网络服务调用。我知道可以进行同步网络请求,但这会阻塞线程,感觉不太对。是否有解决方案或当前不支持呢?
最近,Google 宣布了新的WorkManager
架构组件。它通过在 Worker
类中实现 doWork()
方法,使同步工作调度变得容易,但是如果我想在后台执行一些异步工作怎么办?例如,我想使用 Retrofit 进行网络服务调用。我知道可以进行同步网络请求,但这会阻塞线程,感觉不太对。是否有解决方案或当前不支持呢?
我使用了CountDownLatch并等待它达到0,这只会在异步回调更新它后发生。请查看此代码:
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];
}
顺便提一下,现在有ListenableWorker,它被设计成异步执行。
编辑:这里是一些示例用法的片段。我删去了我认为不具有说明性的大段代码,因此可能存在一些小错误。
这是针对一个任务的,它需要一个 String photoKey,从服务器检索元数据,进行一些压缩工作,然后上传已压缩的照片。这发生在主线程之外。以下是我们如何发送工作请求:
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;
}
}
根据您在应用程序中使用的工具,您也可以扩展RxWorker(如果您正在使用RxJava)或CoroutineWorker(如果您正在使用Coroutines)。它们都继承自ListenableWorker。
SettableFuture.create();
模块仅限于 WorkManager 库组的私有使用,无法使用。 - idish默认情况下,WorkManager在后台线程上运行其操作。如果您已经在后台线程上运行,并需要同步(阻塞)调用WorkManager,则使用synchronous()访问此类方法。
因此,如果您不使用synchronous(),则可以安全地从doWork()执行同步网络调用。从设计角度来看,这也是更好的方法,因为回调很混乱。
话虽如此,如果您真的想要从doWork()触发异步作业,则需要暂停执行线程,并在异步作业完成后恢复它,使用wait/notify机制
(或其他线程管理机制,例如Semaphore
)。这在大多数情况下都不是我推荐的做法。
另外,值得一提的是,WorkManager目前处于非常早期的alpha版本。
.blockingGet()
或.blockingFirst()
,可将Observable<T>
转换为阻塞的T
。
Worker
在后台线程上执行,因此不必担心NetworkOnMainThreadException
。onProgress
回调将在主线程上调用,请注意。 - idish使用协程的能力,您可以像这样“同步”doWork()
:
获取位置的暂停方法(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:
现在可以使用CoroutineWorker
,它有一个暂停的doWork()
方法。
class MySuspendWorker(private val appContext: Context, workerParams: WorkerParameters) : CoroutineWorker(appContext, workerParams) {
override suspend fun doWork(): Result {
//do your async work
}
}
我使用了BlockingQueue
,它简化了线程同步和在线程之间传递结果的过程,你只需要一个对象就行了。
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
}
}
override fun onStopped(cancelled: Boolean) {
disposable.dispose()
}
虽然有点晚了,但这可能会帮助其他人,
您可以使用 CoroutineWorker
,并在其中使用称为 suspendCancellableCoroutine
的东西,在内部进行 doWork(),它专门设计用于此目的。
以下是代码片段:
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,协程将会被暂停,直到你调用cancellableContinuation.resumeWith。
这个示例对于寻找 Firebase 和 Work Manager 的人可能会有用。它使用了 androidx.concurrent
,因此您需要在 Android 项目中 [安装][1] 它。
import android.content.Context;
import androidx.annotation.NonNull;
import androidx.work.ListenableWorker;
import androidx.work.WorkerParameters;
import androidx.concurrent.futures.CallbackToFutureAdapter;
import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.common.util.concurrent.ListenableFuture;
import com.google.firebase.firestore.FirebaseFirestore;
public class MessageWorker extends ListenableWorker
{
// Define the parameter keys:
public static final String MESSAGE_ID = "messageId";
public static final String MESSAGE_STATUS = "messageStatus";
public MessageWorker(@NonNull Context context, @NonNull WorkerParameters
workerParams) {
super(context, workerParams);
}
@NonNull
@Override
public ListenableFuture<Result> startWork() {
return CallbackToFutureAdapter.getFuture(completer -> {
String messageId = getInputData().getString(MESSAGE_ID);
String messageStatus = getInputData().getString(MESSAGE_STATUS);
FirebaseFirestore.getInstance()
.collection("messages")
.document(messageId)
.update("status", messageStatus)
.addOnSuccessListener(new OnSuccessListener<Void>() {
@Override
public void onSuccess(Void unused) {
completer.set(Result.success());
}
})
.addOnFailureListener(new OnFailureListener() {
@Override
public void onFailure(@NonNull Exception e) {
completer.set(Result.retry());
}
});
// This value is used only for debug purposes: it will be used
// in toString() of returned future or error cases.
return "startSomeAsyncStuff";
});
}
}
[1]: https://developer.android.com/jetpack/androidx/releases/concurrent#1.0.0
@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();
}
}