好早之前，项目中有个功能项需要创建一个下载任务，考虑到和界面的无依赖性，所以我选用了WorkManager。在当时来看，WorkManager还算是Android的一个新技术。而那段项目代码，也是简单的使用而已。
今天，我们从头再来重新认识并学习一下WorkManager。
简介
WorkManager is an API that makes it easy to schedule deferrable, asynchronous tasks that are expected to run even if the app exits or the device restarts.
上面是官方关于WorkManager的一句简要说明，从中可以提出几个要点：
- WorkManager可以轻松地生成定时、异步的后台任务
- 即使应用退出，甚至系统重启后，后台任务仍可以执行
WorkManger最小可兼容到API14，是以替代像JobScheduler、AlarmManger等定时任务发布类而存在的，也是官方推荐使用的
基本使用
依赖
首先，在相应的module的build里，添加必要的库依赖
dependencies {
def work_version = "2.4.0"
// ....
implementation "androidx.work:work-runtime-ktx:$work_version"
// ....
}
因为是kotlin工程，所以这里添加的是work-runtime-ktx，Java工程则应该用work-runtime
实现Worker
androidx.work.WorkerA class that performs work synchronously on a background thread provided by WorkManager
Worker就是我们要实现的用于添加任务代码的抽象类
public abstract class Worker extends ListenableWorker {
// Package-private to avoid synthetic accessor.
SettableFuture<Result> mFuture;
@Keep
@SuppressLint("BanKeepAnnotation")
public Worker(@NonNull Context context, @NonNull WorkerParameters workerParams) {
super(context, workerParams);
}
/**
* Override this method to do your actual background processing. This method is called on a
* background thread - you are required to <b>synchronously</b> do your work and return the
* {@link androidx.work.ListenableWorker.Result} from this method. Once you return from this
* method, the Worker is considered to have finished what its doing and will be destroyed. If
* you need to do your work asynchronously on a thread of your own choice, see
* {@link ListenableWorker}.
* <p>
* A Worker is given a maximum of ten minutes to finish its execution and return a
* {@link androidx.work.ListenableWorker.Result}. After this time has expired, the Worker will
* be signalled to stop.
*
* @return The {@link androidx.work.ListenableWorker.Result} of the computation; note that
* dependent work will not execute if you use
* {@link androidx.work.ListenableWorker.Result#failure()} or
* {@link androidx.work.ListenableWorker.Result#failure(Data)}
*/
@WorkerThread
public abstract @NonNull Result doWork();
@Override
public final @NonNull ListenableFuture<Result> startWork() {
mFuture = SettableFuture.create();
getBackgroundExecutor().execute(new Runnable() {
@Override
public void run() {
try {
Result result = doWork();
mFuture.set(result);
} catch (Throwable throwable) {
mFuture.setException(throwable);
}
}
});
return mFuture;
}
}
复写doWork方法，并在其中添加实际任务代码即可。
这里来一个假任务，作为测试：
class DelayWorker(context: Context, workerParams: WorkerParameters) : Worker(context, workerParams) {
override fun doWork(): Result {
Log.d(TAG, "doWork on ${Thread.currentThread().id} started - ${System.currentTimeMillis()}")
// emulated work
Thread.sleep(3000)
Log.d(TAG, "doWork on ${Thread.currentThread().id} ended - ${System.currentTimeMillis()}")
return Result.success()
}
}
WorkRequest
任务建好后，需要一个启动任务的“请求类”，如果需要的话，还可以添加参数 —— WorkRequest上场了。
public abstract class WorkRequest {
// ....
public abstract static class Builder<B extends Builder<?, ?>, W extends WorkRequest> {
// ...
}
// ....
}
可以看出，构造WorkRequest，使用的是Builder模型；但WorkRequest和它的builder都是抽象类啊，别慌，有抽象，就有实现。
WorkRequest有两个实现：OneTimeWorkRequest和PeriodicWorkRequest。望文生义，前者用于非重复的单个任务，而后者用于周期重复性任务。
下面来构造一个非重复任务：
val request = OneTimeWorkRequestBuilder<DelayWorker>().build()
很简单，这里用了ktx的扩展，实际上就是用了OneTimeWorkRequest.Builder来build的。
/**
* Creates a [OneTimeWorkRequest] with the given [ListenableWorker].
*/
inline fun <reified W : ListenableWorker> OneTimeWorkRequestBuilder() =
OneTimeWorkRequest.Builder(W::class.java)
启动任务
worker和request都有了，接下来就要启动任务了，WorkManager上场：
Log.d(TAG, "enqueue on ${Thread.currentThread().id} ${System.currentTimeMillis()}")
WorkManager.getInstance(applicationContext).enqueue(request)
WorkManager以单例形式存在，调用enqueue，添加前面创建的request即可。
执行结果
上述任务的执行日志：
2021-01-09 15:46:43.437 15929-15929/com.jacee.examples.workmanager D/JTest: enqueue on 2 1610178403437
2021-01-09 15:46:43.513 15929-15969/com.jacee.examples.workmanager D/JTest: doWork on 4754 started - 1610178403513
2021-01-09 15:46:46.515 15929-15969/com.jacee.examples.workmanager D/JTest: doWork on 4754 ended - 1610178406514
任务成功异常地在子线程执行了，并耗时3秒（当然，这里是假任务）。
结语
前面的测试代码，算是帮我们基本认识了WorkManager，也大概了解了它的基本使用。有时间，咱再来学习WorkManager的其他功能。
