android - 在服务中使用 Looper 与使用单独的线程一样吗？

Question

在文档 (https://developer.android.com/guide/components/services.html#ExtendingService) 中的这个例子中，我们使用线程的“循环器”，并在服务类中使用它，然后服务将像在单独的线程中一样工作？

public class HelloService extends Service {
  private Looper mServiceLooper;
  private ServiceHandler mServiceHandler;

  // Handler that receives messages from the thread
  private final class ServiceHandler extends Handler {
      public ServiceHandler(Looper looper) {
          super(looper);
      }
      @Override
      public void handleMessage(Message msg) {
          // Normally we would do some work here, like download a file.
          // For our sample, we just sleep for 5 seconds.
          long endTime = System.currentTimeMillis() + 5*1000;
          while (System.currentTimeMillis() < endTime) {
              synchronized (this) {
                  try {
                      wait(endTime - System.currentTimeMillis());
                  } catch (Exception e) {
                  }
              }
          }
          // Stop the service using the startId, so that we don't stop
          // the service in the middle of handling another job
          stopSelf(msg.arg1);
      }
  }

  @Override
  public void onCreate() {
    // Start up the thread running the service.  Note that we create a
    // separate thread because the service normally runs in the process's
    // main thread, which we don't want to block.  We also make it
    // background priority so CPU-intensive work will not disrupt our UI.
    HandlerThread thread = new HandlerThread("ServiceStartArguments",
            Process.THREAD_PRIORITY_BACKGROUND);
    thread.start();

    // Get the HandlerThread's Looper and use it for our Handler 
    mServiceLooper = thread.getLooper();
    mServiceHandler = new ServiceHandler(mServiceLooper);
  }

  @Override
  public int onStartCommand(Intent intent, int flags, int startId) {
      Toast.makeText(this, "service starting", Toast.LENGTH_SHORT).show();

      // For each start request, send a message to start a job and deliver the
      // start ID so we know which request we're stopping when we finish the job
      Message msg = mServiceHandler.obtainMessage();
      msg.arg1 = startId;
      mServiceHandler.sendMessage(msg);

      // If we get killed, after returning from here, restart
      return START_STICKY;
  }

  @Override
  public IBinder onBind(Intent intent) {
      // We don't provide binding, so return null
      return null;
  }

  @Override
  public void onDestroy() {
    Toast.makeText(this, "service done", Toast.LENGTH_SHORT).show(); 
  }
}

谢谢

Answer 1

线程(HandlerThread)开始于onCreate , 当你调用 thread.start(); ，然后您将获得对 Looper 的引用该线程(每个 Looper 只创建一个 HandlerThread)创建一个 Handler和 Handler用于向线程发布消息。 Looper是等待 while(true) 中消息的对象循环。

每次命令发送到Service , Service向 HandlerThread 发送消息通过 Handler .

仔细查看源代码将有助于您更好地理解它的工作原理。有一篇关于 Handler 的优秀帖子s 和 Looper位于 Square Engineering Blog - A journey on the Android Main Thread - Part 1 .

您还可以使用 IntentService以避免实例化您自己的线程。