如何创建一个 Looper 线程，然后立即向它发送消息？

Question

我有一个位于后台处理消息的工作线程。像这样的：

class Worker extends Thread {

    public volatile Handler handler; // actually private, of course

    public void run() {
        Looper.prepare();
        mHandler = new Handler() { // the Handler hooks up to the current Thread
            public boolean handleMessage(Message msg) {
                // ...
            }
        };
        Looper.loop();
    }
}

从主线程（UI线程，没关系）我想做这样的事情：

Worker worker = new Worker();
worker.start();
worker.handler.sendMessage(...);

问题在于，这为我设置了一个漂亮的竞争条件：在读取 worker.handler 时，无法确定工作线程已经分配给该字段！

我不能简单地从Worker的构造函数中创建Handler，因为构造函数在主线程上运行，所以Handler会与错误的线程相关联。

这似乎并不罕见。我可以想出几种变通方法，但都很难看：

1. 类似这样的：

   class Worker extends Thread {
   
       public volatile Handler handler; // actually private, of course
   
       public void run() {
           Looper.prepare();
           mHandler = new Handler() { // the Handler hooks up to the current Thread
               public boolean handleMessage(Message msg) {
                   // ...
               }
           };
           notifyAll(); // <- ADDED
           Looper.loop();
       }
   }
   

   从主线程：

   Worker worker = new Worker();
   worker.start();
   worker.wait(); // <- ADDED
   worker.handler.sendMessage(...);
   

   但这也不可靠：如果notifyAll() 发生在wait() 之前，那么我们将永远不会被唤醒！

2. 将初始的Message 传递给Worker 的构造函数，让run() 方法发布它。临时解决方案不适用于多条消息，或者如果我们不想立即发送，但很快就会发送。

3. 忙于等待 handler 字段不再是 null。是的，最后的手段……

我想代表Worker 线程创建Handler 和MessageQueue，但这似乎是不可能的。最优雅的方法是什么？

Answer 1

最终解决方案（减去错误检查），感谢 CommonsWare：

class Worker extends HandlerThread {

    // ...

    public synchronized void waitUntilReady() {
        d_handler = new Handler(getLooper(), d_messageHandler);
    }

}

从主线程：

Worker worker = new Worker();
worker.start();
worker.waitUntilReady(); // <- ADDED
worker.handler.sendMessage(...);

这要归功于 HandlerThread.getLooper() 的语义，它在循环器初始化之前一直阻塞。

---

顺便说一句，这与我上面的解决方案#1 类似，因为HandlerThread 的实现大致如下（必须喜欢开源）：

public void run() {
    Looper.prepare();
    synchronized (this) {
        mLooper = Looper.myLooper();
        notifyAll();
    }
    Looper.loop();
}

public Looper getLooper() {
    synchronized (this) {
        while (mLooper == null) {
            try {
                wait();
            } catch (InterruptedException e) {
            }
        }
    }
    return mLooper;
}

关键区别在于它不检查工作线程是否在运行，而是它实际上已经创建了一个looper；这样做的方法是将looper存储在一个私有字段中。不错！

Answer 2

    class WorkerThread extends Thread {
            private Exchanger<Void> mStartExchanger = new Exchanger<Void>();
            private Handler mHandler;
            public Handler getHandler() {
                    return mHandler;
            }
            @Override
            public void run() {
                    Looper.prepare();
                    mHandler = new Handler();
                    try {
                            mStartExchanger.exchange(null);
                    } catch (InterruptedException e) {
                            e.printStackTrace();
                    }
                    Looper.loop();
            }

            @Override
            public synchronized void start() {
                    super.start();
                    try {
                            mStartExchanger.exchange(null);
                    } catch (InterruptedException e) {
                            e.printStackTrace();
                    }
            }
    }

Answer 3

看看HandlerThread的源码

@Override
     public void run() {
         mTid = Process.myTid();
         Looper.prepare();
         synchronized (this) {
             mLooper = Looper.myLooper();
             notifyAll();
         }
         Process.setThreadPriority(mPriority);
         onLooperPrepared();
         Looper.loop();
         mTid = -1;
     }

基本上，如果您在 worker 中扩展 Thread 并实现自己的 Looper，那么您的主线程类应该扩展 worker 并在那里设置您的处理程序。

Answer 4

这是我的解决方案： 主要活动：

//Other Code

 mCountDownLatch = new CountDownLatch(1);
        mainApp = this;
        WorkerThread workerThread = new WorkerThread(mCountDownLatch);
        workerThread.start();
        try {
            mCountDownLatch.await();
            Log.i("MsgToWorkerThread", "Worker Thread is up and running. We can send message to it now...");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        Toast.makeText(this, "Trial run...", Toast.LENGTH_LONG).show();
        Message msg = workerThread.workerThreadHandler.obtainMessage();
        workerThread.workerThreadHandler.sendMessage(msg);

WorkerThread 类：

public class WorkerThread extends Thread{

    public Handler workerThreadHandler;
    CountDownLatch mLatch;

    public WorkerThread(CountDownLatch latch){

        mLatch = latch;
    }


    public void run() {
        Looper.prepare();
        workerThreadHandler = new Handler() {
            @Override
            public void handleMessage(Message msg) {

                Log.i("MsgToWorkerThread", "Message received from UI thread...");
                        MainActivity.getMainApp().runOnUiThread(new Runnable() {

                            @Override
                            public void run() {
                                Toast.makeText(MainActivity.getMainApp().getApplicationContext(), "Message received in worker thread from UI thread", Toast.LENGTH_LONG).show();
                                //Log.i("MsgToWorkerThread", "Message received from UI thread...");
                            }
                        });

            }

        };
        Log.i("MsgToWorkerThread", "Worker thread ready...");
        mLatch.countDown();
        Looper.loop();
    }
}