如何创buildLooper线程，然后立即发送消息？

我有一个工作线程，在后台，处理消息。 像这样的东西：

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，并不重要）我想要做这样的事情：

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

麻烦的是，这使我有一个美丽的竞争条件：在读取worker.handler ，没有办法确定工作线程已经分配给这个字段！

我不能简单地从Worker的构造函数中创buildHandler ，因为构造函数在主线程上运行，所以Handler会将自己与错误的线程关联起来。

这似乎不是一个罕见的情况。 我可以想出几个解决方法，他们都是丑陋的：

1. 像这样的东西：

    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(); } } 

   并从主线程：

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

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

2. 将一个初始Message传递给Worker的构造函数，使run()方法发布它。 临时解决scheme不适用于多个邮件，或者如果我们不想马上发送邮件，

3. 忙于等待handler字段不再为null 。 是的，最后的手段…

我想代表Worker线程创build一个Handler和MessageQueue ，但这似乎不可能。 什么是最优雅的方式呢？

Answer 1

最终解决scheme（减去错误检查），感谢CommonsWare：

 class Worker extends HandlerThread { // ... public synchronized void waitUntilReady() { d_handler = new Handler(getLooper(), d_messageHandler); } } 

并从主线程：

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

这工作得益于HandlerThread.getLooper()的语义，它会阻塞到Looper初始化。

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顺便说一句，这与我上面的解决scheme＃1类似，因为HandlerThread大致如下实现（得爱开源）：

 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; } 

关键的区别在于它不检查工作线程是否正在运行，但它确实创build了一个活套; 而这样做的方式是将活套存放在私人领域。 太好了！

Answer 2

看看HandlerThread的源代码

 @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，如果你在Worker中扩展Thread并实现自己的Looper，那么你的主线程类应该扩展worker并在那里设置你的handler。

Answer 3

这是我的解决scheme：MainActivity：

 //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(); } } 

Answer 4

  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(); } } }