Handler使用小结

我的概念里面handler用来更新UI。一直有一个问题困恼我，为何我在主线程里面建立一个Handler不须要传递传递Looper，而在一个子线程里面必须调用Looper.prepare, Looper.loop。今天看了看源码，终于知道里面的原委。我的以为一切和ThreadLocal有关，关于ThreadLocal，请阅读以下博客:Android的消息机制之ThreadLocal的工做原理。 简而言之，ThreadLocal和当前线程绑定，若是handler在UI线程里面建立，ThreadLocal已经和主线程绑定，即便handler传入为空，也能够拿到主线程的looper，代码以下，

mLooper = Looper.myLooper();
if (mLooper == null) {
    throw new RuntimeException(
        "Can't create handler inside thread that has not called Looper.prepare()");
}
public static @Nullable Looper myLooper() {
    return sThreadLocal.get();
}

 

咱们知道，在app启动时候，在ActivityThread里面的main函数被执行：

Looper.prepareMainLooper();

ActivityThread thread = new ActivityThread();
thread.attach(false);

if (sMainThreadHandler == null) {
    sMainThreadHandler = thread.getHandler();
}

if (false) {
    Looper.myLooper().setMessageLogging(new
            LogPrinter(Log.DEBUG, "ActivityThread"));
}

// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();

 

综合上面代码，looper已经在app启动的时候建立，因此 Looper.myLooper()取得的是UI主线程对应的looper，没有错误抛出。若是handler实在子线城里面建立：

new Thread(new Runnable() {
    @Override
    public void run() {
          Handler handler = new Handler() {
              @Override
              public void handleMessage(Message msg) {
                  super.handleMessage(msg);
              }
          };
    }
}).start();

 

能够很容易得出结论，子线程没有和ThreadLocal绑定，因此ThreadLocal里面的looper为空，以下异常抛出：

if (mLooper == null) {
    throw new RuntimeException(
        "Can't create handler inside thread that has not called Looper.prepare()");
}

既然有上面问题，如何在子线程里面建立handler呢？

new Thread(new Runnable() {
    @Override
    public void run() {
          Looper.prepare();
          Handler handler = new Handler(Looper.myLooper()) {
                 public void handleMessage(android.os.Message msg) {
                       // XXX
                 }
          };
         Looper.loop();
    }
}).start();

 

继续看Looper.prepare:

private static void prepare(boolean quitAllowed) {
    if (sThreadLocal.get() != null) {
        throw new RuntimeException("Only one Looper may be created per thread");
    }
    sThreadLocal.set(new Looper(quitAllowed));
}

 ThreadLocal的set方法以下， 能够看到内部ThreadLocalMap已当前线程为key进行绑定。

public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

 

这样当前线程和 sThreadLocal就绑定了，Looper.myLooper()获得的就是上面建立的 new Looper(quitAllowed)。
固然聪明的google意识到这个问题，因此有个HandlerThread能够省去手动调用prepare和loop的烦恼，核心代码以下：

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