Handler使用小结

个人概念里面handler用来更新UI。一直有一个问题困恼我，为什么我在主线程里面创建一个Handler不需要传递传递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。个人觉得一切和ThreadLocal有关，关于ThreadLocal，请阅读如下博客:Android的消息机制之ThreadLocal的工作原理。 简而言之，ThreadLocal和当前线程绑定，如果handler在UI线程里面创建，ThreadLocal已经和主线程绑定，即使handler传入为空，也可以拿到主线程的looper，代码如下，

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();
}

我们知道，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启动的时候创建，所以 Looper.myLooper()取得的是UI主线程对应的looper，没有错误抛出。如果handler实在子线城里面创建：

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

可以很容易得出结论，子线程没有和ThreadLocal绑定，所以ThreadLocal里面的looper为空，如下异常抛出：

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，如何在子线程里面创建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方法如下， 可以看到内部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()得到的就是上面创建的 new Looper(quitAllowed)。
当然聪明的google意识到这个问题，所以有个HandlerThread可以省去手动调用prepare和loop的烦恼，核心代码如下：

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;
    }

}