public Handler(Callback callback, boolean async) {
    
    //检查潜在的泄漏
    if (FIND_POTENTIAL_LEAKS) {
        final Class<? extends Handler> klass = getClass();
        if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
                (klass.getModifiers() & Modifier.STATIC) == 0) {
            Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
                klass.getCanonicalName());
        }
    }

    //获取当前线程的Looper
    mLooper = Looper.myLooper();
    //Handler需要Looper,如果为null，抛出异常
    if (mLooper == null) {
        throw new RuntimeException(
            "Can't create handler inside thread " + Thread.currentThread()
                    + " that has not called Looper.prepare()");
    }
    //设置当前Handler的MessageQueue 等于Looper的MessageQueue
    mQueue = mLooper.mQueue;
    mCallback = callback;
    mAsynchronous = async;
}