android 中handler的用法分析

1、直接再ui线程中初始化，这种情况handlermessage会在 ui线程中执行

2、传入Looper 参数，这种情况会在ht线程中执行 handlermessage。

HandlerThread ht = new HandlerThread("handler");
ht.start();
Handler handler = new Handler(ht.getLooper());

步骤分析：

1、HandlerThread 继承Thread

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

2、在run 分别初始化 ，绑定Looper 到当前线程，并且执行looper 循环处理MessageQuene 中的message

//初始化Looper
a) Looper.prepare();

b) if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper());

c) private Looper() {
        mQueue = new MessageQueue();
        mRun = true;
        mThread = Thread.currentThread();
    }

//绑定 Looper 到当前线程
 a) sThreadLocal.set(new Looper());

 b) public void set(T value) {
        Thread currentThread = Thread.currentThread();
//取出当前线程中的ThreadLocal.Values变量
     Values values = values(currentThread);
        if (values == null) {
            values = initializeValues(currentThread);
        }
//将Looper变量保存到ThreadLocal.Values 变量中
        values.put(this, value);
    }

c) Values values(Thread current) {
        return current.localValues;
    }

//循环处理Message
Looper.looper() 

所以线程ht.getLooper() 可以用 ThreadLocal 从 当前线程中获取 Looper

3 、Handler 中的MessageQuene 为 Looper中 MessageQuene 的引用 ，sendmessage将Message保存到 MessageQuene，然后 再loop中取用。

public boolean sendMessageAtTime(Message msg, long uptimeMillis)
    {
        boolean sent = false;
        MessageQueue queue = mQueue;
        if (queue != null) {
            msg.target = this;
            sent = queue.enqueueMessage(msg, uptimeMillis);
        }
        else {
            RuntimeException e = new RuntimeException(
                this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
        }
        return sent;
    }

 Handler 另外用法 在service（或者非UI线程）中调用Toast等UI操作

looper = Looper.getMainLooper();
handler = new Handler(looper);
handler.post(new Runnable()
{
   public void run()
   {
       Toast.makeText().show();  
   }
});

同样可以使用Activity.runOnUiThread(Runnable r);