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Android的UI操作是线程不安全的，如果在UI线程中进行耗时操作，会导致ANR（Application Not Responding ），Android平台只允许在UI线程里修改UI组件，那么既要在改变UI又不能在UI线程中进行耗时操作就需要借助于Handler。

Handler 的底层由MessageQueue（内部采用单链表的数据结构来存储消息） 和Looper进行支撑` MessageQueue采用先进先出的方式来管理Message 下边是MessagQueue的next的实现

Message next() {

// Return here if the message loop has already quit and been disposed.（直接返回如果message loop 已经停止并且已经进行了处理） // This can happen if the application tries to restart a looper after quit(一般发生在程序停止并尝试重新开始looper) // which is not supported. //没有被用过的 final long ptr = mPtr; if (ptr == 0) { return null; }

int pendingIdleHandlerCount = -1; // -1 only during first iteration    int nextPollTimeoutMillis = 0;    //通过这个死循环不断的从Queue中取消息    for (;;) {        //如果取出消息超时，则进行刷新        if (nextPollTimeoutMillis != 0) {            Binder.flushPendingCommands();        }        nativePollOnce(ptr, nextPollTimeoutMillis);        synchronized (this) {            // Try to retrieve the next message.  Return if found.(取出下一个消息)            final long now = SystemClock.uptimeMillis();            Message prevMsg = null;            Message msg = mMessages;            //当msg不为空，msg的target(Handler)为空时            if (msg != null && msg.target == null) {                // Stalled by a barrier.  Find the next asynchronous message in the queue.                do {                    prevMsg = msg;                    msg = msg.next;                } while (msg != null && !msg.isAsynchronous());            }            if (msg != null) {                //下一条消息还未准备好                if (now < msg.when) {                    // Next message is not ready.  Set a timeout to wake up when it is ready.                    nextPollTimeoutMillis = (int) Math.min(msg.when - now, Integer.MAX_VALUE);                }                //取出下一个消息，并设置为“未阻塞”状态                 else {                    // Got a message.                    mBlocked = false;                    if (prevMsg != null) {                        prevMsg.next = msg.next;                    } else {                        mMessages = msg.next;                    }                    msg.next = null;                        if (DEBUG) Log.v(TAG, "Returning message: " + msg);                  //将消息标记为使用                    msg.markInUse();                    return msg;                }            } else {                // No more messages.                nextPollTimeoutMillis = -1;            }            // Process the quit message now that all pending messages have been handled.            if (mQuitting) {                dispose();                return null;            }            // If first time idle, then get the number of idlers to run.            // Idle handles only run if the queue is empty or if the first message            // in the queue (possibly a barrier) is due to be handled in the future.            if (pendingIdleHandlerCount < 0                    && (mMessages == null || now < mMessages.when)) {                pendingIdleHandlerCount = mIdleHandlers.size();            }            if (pendingIdleHandlerCount <= 0) {                // No idle handlers to run.  Loop and wait some more.                mBlocked = true;                continue;            }            if (mPendingIdleHandlers == null) {                mPendingIdleHandlers = new IdleHandler[Math.max(pendingIdleHandlerCount, 4)];            }            mPendingIdleHandlers = mIdleHandlers.toArray(mPendingIdleHandlers);        }        // Run the idle handlers.        // We only ever reach this code block during the first iteration.        for (int i = 0; i < pendingIdleHandlerCount; i++) {            final IdleHandler idler = mPendingIdleHandlers[i];            mPendingIdleHandlers[i] = null; // release the reference to the handler            boolean keep = false;            try {                keep = idler.queueIdle();            } catch (Throwable t) {                Log.wtf(TAG, "IdleHandler threw exception", t);            }            if (!keep) {                synchronized (this) {                    mIdleHandlers.remove(idler);                }            }        }        // Reset the idle handler count to 0 so we do not run them again.        pendingIdleHandlerCount = 0;        // While calling an idle handler, a new message could have been delivered        // so go back and look again for a pending message without waiting.        nextPollTimeoutMillis = 0;    }}`

上边是不断从Queue中取出消息的源码

这里是将消息插入到队列当中

boolean enqueueMessage(Message msg, long when) {        if (msg.target == null) {            throw new IllegalArgumentException("Message must have a target.");        }        if (msg.isInUse()) {            throw new IllegalStateException(msg + " This message is already in use.");        }        synchronized (this) {            if (mQuitting) {                IllegalStateException e = new IllegalStateException(                        msg.target + " sending message to a Handler on a dead thread");                Log.w(TAG, e.getMessage(), e);                msg.recycle();                return false;            }            msg.markInUse();            msg.when = when;            Message p = mMessages;            boolean needWake;            //如果队列阻塞，进行wakeup            if (p == null || when == 0 || when < p.when) {                // New head, wake up the event queue if blocked.                msg.next = p;                mMessages = msg;                needWake = mBlocked;            } else {                // Inserted within the middle of the queue.  Usually we don't have to wake                // up the event queue unless there is a barrier at the head of the queue                // and the message is the earliest asynchronous message in the queue.                needWake = mBlocked && p.target == null && msg.isAsynchronous();                Message prev;                //将Message插入到队列                for (;;) {                    prev = p;                    p = p.next;                    if (p == null || when < p.when) {                        break;                    }                    if (needWake && p.isAsynchronous()) {                        needWake = false;                    }                }                msg.next = p; // invariant: p == prev.next                prev.next = msg;            }            // We can assume mPtr != 0 because mQuitting is false.            if (needWake) {                nativeWake(mPtr);            }        }        return true;    }

Looper的构造方法是是private，因此不能通过new来创建一个Looper。通过prepare方法进行创建。在Looper的构造方法中创建一个Message,并且保存当前Thread对象

private Looper(boolean quitAllowed) {        mQueue = new MessageQueue(quitAllowed);        mThread = Thread.currentThread();    }

Looper的源码中有这样的一个ThreadLocal。

ThreadLocal从名字可以看出局部线程，她是一个线程内部的数据存储类，通过ThreadLocal可以在指定线程中存储数据,因为Looper的作用于为当前所在线程，所以用ThreadLocal来存放Looper再合适不过了。

static final ThreadLocal
   
     sThreadLocal = new ThreadLocal
    
     ();
    
   

通过preppare()方法为当前线程创建一个Looper，如果当前线程已经存在了Looper，那么抛出异常.

public static void prepare() {        prepare(true);    }    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));    }

loop是Looper中最主要的方法。来看下loop的源码

public static void loop() { //获取当前线程的Looper        final Looper me = myLooper();        if (me == null) {            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");        }        //获取当前线程的MessageQueue        final MessageQueue queue = me.mQueue;        // Make sure the identity of this thread is that of the local process,        // and keep track of what that identity token actually is.        Binder.clearCallingIdentity();        final long ident = Binder.clearCallingIdentity();    //用死循环从Message中取出消息        for (;;) {            Message msg = queue.next(); // might block            //当消息为空时，跳出这个死循环            if (msg == null) {                // No message indicates that the message queue is quitting.                return;            }            // This must be in a local variable, in case a UI event sets the logger            Printer logging = me.mLogging;            if (logging != null) {                logging.println(">>>>> Dispatching to " + msg.target + " " +                        msg.callback + ": " + msg.what);            }            //Handler进行事件的分发            msg.target.dispatchMessage(msg);            if (logging != null) {                logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);            }            // Make sure that during the course of dispatching the            // identity of the thread wasn't corrupted.            final long newIdent = Binder.clearCallingIdentity();            if (ident != newIdent) {                Log.wtf(TAG, "Thread identity changed from 0x"                        + Long.toHexString(ident) + " to 0x"                        + Long.toHexString(newIdent) + " while dispatching to "                        + msg.target.getClass().getName() + " "                        + msg.callback + " what=" + msg.what);            }            msg.recycleUnchecked();        }    }

Handler的发送消息,最后通过sendMessageAtTime将Message放到MessageQueue中

public final boolean sendMessage(Message msg)    {        return sendMessageDelayed(msg, 0);    }    public final boolean sendEmptyMessage(int what)    {        return sendEmptyMessageDelayed(what, 0);    }    public final boolean sendEmptyMessageDelayed(int what, long delayMillis) {        Message msg = Message.obtain();        msg.what = what;        return sendMessageDelayed(msg, delayMillis);    }    public final boolean sendEmptyMessageAtTime(int what, long uptimeMillis) {        Message msg = Message.obtain();        msg.what = what;        return sendMessageAtTime(msg, uptimeMillis);    }    public final boolean sendMessageDelayed(Message msg, long delayMillis)    {        if (delayMillis < 0) {            delayMillis = 0;        }        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);    }    public boolean sendMessageAtTime(Message msg, long uptimeMillis) {        MessageQueue queue = mQueue;        if (queue == null) {            RuntimeException e = new RuntimeException(                    this + " sendMessageAtTime() called with no mQueue");            Log.w("Looper", e.getMessage(), e);            return false;        }        return enqueueMessage(queue, msg, uptimeMillis);    }

Handler的dispatchMessage，callback是一个Runnable对象

public void dispatchMessage(Message msg) {        if (msg.callback != null) {            handleCallback(msg);        } else {            if (mCallback != null) {                if (mCallback.handleMessage(msg)) {                    return;                }            }            handleMessage(msg);        }    }    //handlerMessage是一个空方法，在用Handler时重写该方法，来处理消息    public void handleMessage(Message msg) {    }

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