Android每天面试一题--Android的Handler机制

我们在面试的时候经常会被问到Handler机制，我们一般回答的很简单，比如答到用来发送及处理消息，处理子线程向主线程发出的消息。对于面试官而言，这或许并不是他们想要的答案。今天，我打算从源码的角度去追溯一下，Handler究竟是怎么样的一个执行机制。

首先，我们知道Handler里面发送消息包含了下面的几种方式
例如：

• sendMessage(Message msg)
• sendEmptyMessage(int what)
• obtainMessage(int what, Object obj).sendToTarget();

首先，我们进入到Handler.jave类，查看sendMessage(Message msg)方法

    public final boolean sendMessage(Message msg)
    {
        return sendMessageDelayed(msg, 0);
    }

上面的代码依次又调用了sendMessageDelayed(msg, 0);

    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

做了一个判断，由于传进来的是0，我们可以忽略掉上面的判断，直接看下面的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);
    }

上面我们看到的是拿了一个MessageQueue对象，假设我们的这个对象不为空，跳过判断，直接我们在接着看下面的enqueueMessage(queue, msg, uptimeMillis);

    private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

queue.enqueueMessage(msg, uptimeMillis);主要做的就是将msg消息装在MessageQueue这个对象中

---

下面我们看一下，怎么把Message拿出来？

Handler.java类的构造方法

   public Handler(Callback callback, boolean async) {
       ```
        mLooper = Looper.myLooper();
        if (mLooper == null) {
            throw new RuntimeException(
                "Can't create handler inside thread that has not called Looper.prepare()");
        }
        mQueue = mLooper.mQueue;
        mCallback = callback;
        mAsynchronous = async;
    }

不错， mQueue = mLooper.mQueue; 一看，是Looper里面来的。
下面我们就要分析一下，Looper是个啥？跟MessageQueue有什么关系？

mLooper = Looper.myLooper();

   static final ThreadLocal sThreadLocal = new ThreadLocal();
 
    public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

Looper的实例对象，在每一个线程中独立存在。也就是Looper拿到Thread的实例对象引用

ThreadLocal是什么呢？

ThreadLocal也是为了解决共享数据（对象）的问题，同步锁的思路是线程不能同时访问一片内存区域，而ThreadLocal的思路是，干脆给每个线程Copy一份一抹一样的对象，各自玩自己的，互相不影响。

这样，就拿到mLooper,然后mLooper调用loop()方法，获取到MessageQueue,拿出Message
Looper.java

    public static void loop() {
        final Looper me = myLooper();
        if (me == null) {
            throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
        }
        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();

        for (;;) {
            Message msg = queue.next(); // might block
            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
            final Printer logging = me.mLogging;
            if (logging != null) {
                logging.println(">>>>> Dispatching to " + msg.target + " " +
                        msg.callback + ": " + msg.what);
            }

            final long traceTag = me.mTraceTag;
            if (traceTag != 0 && Trace.isTagEnabled(traceTag)) {
                Trace.traceBegin(traceTag, msg.target.getTraceName(msg));
            }
            try {
                msg.target.dispatchMessage(msg);
            } finally {
                if (traceTag != 0) {
                    Trace.traceEnd(traceTag);
                }
            }

            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();
        }
    }

上面我们还遗留了一个问题，MessageQueue是怎么来的？
在看代码的时候，发现 MessageQueue queue = mQueue;，那mQueue是怎么赋值的？

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

大致我们清楚了，我们在handler发送Meesage，有这个几个过程？

1、当我们调用snedMessage(msg)的时候，Handler类里面会启用Looper类里面的Looper方法,创建MessageQueue，并获取当前线程的引用。然后将msg存进去

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

2、当我们去拿数据的时候，由于我们的Looper.loop()方法在回调handleMessage(Message msg)方法后执行。它会不停的拿到当前的MessageQueue,从而遍历消息。