我們在new Handler()時候，實際上調(diào)用的是兩個參數(shù)的構(gòu)造方法，我們看下

 public Handler() {
        this(null, false);
    }

   public Handler(Callback callback, boolean async) {
        if (FIND_POTENTIAL_LEAKS) {
            final Class 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());
            }
        }

        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 null
        mCallback = callback;
        // mAsynchronous false
        mAsynchronous = async;
    }

我們看下myLooper()方法，

 public static @Nullable Looper myLooper() {
        return sThreadLocal.get();
    }

sThreadLocal是什么我們看下：

 // sThreadLocal.get() will return null unless you"ve called prepare().
    static final ThreadLocal sThreadLocal = new ThreadLocal();

在沒有調(diào)用Looper的prepare()情況下回返回null，我們看下prepare()方法的實現(xiàn)：

private static void prepare(boolean quitAllowed) {
        //一個Thread只能有一個Looper綁定
        if (sThreadLocal.get() != null) {
            throw new RuntimeException("Only one Looper may be created per thread");
        }
        sThreadLocal.set(new Looper(quitAllowed));
    }

現(xiàn)在終于可以看下Looper是構(gòu)造方法了

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

到這里Handler的mLooper和mQueue就找到出處了

我們看下sendMessage()做了什么：

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

 public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        //uptimeMillis() 從開機(jī)到現(xiàn)在的毫秒數(shù)
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
        //looper中創(chuàng)建的queue
        MessageQueue queue = mQueue;
        if (queue == null) {
            RuntimeException e = new RuntimeException(
                    this + " sendMessageAtTime() called with no mQueue");
            Log.w("Looper", e.getMessage(), e);
            return false;
        }
        // 加入隊列
        return enqueueMessage(queue, msg, uptimeMillis);
    }

  private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
        // 在這里我們注意下我們給msg添加了一個target是handler對象
        msg.target = this;
        if (mAsynchronous) {
            msg.setAsynchronous(true);
        }
        return queue.enqueueMessage(msg, uptimeMillis);
    }

這里調(diào)用了MessageQueue的enqueueMessae()方法，把我們的msg添加到queue里

我們再來看下Looper.loop()方法：

 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 {
                //==================================================================
                // 重點代碼在這里
                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();
        }
    }

重點代碼是 msg.target.dispatchMessage(msg);這句代碼，msg的target對象實際就是Handler，我們看下Handler的dispatchMessage()方法。

 public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
           
            handleMessage(msg);
        }
    }

看到了我們熟悉的handleMessaee()方法