Handler是一套消息机制，可以通过其向MessageQueue中发送消息，然后Looper则会无限循环MessageQueue中的消息来进行操作

Handler机制的几个名词概念：

1. Message :消息，理解为线程间的通讯单元；里面包含了更新UI的信息，通常将其发送给UI线程，然后进行更新UI
   
2. Message Queue :消息队列，用于存放通过Handler发布的消息，按照先进先出执行
   
3. Handler :Handler是Message的主要处理者，负责将Message添加到消息队列以及对消息队列中的Message进行处理
   
4. Looper :循环器，扮演Message Queue和Handler之间桥梁的角色，循环去除Message Queue里面的Message并交付给相应的Handler进行处理
   
5. UI线程 :UI线程通常指的就是Main线程。而主线程启动的时候，系统会默认为其建立一个Message Queue和对应的Looper

每个线程都可以包含多个Handler对象，但是这些Handler对象共享的是一套MessageQueu和Looper。原因如下：

①Looper的初始化操作是由Looper.prepare()来进行初始化的。

  /** Initialize the current thread as a looper.
      * This gives you a chance to create handlers that then reference
      * this looper, before actually starting the loop. Be sure to call
      * {@link #loop()} after calling this method, and end it by calling
      * {@link #quit()}.
      */
    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));
    }

此段代码的作用在于会调用当前线程所对应的ThreadLocal变量，判断其中是否已经有存在的Looper，如果有，则会抛出异常；如果没有，则会往ThreadLocal中放入一个新的Looper对象；

   /**
     * Return the {@link MessageQueue} object associated with the current
     * thread.  This must be called from a thread running a Looper, or a
     * NullPointerException will be thrown.
     */
    public static @NonNull MessageQueue myQueue() {
        return myLooper().mQueue;
    }

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

当Looper一个新对象建立的时候，对应的唯一一个MessageQueue也会被建立。所以在非Main线程之中新建Handler对象时必须手动调用Looper.prepare()方法来新建对应的一套流程（Looper,MessageQueue）
如果没有进行手动初始化Looper:

public Handler(Callback callback, boolean async) {
    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());
        }
    }

    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;
}

初始化Handler的时候回调用Looper.myLooper()方法，如果Looper为空，则会抛出异常

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

Handler发送消息给MessageQueue和Looper从MessageQueue中取出消息交由Handler

发送消息

翻阅源码可知通过Handler发送消息的时候，首先是将当前的Handler对象与Message绑定起来，

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

msg：需要发送的Message对象

uptimeMillis:则表示要发送消息的时间，它的值等于自系统开机到当前时间的毫秒数加上延迟时间

然后调用MessageQueue的enqueueMessage()方法将此Message插入队列之中

 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;
            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;
                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;
    }

上段代码还未分析！功能是将Message按照时间长短进行入队！！

取出消息

出队操作是由Looper.loop()方法来进行实现的 此方法会无限制地循环MessageQueue队列，如果其中有Message，则进行下发；要么阻塞！

/**
     * Run the message queue in this thread. Be sure to call
     * {@link #quit()} to end the loop.
     */
    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();
        }
    }

上面的方法就是进入一个死循环，进行调用MessageQueue的next()方法来判断是否存在待处理消息，如果有的话，则调用msg.target.dispatchMessage()来用其绑定的Handler对象来进行下发此Message

    /**
     * Handle system messages here.
     */
    public void dispatchMessage(Message msg) {
        if (msg.callback != null) {
            handleCallback(msg);
        } else {
            if (mCallback != null) {
                if (mCallback.handleMessage(msg)) {
                    return;
                }
            }
            handleMessage(msg);
        }
    }

 private static void handleCallback(Message message) {
        message.callback.run();
    }

几种可以在子线程中进行UI操作

1. Handler的post()方法

 public final boolean post(Runnable r)
  {return  sendMessageDelayed(getPostMessage(r), 0);}

   private static Message getPostMessage(Runnable r) {
        Message m = Message.obtain();
        m.callback = r;
        return m;}

是将此Message转换成Runnable对象 然后传入队列

2. View的post()方法

 public boolean post(Runnable action) {
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            return attachInfo.mHandler.post(action);
        }

        // Postpone the runnable until we know on which thread it needs to run.
        // Assume that the runnable will be successfully placed after attach.
        getRunQueue().post(action);
        return true;
    }

    /**
     * Returns the queue of runnable for this view.
     *
     * @return the queue of runnables for this view
     */
    private HandlerActionQueue getRunQueue() {
        if (mRunQueue == null) {
            mRunQueue = new HandlerActionQueue();
        }
        return mRunQueue;
    }

调用了MessageQueue的post()方法

3. runOnUIThread

    /**
     * Runs the specified action on the UI thread. If the current thread is the UI
     * thread, then the action is executed immediately. If the current thread is
     * not the UI thread, the action is posted to the event queue of the UI thread.
     *
     * @param action the action to run on the UI thread
     */
    public final void runOnUiThread(Runnable action) {
        if (Thread.currentThread() != mUiThread) {
            mHandler.post(action);
        } else {
            action.run();
        }
    }

源码分析：传入试图对UI进行改变的动作action，进行对当前的线程进行判断是否为主线程（UI线程），是的话直接进行运行action，否则使用了Handler的post()方法