Handler机制原理解析（二）prepare，loop，post

上一篇已经介绍了Handler机制的原理，如果不熟悉可以看Handler机制原理解析（一）。这一篇，介绍下Handler周边的知识点。我们已经知道，要获得looper对象，必须要先执行prepare。

1，Looper.prepare

    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));}/*** Initialize the current thread as a looper, marking it as an* application's main looper. The main looper for your application* is created by the Android environment, so you should never need* to call this function yourself.  See also: {@link #prepare()}*/public static void prepareMainLooper() {prepare(false);synchronized (Looper.class) {if (sMainLooper != null) {throw new IllegalStateException("The main Looper has already been prepared.");}sMainLooper = myLooper();}}

从prepareMainLooper的注释我们能看到，初始化当前线程为looper，并且标记为应用的主looper，这个主looper是Android帮忙创建的，所有不需要手动调用。我们跟踪这个方法。在ActivityThread中我们看到

public static void main(String[] args) {....Looper.prepareMainLooper();....Looper.loop();....
}

这里我们只需知道，这个方法会在创建应用的时候调用，并且在主Activity的onCreate前已经调用了即可。至于Activity的启动流程，这里不做详述。这里我们同时也看到了另外一个方法，loop。需要注意的是，如果想在工作线程使用Handler，需要手动调用这两个方法，至于顺序，相信你一定知道。

2，post(Runnable)

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

只有这一句，接着看吧，先看getPostMessage

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

这里将Runnable对象r封装进Message对象了。后面就不用再分析了，因为现在已经变成了send(message)，前面我们说过，最终都会调用到一个方法上。发送是相同的，可是处理是不同的。还是贴一下处理的代码吧

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

如果是发送的runnable，则会走第一个分支

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

这里也没啥，就是让其run起来。再回顾一下，无论在哪个线程post一个Runnable，最终都是会通过dispatch方法，让其run起来，而我们知道，dispatch方法是在主线程。那么View的post方法呢，应该也是封装了Handler的post吧，我们一起看看

    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;}

先看第一个分支，如果有attachinfo，就通过handler post出去。这个应该跟handler的方法一样。

        /*** A Handler supplied by a view's {@link android.view.ViewRootImpl}. This* handler can be used to pump events in the UI events queue.*/final Handler mHandler;

看一下Handler，果然一样。这种是view已经attach到window上了，如果没有呢？看另一个分支

    public void post(Runnable action) {postDelayed(action, 0);}public void postDelayed(Runnable action, long delayMillis) {final HandlerAction handlerAction = new HandlerAction(action, delayMillis);synchronized (this) {if (mActions == null) {mActions = new HandlerAction[4];}mActions = GrowingArrayUtils.append(mActions, mCount, handlerAction);mCount++;}}

这里还是不清楚，继续往下看

    public static <T> T[] append(T[] array, int currentSize, T element) {assert currentSize <= array.length;if (currentSize + 1 > array.length) {@SuppressWarnings("unchecked")T[] newArray = ArrayUtils.newUnpaddedArray((Class<T>) array.getClass().getComponentType(), growSize(currentSize));System.arraycopy(array, 0, newArray, 0, currentSize);array = newArray;}array[currentSize] = element;return array;}

这里能看明白，就是把handlerAction加入到数组中了（Runnable封装在handlerAction中）。这似乎也没得到执行啊。确实

，没办法，既然有post方法，我们姑且尝试看看有没有handle什么之类名字的方法吧。

    public void executeActions(Handler handler) {synchronized (this) {final HandlerAction[] actions = mActions;for (int i = 0, count = mCount; i < count; i++) {final HandlerAction handlerAction = actions[i];handler.postDelayed(handlerAction.action, handlerAction.delay);}mActions = null;mCount = 0;}}

果不其然，能executeAction，并且通过handler post出去了，猜测应该没问题，那就往上找找看吧。在View的dispatchAttachedToWindow中调用的

        if (mRunQueue != null) {mRunQueue.executeActions(info.mHandler);mRunQueue = null;}

哦，原来当view被attach到window上时，就可以通过handler post出去了。其实呢，跟View post方法的第一个分支一样，只不过保证view一定要执行过attach。