View之Handler的应用

View中Handler的应用

==问题==：View中mHandler实例是如何复用？在整个布局中所有的View都持有同一个View Handler么？还是同一个Activity下持有同一个实例？

1. android.view.View中有成员方法post(Runnable)，其实质就是通过Handler的post方法，进行消息传递。通过包属性mAttachInfo持有的mHandler.post();

   /**
    * <p>Causes the Runnable to be added to the message queue.
    * The runnable will be run on the user interface thread.</p>
    * 
    * @param action The Runnable that will be executed.
    *
    */
   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;
   }
   AttachInfo mAttachInfo;
   

2. AttachInfo，是View中的一个静态内部类，是View相关信息的存储集合，其中就有mHandler属性。

   /**//View中的静态内部类
    * A set of information given to a view when it is attached to its parent
    * window.
    */
   final static class AttachInfo {
       /**
        * 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;
   }
   

3. View的mAttachInfo属性是在View的包成员方法dispatchAttachedToWindow（）中被赋值的。

   /**
    * @param info the {@link android.view.View.AttachInfo} to associated with
    *        this view
    */
   void dispatchAttachedToWindow(AttachInfo info, int visibility) {
       mAttachInfo = info;
       ...
   }
   

4. android.view.ViewRootImpl中有performTraversals（），通过host调用了View的dispatchAttachedToWindow方法，同时也为View传入了mAttachInfo对象，其中此方法的调用是在doTraversal()中进行的，同时也因为performTraversals()是私有方法，要想外部调用只能在别处有引用才能起作用。

   View mView;
   private void performTraversals() {
   // cache mView since it is used so much below...
       final View host = mView;
       ...
       host.dispatchAttachedToWindow(mAttachInfo, 0);
       ...
   }
   void doTraversal() {
       if (mTraversalScheduled) {
           mTraversalScheduled = false;
           mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
   
           if (mProfile) {
               Debug.startMethodTracing("ViewAncestor");
           }
   
           performTraversals();
   
           if (mProfile) {
               Debug.stopMethodTracing();
               mProfile = false;
           }
       }
   }
   

   final class TraversalRunnable implements Runnable {
       @Override
       public void run() {
           doTraversal();
       }
   }
   final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
   

5. android.view.ViewRootImpl，在构造器中就进行View.AttachInfo的初始化，并将mHandler对象传入，其中mHandler的实际类型是包权限内部类ViewRootHandler。

   public ViewRootImpl(Context context, Display display) {
       mContext = context;
       ...
       mAttachInfo = new View.AttachInfo(mWindowSession, mWindow, display, this, mHandler, this);
       ...
   }
   //初始化mHandler
   final ViewRootHandler mHandler = new ViewRootHandler();
   //ViewRootHandler是ViewRootImpl的一个内部类
   final class ViewRootHandler extends Handler {}
   

6. ViewRootImpl中方法setView，为其属性View mView 赋值

   /**
    * We have one child
    */
   public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
       synchronized (this) {
           if (mView == null) {
               mView = view;
               ...
           }
       }
   }