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view.post为啥可以拿到view的宽高

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我们都知道可以通过View去post一个Runnable对象，在其中就可以通过getWidth和getHeight获取到View的宽高，那么为什么呢？

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//View#post
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;
}

这边判断了attachInfo对象是否为空，不为空时通过attachInfo.mHandler去post该Runnable，为空时则调用getRunQueue()去post该Runnable，我们先看看getRunQueue().post方法

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private HandlerActionQueue getRunQueue() {
    if (mRunQueue == null) {
        mRunQueue = new HandlerActionQueue();
    }
    return mRunQueue;
}
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//HandlerActionQueue
private HandlerAction[] mActions;
...
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++;
    }
}

可以看到getRunQueue方法返回了一个HandlerActionQueue对象，该对象的post方法会调用到postDelayed方法，新建了一个HandlerAction对象，并执行了GrowingArrayUtils.append把handlerAction添加到数组中，然后就没了。那我们返回去看attachInfo是什么时候赋值的

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//View#dispatchAttachedToWindow
void dispatchAttachedToWindow(AttachInfo info, int visibility) {
    mAttachInfo = info;
    ...
}

可以看到在View的dispatchAttachedToWindow中传入了AttachInfo对象

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//ViewRootImpl
public ViewRootImpl(Context context, Display display) {
    ...
        mAttachInfo = new View.AttachInfo(mWindowSession, mWindow, display, this, mHandler, this, context);
    ...
}
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//ViewRootImpl
...
View mView;
...
private void performTraversals() {
    final View host = mView;
    ...
    host.dispatchAttachedToWindow(mAttachInfo, 0);
    ...
    getRunQueue().executeActions(mAttachInfo.mHandler);
    ...
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
    ...
    performLayout(lp, mWidth, mHeight);
    ...
    performDraw();
    ...
}
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;
    }
}

在ViewRootImpl构造函数中创建了AttachInfo对象，并于performTraversals中调用了host.dispatchAttachedToWindow，接着调用了getRunQueue().executeActions方法，该方法中可以看到是将mActions数组中的Runnable对象用handler的postDelayed方法放入了消息队列中，这个mActions数组就是上面我们分析的在attachInfo为空的情况下会将Runnable对象添加到该数组中。再后面执行的就是我们非常熟悉的测量、布局和绘制过程，在performLayout之后就能拿到view的宽高了。虽然host.dispatchAttachedToWindow和getRunQueue().executeActions的调用都在测量之前，但是因为他们实际上都是在消息队列中加入了一个消息，所以实际执行到Runnable的时候view已经展示了出来，也就能获取到宽高了。

拓展知识：

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ApplicationThread#scheduleResumeActivity->ActivityThread#handleResumeActivity-> Activity#makeVisible->WindowManager#addView->WindowManagerImpl#addView->WindowManagerGlobal#addView->ViewRootImpl#setView->ViewRootImpl#requestLayout->ViewRootImpl#scheduleTraversals->ViewRootImpl#performTraversals

一些关键方法的执行顺序，视图是在Activity#onResume之后才被添加进window并开始测量布局的，所以onResume也没办法直接获取到宽高。