android之View绘制到窗口上的过程

以前在研究自定义空间的时候，查看过View和ViewGroup绘制的流程的。只是定性的知道会经过onMeasure—onLayout—onDraw这些流程。上一篇Android视图加载到窗口的过程分析中分析了视图加载到窗口的过程。主要就是一系列的addView操作，这篇就从addView方法开始看看View是如何绘制到窗口上的。

ActivityThread#handleResumeActivity方法

final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward,
            boolean reallyResume) {
……
ActivityClientRecord r = performResumeActivity(token, clearHide);
	……
if (r.window == null && !a.mFinished && willBeVisible) {
                r.window = r.activity.getWindow();
                View decor = r.window.getDecorView();//前面分析过，这个是Window对象所维护的装饰窗口，最顶层的窗口
                decor.setVisibility(View.INVISIBLE);
                ViewManager wm = a.getWindowManager();//获取WindowManager，继承自ViewManager，不可实例化，是个接口
                WindowManager.LayoutParams l = r.window.getAttributes();
                a.mDecor = decor;
                l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
                l.softInputMode |= forwardBit;
                if (a.mVisibleFromClient) {
                    a.mWindowAdded = true;
                    wm.addView(decor, l);// 通过WindowManager添加到窗口
                }
	……
}

可以看到最顶层的装饰窗口在activity resume的时候通过windowManager#addView方法添加。

WindowManagerImpl# addView

public void addView(View view, ViewGroup.LayoutParams params) {
        mGlobal.addView(view, params, mDisplay, mParentWindow);
}

    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {

        ViewRootImpl root;
        View panelParentView = null;
		......
            root = new ViewRootImpl(view.getContext(), display); // 创建一个ViewRoot对象

            view.setLayoutParams(wparams);

            if (mViews == null) {
                index = 1;
                mViews = new View[1];
                mRoots = new ViewRootImpl[1];
                mParams = new WindowManager.LayoutParams[1];
            } else {
                index = mViews.length + 1;
                Object[] old = mViews;
                mViews = new View[index]; 
                System.arraycopy(old, 0, mViews, 0, index-1);
                old = mRoots;
                mRoots = new ViewRootImpl[index];
                System.arraycopy(old, 0, mRoots, 0, index-1);
                old = mParams;
                mParams = new WindowManager.LayoutParams[index];
                System.arraycopy(old, 0, mParams, 0, index-1);
            }
            index--;

            mViews[index] = view;
            mRoots[index] = root;// 将view和ViewRootImp关联起来  ViewRootImp是链接View和WindowManagerService的桥梁
            mParams[index] = wparams;
        }
        try {
            root.setView(view, wparams, panelParentView);// 调用ViewRoot的setView方法
        } 
		......
    }

setView:

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {  
	requestLayout(); // 请求UI开始绘制重新绘制View树
	......
        try {
           mOrigWindowType = mWindowAttributes.type;
           mAttachInfo.mRecomputeGlobalAttributes = true;
           collectViewAttributes();
	// 通知WindowManagerService添加一个窗口  会调用到addWindow方法
           res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
                     getHostVisibility(), mDisplay.getDisplayId(),
                     mAttachInfo.mContentInsets, mInputChannel);
        }
}

两种情况会导致调用到requestLayout，改变视图显示属性，比如setVisibility,是直接或者间接调用该函数。

@Override
    public void requestLayout() {
        checkThread();// 本次调用是否是在UI线程调用的。
        mLayoutRequested = true;
        scheduleTraversals();
    }

    void scheduleTraversals() {
        if (!mTraversalScheduled) {
            mTraversalScheduled = true;
            mTraversalBarrier = mHandler.getLooper().postSyncBarrier();
//分发一个异步消息，处理函数中调用performTraversals()对View进行重新遍历。
            mChoreographer.postCallback(
                    Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);// mTraversalRunnable是一个Runnable对象
            scheduleConsumeBatchedInput();
        }
    }

	final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

	    final class TraversalRunnable implements Runnable {
        @Override
        public void run() {
            doTraversal();//执行doTraversal()
        }
}

	void doTraversal() {
        if (mTraversalScheduled) {
            mTraversalScheduled = false;
            mHandler.getLooper().removeSyncBarrier(mTraversalBarrier);
	……
            try {
                performTraversals();
            } finally {
                Trace.traceEnd(Trace.TRACE_TAG_VIEW);
            }
		……
        }
}

View树遍历的核心函数  measure—layout--draw

private void performTraversals() {
	......
    // Ask host how big it wants to be
    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
	......
performLayout();
……
performDraw();
……
     mIsInTraversal = false;
}

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
        try {
            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

mView.measure(childWidthMeasureSpec,childHeightMeasureSpec);就执行到了熟悉onMeasure(childWidthMeasureSpec,childHeightMeasureSpec);里面。