Android View绘制流程

众所周知View树的绘制入口在ViewRootImpl. performTraversals()方法。这里只挑重点看流程，不纠结于Measure()、layout()、draw()的详细过程。

    private void performTraversals() {
        //测量
        performMeasure(...);
        //布局
        performLayout(...);
        //绘制
        performDraw();
    }

• performMeasure()

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

View显示时调用到ViewRootImpl.setView()

    public void setView(View view...){
        mView = view;
    }

mView就是包含content的DecorView

    public final void measure(...){
        onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

DecorView继承自FrameLayout重写了onMeasure()，DecorView.onMeasure()

    protected void onMeasure(...){
        ...
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

调用到父类FrameLayout.onMeasure()

    protected void onMeasure(...){
        ...
        for (int i = 0; i < count; i++) {
            ...
            //测量子View
            measureChildWithMargins()
        }
        
        //设置大小
        setMeasuredDimension(...)

        for (int i = 0; i < count; i++) {
            ...
            //遍历子view，调用measure()完成View树测量
            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        }
        ...
    }

ViewGroup.measureChildWithMargins()

    protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                        + heightUsed, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

ViewRootImpl.performMeasure()调用到DecorView.onMeasure()；DecorView继承自FrameLayout也就是Viewgroup，遍历子View调用子View.Measure()，内部调用onMeasure()完成测量。

• performLayout()

    private void performLayout(){
        final View host = mView;
        ...
        host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
    }

DecorView.onLayout()

    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        ...
    }

FrameLayout.onLayout()

    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        layoutChildren(left, top, right, bottom, false /* no force left gravity */);
    }

FrameLayout.layoutChildren()

   void layoutChildren(...){
        ...
        child.layout(...)
   }

View.layout()确定自身位置。ViewRootImpl.performLayout()调用到DecorView.onLayout()。ViewGroup.onLayout()是抽象方法，实现类必需重写。ViewGroup需先遍历子View，调用View.onLayout()确定子View位置，然后才能确定自身位置。子View如果是View不是ViewGroup，其onLayou()是空方法，因为View不会有子View。

• performDraw()

    private void performDraw() {
        boolean canUseAsync = draw(fullRedrawNeeded);
    }

ViewRootImpl.draw()

    private boolean draw(...){
        if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
                        scalingRequired, dirty, surfaceInsets)) {
                    return false;
        }
    }

ViewRootImpl.drawSoftware()

    private boolean drawSoftware(...){
        mView.draw(canvas);
    }

DecorView.draw()

    public void draw(Canvas canvas) {
        super.draw(canvas);

        if (mMenuBackground != null) {
            mMenuBackground.draw(canvas);
        }
    }

super.draw(canvas)调用到View.draw()

    public void draw(Canvas canvas) {
        ...
        //背景
        drawBackground(canvas);
        //绘制
        onDraw(canvas)
        //遍历调用子View.draw()
        dispatchDraw(canvas);
        //前景
        onDrawForeground(canvas);
        //默认焦点
        drawDefaultFocusHighlight(canvas)
    }

View.dispatchDraw()是空方法，因为View不会有子View

    protected void dispatchDraw(Canvas canvas) {

    }

ViewGroup.dispatchDraw()遍历子View调用ViewGroup.drawChild()，间接调用子View.draw()

    protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
        return child.draw(canvas, this, drawingTime);
    }

ViewRootImpl.performDraw()方法调用到DecorView.draw()。draw()方法中绘制背景、调用onDraw()方法(DecorView是ViewGroup，自身并没有内容，所以没有重写onDraw()，其父类View.onDraw()也是空方法，也就是说ViewGroup不用绘制自身)、遍历调用子View.draw()、绘制前景和默认焦点，直至View树绘制完成。

总结下：setContentView()方法通过ViewRootImpl. performTraversals()开启绘制流程。该方法内部连续调用performMeasure()、performLayout()、performDraw()，最终对应调用到View的onMeasure()、onLayout()、onDraw()。布局解析出来后是树形结构，顶层View是DecorView这个继承自FrameLayout的ViewGroup。而ViewGroup需要遍历所有子View，对应调用三个方法，直至整个View树绘制完毕。不同的是ViewGroup是容器，自身并不需要绘制，所以onDraw()是空方法；View的位置由其父View->ViewGroup决定，自身没有子View，所以不需要onLayout()。