onMeasure(int widthMeasureSpec, int heightMeasureSpec)

宽和高的测量规格是哪里来的？

无论是View还是ViewGroup onMeasure方法参数中的测量规格都是其父View的measureSpce和它自身的size计算出来的

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

但DecorView是安卓视图中的顶层View,它的measureSpec是通过window算出来的，看过之前绘制流程的应该知道下面这个方法,在android.view.ViewRootImpl#performTraversals方法里 getRootMeasureSpec(mWidth, lp.width)

 private static int getRootMeasureSpec(int windowSize, int rootDimension) {
        int measureSpec;
        switch (rootDimension) {

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view.
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size.
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

                    int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
                    int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
                     // Ask host how big it wants to be
                    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);