FrameLayout的onMeasure方法

     /**
     * {@inheritDoc}
     */
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();
        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        //当FrameLayout的宽和高，只有同时设置为match_parent或者指定的size，那么这个

        //measureMatchParentChlidren = false，否则为true。下面会用到这个变量
        
        mMatchParentChildren.clear();
        int maxHeight = 0;     
        int maxWidth = 0;
        int childState = 0;    //宽高的期望类型

        for (int i = 0; i < count; i++) {    //一次遍历每一个不为GONE的子view
    
            final View child = getChildAt(i);    
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                //去掉FrameLayout的左右padding，子view的左右margin，这时候，再去

                //计算子view的期望的值
                
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                
                
                /*maxWidth找到子View中最大的宽，高同理，为什么要找到他，因为在这里，FrameLayout是wrap
                -content.他的宽高肯定受子view的影响*/
                
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState, child.getMeasuredState());
               
                /*下面的判断，只有上面的FragLayout的width和height都设置为match_parent 才不会执行
                此处的mMatchParentChlidren的list里存的是设置为match_parent的子view。
                结合上面两句话的意思，当FrameLayout设置为wrap_content，这时候要把所有宽高设置为
                match_parent的子View都记录下来，记录下来干什么呢？
                这时候FrameLayout的宽高同时受子View的影响*/
                   
                 if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child);
                    }
                }
            }
        }
        
        // Account for padding too
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
        
        // Check against our minimum height and width
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
        // Check against our foreground‘s minimum height and width
        final Drawable drawable = getForeground();
        if (drawable != null) {
            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
        }

        //设置测量过的宽高
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));
        count = mMatchParentChildren.size();//这个大小就是子view中设定为match_parent的个数

        if (count > 1) {
            for (int i = 0; i < count; i++) {
                //这里看上去重新计算了一遍

                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
                int childWidthMeasureSpec;
                int childHeightMeasureSpec;
                /*如果子view的宽是match_parent，则宽度期望值是总宽度-padding-margin
                 如果子view的宽是指定的比如100dp，则宽度期望值是padding+margin+width
                 这个很容易理解,下面的高同理*/
                if (lp.width == LayoutParams.MATCH_PARENT) {
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
                            getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
                            lp.leftMargin - lp.rightMargin,
                            MeasureSpec.EXACTLY);
                } else {
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                }
                
                if (lp.height == LayoutParams.MATCH_PARENT) {
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -
                            getPaddingTopWithForeground() - getPaddingBottomWithForeground() -
                            lp.topMargin - lp.bottomMargin,
                            MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }
                //把这部分子view重新计算大小

                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }