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;
        //當FrameLayout的寬和高，只有同時設置爲match_parent或者指定的size，那麼這個

        //measureMatchParentChlidren = false，不然爲true。下面會用到這個變量
        
        mMatchParentChildren.clear();
        int maxHeight = 0;     
        int maxWidth = 0;
        int childState = 0;    //寬高的指望類型

        for (int i = 0; i < count; i++) {    //一次遍歷每個不爲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，這時候，再去

                //計算子view的指望的值
                
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                
                
                /*maxWidth找到子View中最大的寬，高同理，爲何要找到他，由於在這裏，FrameLayout是wrap
                -content.他的寬高確定受子view的影響*/
                
                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());
               
                /*下面的判斷，只有上面的FragLayout的width和height都設置爲match_parent 纔不會執行
                此處的mMatchParentChlidren的list裏存的是設置爲match_parent的子view。
                結合上面兩句話的意思，當FrameLayout設置爲wrap_content，這時候要把全部寬高設置爲
                match_parent的子View都記錄下來，記錄下來幹什麼呢？
                這時候FrameLayout的寬高同時受子View的影響*/
                   
                 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());
        }

        //設置測量過的寬高
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));
        count = mMatchParentChildren.size();//這個大小就是子view中設定爲match_parent的個數

        if (count > 1) {
            for (int i = 0; i < count; i++) {
                //這裏看上去從新計算了一遍

                final View child = mMatchParentChildren.get(i);
                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
                int childWidthMeasureSpec;
                int childHeightMeasureSpec;
                /*若是子view的寬是match_parent，則寬度指望值是總寬度-padding-margin
                 若是子view的寬是指定的好比100dp，則寬度指望值是padding+margin+width
                 這個很容易理解,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);
                }
                //把這部分子view從新計算大小

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