安卓高手之路 图形系统(4 Measure的算法)

 	    


 /**
     * Does the hard part of measureChildren: figuring out the MeasureSpec to
     * pass to a particular child. This method figures out the right MeasureSpec
     * for one dimension (height or width) of one child view.
     *
     * The goal is to combine information from our MeasureSpec with the
     * LayoutParams of the child to get the best possible results. For example,
     * if the this view knows its size (because its MeasureSpec has a mode of
     * EXACTLY), and the child has indicated in its LayoutParams that it wants
     * to be the same size as the parent, the parent should ask the child to
     * layout given an exact size.
     *
     * @param spec The requirements for this view
     * @param padding The padding of this view for the current dimension and
     *        margins, if applicable
     * @param childDimension How big the child wants to be in the current
     *        dimension
     * @return a MeasureSpec integer for the child
     */
    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding);

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size, but our size is not fixed.
                // Constrain child to not be bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent asked to see how big we want to be
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

 这个Measure算法主要是算child的测量方式和大小的。主要根据两种根据：

1. 自身的测量方式（MeasureSpec EXACTLY，AT_MOST,UNSPECIFIED）【这个是parent传过来的】
2. 自身的大小 【这个是parent传过来的】
3. 自己的padding【计算得到】
4. child的大小【child的LayoutParam得到】

根据以上四个推测出child的MeasureSpec 。也就是大小和测量方式。

child的请求大小来确定到底child应该有多宽以及应该怎么测量。

下面的逻辑比较多。

1.如果当前View是Exactly。那么也就是说有了固定的大小。
  
  那么Child如果说是一个固定的LayoutParam.width 或者 height，那么返回结果resultSize 就设置为这个固定值。
 
 
 
 

如果child是wrapContent，那么resultSize 就设置为当前这个View的工作区大小。也就是去除pad和margin
  的大小。同时，测量方式设置为AT_MOST

如果child是match_parent，那么由于当前View是固定大小的。那么resultSize 就设置为当前这个View的工作区大小。也就是去除pad和margin
  的大小。同时，测量方式设置为EXACTLY

2.如果当前View是MeasureSpec.AT_MOST。也就是说有个最大值。也是这三种情况
  a.fixed
     这个时候，当前View可以无限大，并且child有个固定值。那么就设置这个值吧。   这个地方
，没有做越界判断，因此有时候会出现一个子View非常大，撑破了父View的情况。google在这里
算不算是一个bug呢？也就是当前的currrentView为wrap_content.但是parentView却有一个fixed value。 同时，childView也有一个fixed Value。这个时候，如果childView的大小超过了parentView。 则会出现childView显示不全的现象。
  b.wrap_content
    
     同理，传入AT_MOST.但是不超过当前View的大小。【这个是与当前View为EXACTLY的情况一样】
    
  c.fill_parent

       这种情况也非常奇怪。发生在
currentView是wrap_content.而childView是fill_parent的情况。这个时候就没办法搞了。
这个时候，就告诉子View你也没有固定的大小。也就是说循环依赖产生了。唯一的办法是让childView 继续wrap_content.也就是废掉这个fill_parent。从而打破了这种循环依赖。