自定义View

MeasureSpec哪里来的

ViewGroup.java getChildMeasureSpec()

    //测量子view
    protected void measureChild(View child, int parentWidthMeasureSpec,
            int parentHeightMeasureSpec) {
        //子view的宽高 match_parent wrap_content 或者具体数值
        final LayoutParams lp = child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

    //Spec父容器对子view的要求
    //padding子view的padding
    //childDimension 子view想要的尺寸
    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        int specMode = MeasureSpec.getMode(spec);//父类对子类的模式要求
        int specSize = MeasureSpec.getSize(spec);//父类告诉子类的尺寸

        int size = Math.max(0, specSize - padding);//如果padding的尺寸比父类给的尺寸还大就用0

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        //父容器为精确模式 知道自己的具体尺寸
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY:
            //子view设置了具体的值 入layout_width = "100dp"
            // 则使用子类设置的值 测量模式使用精确
            if (childDimension >= 0) {
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            }
            //子view想和父容器一样大
            //使用父容器的尺寸 测量模式为精确
            else if (childDimension == LayoutParams.MATCH_PARENT) {
                resultSize = size;
                resultMode = MeasureSpec.EXACTLY;
            }
            //子view想用自己的尺寸 但是他的大小不能超过父容器大小 所以resultSize使用父容器大小
            //子view的模式就是AT_MOST 即不能超过我给你的大小
            else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size. It can't be
                // bigger than us.
                resultSize = size;//给他父容器尺寸，最大尺寸不能超过父类尺寸
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        //父容器 测量模式为最大模式的话，即父容器不知道自己的尺寸，但是大小不能超过size
        case MeasureSpec.AT_MOST:
            //子View知道自己的尺寸 使用子view的尺寸 精确模式
            if (childDimension >= 0) {
                // Child wants a specific size... so be it
                resultSize = childDimension;
                resultMode = MeasureSpec.EXACTLY;
            }
            //子viwe想和父容器一样大，使用父容器的尺寸 最大模式
            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;
            }
            //子view想使用自己的最小值
            //让子view自己测量 但最大不能超过父容器size
            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
        //父容器未定义尺寸
        case MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                // Child wants a specific size... let him have it
                resultSize = childDimension;//让子view自己决定
                resultMode = MeasureSpec.EXACTLY;
            }
            //子类想和父容器一样大，但是父容器不知道知己的尺寸，所以子view设置为0
            else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size... find out how big it should
                // be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            //child 希望尺寸由自己决定，一般这个时候，父容器 会给它一个 Size 作为最大值限制，  
            //但是 父容器 本身也需要计算，所以只能设置为0，并且没有给child最大值的设定  
            //将child的测量模式设置为 UNSPECIFIED
            else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        //noinspection ResourceType
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }