之前面試的時候遇到一道問題，父View的屬性(match_parent和wrap_content)對測量子view的影響，當時很蒙，沒有回答上來，最近在看View原始碼的時候發現裡面早已給出了標準答案，

先上原始碼

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {

    //父View的寬/高測量模式
    int specMode = MeasureSpec.getMode(spec);
    //父View的寬/高大小
    int specSize = MeasureSpec.getSize(spec);

    //父View剩下的可用區域
    int size = Math.max(0, specSize - padding);

    int resultSize = 0;
    int resultMode = 0;

    switch (specMode) {
    //父View_EXACTLY
    case MeasureSpec.EXACTLY:
        //如果子View寫si了寬/高
        if (childDimension >= 0) {
            //子View的MeasureSpec=EXACTLY+寫si的寬/高(si說多了不吉利)
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            //子View的MeasureSpec=EXACTLY+父View剩下的區域
            resultSize = size;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {

            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        }
        break;
    //父View_AT_MOST
    case MeasureSpec.AT_MOST:
        //如果子View寫死了寬高
        if (childDimension >= 0) {
            //子View的MeasureSpec=EXACTLY+寫si的寬/高
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            //子View的MeasureSpec=AT_MOST+父View剩下的區域
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        }
        break;

    //父View_UNSPECIFIED從來沒有用到，不做分析
    case MeasureSpec.UNSPECIFIED:
        if (childDimension >= 0) {

            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {

            resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
            resultMode = MeasureSpec.UNSPECIFIED;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {

            resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
            resultMode = MeasureSpec.UNSPECIFIED;
        }
        break;
    }

    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
 

從原始碼中不難看出，子View固定寬高：測量模式不受父View影響，全都為EXACTLY，寬高固定。 
當子View沒有固定寬高:如果父View都為AT_MOST，子View想都別想還是為AT_MOST，如果父View為EXACTLY且子View的LayoutParams為match_parent,才為EXACTLY。寬高都為父View剩下的區域。這就很好的明白了為什麼我們自定義View時，如果沒對View的寬高進行處理，View即使是wrap_content也會撐滿整個螢幕了。

綜上可以得出結論：

父View在幫助計運算元View的MeasureSpec時有著固定的套路: 
1.受父View的MeasureSpec影響

參考文件：

https://blog.csdn.net/sinat_35938012/article/details/81055380