一起Talk Android吧（第一百三十九回：Android自定义View之Measure三）

各位看官们，大家好，上一回中咱们说的是Android中自定义View之Measure的例子，这一回咱们继续说该例子。闲话休提，言归正转。让我们一起Talk Android吧！

看官们，我们在前面章回中介绍了MeasureSpec类的变量和方法，这一回中，我们将介绍如何获取到View的MeasureSpec。

大家都知道，我们在使用控件时都会在配置文件中设定控件的长度和宽度，这些长度和宽度都会存储到MeasureSpec变量中，以便在Measure的时候去使用它们。那么View是如何获取到MeasureSpec呢？答案是通过父View的MeasureSpec和子View自身的长度、宽度以及边距属性。这个答案是通过分析源代码得到的，我们以ViewGroup类的measureChild()方法为例进行说明，下面是具体的源代码，请大家参考：

protected void measureChild(View child, int parentWidthMeasureSpec,
        int parentHeightMeasureSpec) {
    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);
}


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 = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
            resultMode = MeasureSpec.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);
}

源代码中measureChilda()方法调用了getChildMeasureSpec()方法，因此主要分析这个方法的源代码就可以。

从源代码中可以看到不管父View使用哪种MeasureSpec,如果子View使用固定的长度和高度，那么不论父View是哪种测量模式，子View的测量模式都是EXACTLY。(这个结论对应于每个case下的第一个if条件)。

如果子View的长度或者宽度属性配置为:“MATCH_PARENT”，那么子View的测量模式就和父View的测量模式操持一致。(这个结论对应于每个case下的第二个if条件)。

如果子View的长度或者宽度属性配置为:“MATCH_CONTENT”，那么不论父View是哪种测量模式，子View的测量模式就是AT_MOST。(这个结论对应于每个case下的第三个if条件)。

测量模式确定后，再结合子View在配置文件中设定长度、宽度和边距组成新的MeasureSpec。有看官问如何组成？直接使用MeasureSpec类的makeMeasureSpec()方法就可以。上一章回刚刚介绍的内容，这么快就忘记了？

各位看官，关于Androd中自定义View之Measure的例子咱们就介绍到这里，欲知后面还有什么例子，且听下回分解！