以LinearLayout为例，子View是一个自定义View。这样可以在onMeasure方法
重写打印日志便于分析

如果LinearLayout是第一层View，并且填满屏幕；
在LinearLayout中，onMeasure接收的参数就是exactly模式；
具体分析如下：

 @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

高度参数heightMeasureSpec：
32位整数
前2位：模式,有三种情况{UNSPECIFIED, EXACTLY, AT_MOST}
后30位：具体大小
这样1个int值可以用于2种用途；
工具类代码如下：
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
其实就是位运算：

public static int getMode(int measureSpec) {
            //noinspection ResourceType
            return (measureSpec & MODE_MASK);
        }

跟踪LinearLayout.onMeasure

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        if (mOrientation == VERTICAL) {
            measureVertical(widthMeasureSpec, heightMeasureSpec);
        } else {
            measureHorizontal(widthMeasureSpec, heightMeasureSpec);
        }
    }

     measureVertical() {
        for (int i = 0; i < count; ++i) {
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
                        heightMeasureSpec, usedHeight);
}

 void measureChildBeforeLayout(View child, int childIndex,
            int widthMeasureSpec, int totalWidth, int heightMeasureSpec,
            int totalHeight) {

        measureChildWithMargins(child, widthMeasureSpec, totalWidth,
                heightMeasureSpec, totalHeight);
    }

大致就是依次对每一个子View测量
对于measureChildWithMargins
是ViewGroup的方法

protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                        + widthUsed, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                        + heightUsed, lp.height);

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

这里父布局测量，需要把父亲自己的模式和子控件的LayoutParam参数合并出来，
结合三种父布局模式，算出子控件建议大小，建议模式，然后调用child.measure

 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;

        case MeasureSpec.UNSPECIFIED:
            ...
            break;
        }
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

setMeasureDimension()就是设置当前view的大小

 private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
        mMeasuredWidth = measuredWidth;
        mMeasuredHeight = measuredHeight;

        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
    }

当然不重写则是默认配置：

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

默认测量规则如下：也是根据模式来的，当然也有一个最小值，具体分析一下吧

public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
        case MeasureSpec.UNSPECIFIED:
            result = size;
            break;
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY:
            result = specSize;
            break;
        }
        return result;
    }

总结：父布局测量的时候，会有参数宽和参数高，依次
对子view测量，如果调用measureChildWithMargin,则根据自己本身的
和子控件的LayoutParam参数共同来算出子控件的建议高度，宽度同理；
具体分析，如果一个matchParent的linearlayout里面有一个wrapcontent的子view
本身在linearlayout测量是Exactly则Exactly+wrapcontent=AT_MOST+父布局高度
所以这时候子控件不重写测量方法高度默认就是和父布局一样大小