以LinearLayout為例瘦麸，子View是一個自定義View壕探。這樣可以在onMeasure方法
重寫打印日志便于分析

如果LinearLayout是第一層View切揭，并且填滿屏幕；
在LinearLayout中，onMeasure接收的參數(shù)就是exactly模式伪煤；
具體分析如下：

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

高度參數(shù)heightMeasureSpec：
32位整數(shù)
前2位：模式,有三種情況{UNSPECIFIED, EXACTLY, AT_MOST}
后30位：具體大小
這樣1個int值可以用于2種用途；
工具類代碼如下：
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
其實就是位運算：

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

跟蹤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);
    }

大致就是依次對每一個子View測量
對于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);
    }

這里父布局測量凛辣，需要把父親自己的模式和子控件的LayoutParam參數(shù)合并出來抱既，
結(jié)合三種父布局模式，算出子控件建議大小扁誓，建議模式防泵，然后調(diào)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()就是設置當前view的大小

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

        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
    }

當然不重寫則是默認配置：

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

默認測量規(guī)則如下：也是根據(jù)模式來的，當然也有一個最小值蝗敢，具體分析一下吧

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;
    }

總結(jié)：父布局測量的時候捷泞，會有參數(shù)寬和參數(shù)高，依次
對子view測量寿谴，如果調(diào)用measureChildWithMargin,則根據(jù)自己本身的
和子控件的LayoutParam參數(shù)共同來算出子控件的建議高度锁右，寬度同理；
具體分析，如果一個matchParent的linearlayout里面有一個wrapcontent的子view
本身在linearlayout測量是Exactly則Exactly+wrapcontent=AT_MOST+父布局高度
所以這時候子控件不重寫測量方法高度默認就是和父布局一樣大小