Android View 测量原理

版权声明：本文为博主原创文章，转载请注明出处。 https://blog.csdn.net/wangwei708846696/article/details/80534543

我想了想，如果直接从ViewGroup里面的方法谈起，可能和网上很多博客一样了，但是如果只是向framework开发者分析哪些，又分析不到应用层，所以我觉得应该从performMeasure()这个方法开始分析测量，因为如果在向framework层深入，那就会接触到WindowManagerService，这个过程需要掌握Binder知识，但是Binder知识很多人一时半会掌握不了，尤其是对于application开发者，不关注这些，所以从performMeasure()说起。

基本概念

1. MeasureSpec封装了父布局传递给子View的布局要求。
2. MeasureSpec可以表示宽和高
3. MeasureSpec由size和mode组成

//获取大小或模式
int specSize = MeasureSpec.getSize(measureSpec)
int specMode = MeasureSpec.getMode(measureSpec)
//设置
int measureSpec=MeasureSpec.makeMeasureSpec(size, mode);

SpecMode一共有三种:
- MeasureSpec.EXACTLY : 父容器已经检测出子View所需要的精确大小
- MeasureSpec.AT_MOST : 父容器未能检测出子View所需要的精确大小，但是指定了一个可用大小即specSize在该模式下，View的测量大小不能超过SpecSize
- MeasureSpec.UNSPECIFIED : 父容器不对子View的大小做限制

深入流程

private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
    try {
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

mView是根View，也就是decorView，decorView对应的布局是一个FrameLayout，所以我们进入FrameLayout的measure方法

  public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    // measure ourselves, this should set the measured dimension flag back
    onMeasure(widthMeasureSpec, heightMeasureSpec);
    ......
  }

在View.measure中又调用了onMeasure(widthMeasureSpec, heightMeasureSpec)方法。
并且DecorView重写了onMeasure方法，在DecorView.onMeasure方法中主要是进一步确定自己的widthMeasureSpec、heightMeasureSpec，并调用super.onMeasure(widthMeasureSpec, heightMeasureSpec)即FrameLayout.onMeasure方法。

/**
 * {@inheritDoc}
 */
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int count = getChildCount();
    final boolean measureMatchParentChildren =
            MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
            MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
    //当FrameLayout的宽和高，只有同时设置为match_parent或者指定的size，那么这个
    //measureMatchParentChlidren = false，否则为true。下面会用到这个变量

    mMatchParentChildren.clear();
    int maxHeight = 0;
    int maxWidth = 0;
    int childState = 0;    //宽高的期望类型

    for (int i = 0; i < count; i++) {    //一次遍历每一个不为GONE的子view

        final View child = getChildAt(i);    
        if (mMeasureAllChildren || child.getVisibility() != GONE) {
            //去掉FrameLayout的左右padding，子view的左右margin，这时候，再去

            //计算子view的期望的值

            measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();


            /*maxWidth找到子View中最大的宽，高同理，为什么要找到他，因为在这里，FrameLayout是wrap
            -content.他的宽高肯定受子view的影响*/

            maxWidth = Math.max(maxWidth,
                    child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
            maxHeight = Math.max(maxHeight,
                    child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
            childState = combineMeasuredStates(childState, child.getMeasuredState());

            /*下面的判断，只有上面的FragLayout的width和height都设置为match_parent 才不会执行
            此处的mMatchParentChlidren的list里存的是设置为match_parent的子view。
            结合上面两句话的意思，当FrameLayout设置为wrap_content，这时候要把所有宽高设置为
            match_parent的子View都记录下来，记录下来干什么呢？
            这时候FrameLayout的宽高同时受子View的影响*/

             if (measureMatchParentChildren) {
                if (lp.width == LayoutParams.MATCH_PARENT ||
                        lp.height == LayoutParams.MATCH_PARENT) {
                    mMatchParentChildren.add(child);
                }
            }
        }
    }

    // Account for padding too
    maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
    maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

    // Check against our minimum height and width
    maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
    maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
    // Check against our foreground's minimum height and width
    final Drawable drawable = getForeground();
    if (drawable != null) {
        maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
        maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
    }

    //设置测量过的宽高
    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
            resolveSizeAndState(maxHeight, heightMeasureSpec,
                    childState << MEASURED_HEIGHT_STATE_SHIFT));
    count = mMatchParentChildren.size();//这个大小就是子view中设定为match_parent的个数

    if (count > 1) {
        for (int i = 0; i < count; i++) {
            //这里看上去重新计算了一遍

            final View child = mMatchParentChildren.get(i);
            final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
            int childWidthMeasureSpec;
            int childHeightMeasureSpec;
            /*如果子view的宽是match_parent，则宽度期望值是总宽度-padding-margin
             如果子view的宽是指定的比如100dp，则宽度期望值是padding+margin+width
             这个很容易理解,下面的高同理*/
            if (lp.width == LayoutParams.MATCH_PARENT) {
                childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
                        getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
                        lp.leftMargin - lp.rightMargin,
                        MeasureSpec.EXACTLY);
            } else {
                childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                        getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                        lp.leftMargin + lp.rightMargin,
                        lp.width);
            }
            if (lp.height == LayoutParams.MATCH_PARENT) {
                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -
                        getPaddingTopWithForeground() - getPaddingBottomWithForeground() -
                        lp.topMargin - lp.bottomMargin,
                        MeasureSpec.EXACTLY);
            } else {
                childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                        getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                        lp.topMargin + lp.bottomMargin,
                        lp.height);
            }
            //把这部分子view重新计算大小

            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        }
    }
}

上面可以看出使用measureChildWithMargins测量子view的大小

仔细分析

我们如何入手进行学习呢？我的想法是从上到下，自下而上。意思就是我们不知道如何下手，所以从最大的开始了解，比如从ViewGroup的measure开始了解流程，自下而上是因为最大的难度大，当我们知道大致流程之后，我们就可以自下而上从View出发深入学习，然后反推至上。下面是ViewGroup对应measure调用顺序图

LinearLayout.onMeasure(int, int)  (android.widget)//从ViewGroup容器开始向下调用
    LinearLayout.measureVertical(int, int)  (android.widget)//使用measureVertical测量垂直的情况
        LinearLayout.measureChildBeforeLayout(View, int, int, int, int, int)  (android.widget)//测量子view的情况
            ViewGroup.measureChildWithMargins(View, int, int, int, int)  (android.view)//测量子view

在ViewGroup中测量子View时会调用到measureChildWithMargins()方法

/**
 * @param child
 * 子View
 * @param parentWidthMeasureSpec
 * 父容器(比如LinearLayout)的宽的MeasureSpec
 * @param widthUsed
 * 父容器(比如LinearLayout)在水平方向已经占用的空间大小
 * @param parentHeightMeasureSpec
 * 父容器(比如LinearLayout)的高的MeasureSpec
 * @param heightUsed
 * 父容器(比如LinearLayout)在垂直方向已经占用的空间大小
 */
protected void measureChildWithMargins(View child, int parentWidthMeasureSpec, int widthUsed, int parentHeightMeasureSpec, int heightUsed) {
    //得到子View的LayoutParams
    final MarginLayoutParams lp = (ViewGroup.MarginLayoutParams) child.getLayoutParams();
    //得到子View的宽的MeasureSpec
    final int childWidthMeasureSpec =
              getChildMeasureSpec(parentWidthMeasureSpec, mPaddingLeft + mPaddingRight +
                                  lp.leftMargin + lp.rightMargin + widthUsed, lp.width);
    //得到子View的高的MeasureSpec
    final int childHeightMeasureSpec =
              getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop + mPaddingBottom +
                                  lp.topMargin + lp.bottomMargin + heightUsed, lp.height);
    //测量子View
    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

对getChildMeasureSpec()做分析

//传递的参数parentWidthMeasureSpec
//mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin + widthUsed = 父view用的size
//(ViewGroup.MarginLayoutParams) child.getLayoutParams().width  = 如果是精准那就是一个大小值，如果是wrap_content或者match_parent对应为负数
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
    int specMode = View.MeasureSpec.getMode(spec);//父容器的模式
    int specSize = View.MeasureSpec.getSize(spec);//父容器的宽或者高的大小
    //父容器宽 -（父容器(比如LinearLayout)在水平方向已经占用的空间大小 + mPaddingLeft和mPaddingRight表示父容器左右两内侧的padding + lp.leftMargin和lp.rightMargin表示子View左右两外侧的margin + 父容器在水平已经使用的宽）
    //表示子view占用的宽
    int size = Math.max(0, specSize - padding);

    int resultSize = 0;
    int resultMode = 0;
    //根据父容器的测量模式
    switch (specMode) {
        case View.MeasureSpec.EXACTLY:
            //我们首先看到一个if判断if (childDimension >= 0)，或许看到这有点懵了：childDimension>=0是啥意思？难道还有小于0的情况？是的，请注意两个系统常量:LayoutParams.MATCH_PARENT=-1和LayoutParams.WRAP_CONTENT=-2 
            if (childDimension >= 0) {
                resultSize = childDimension;//表示子View的宽或高不是match_parent，也不是wrap_content而是一个具体的数值，比如100px。
                resultMode = View.MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                //子View的size就是父容器在水平方向或垂直方向可用的最大空间值即size，子View的mode也为MeasureSpec.EXACTLY
                resultSize = size;
                resultMode = View.MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                //子View的size就是父容器在水平方向或垂直方向可用的最大空间值即size，子View的mode为MeasureSpec.AT_MOST
                resultSize = size;
                resultMode = View.MeasureSpec.AT_MOST;
            }
            break;

        case View.MeasureSpec.AT_MOST:
            if (childDimension >= 0) {
                //表示子View的宽或高不是match_parent，也不是wrap_content而是一个具体的数值，比如100px
                //子View的size就是childDimension，子View的mode也为MeasureSpec.EXACTLY
                resultSize = childDimension;
                resultMode = View.MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                //子View的size就是父容器在水平方向或垂直方向可用的最大空间值即size，子View的mode也为MeasureSpec.AT_MOST
                resultSize = size;
                resultMode = View.MeasureSpec.AT_MOST;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                //子View的size就是父容器在水平方向或垂直方向可用的最大空间值即size，子View的mode也为MeasureSpec.AT_MOST
                resultSize = size;
                resultMode = View.MeasureSpec.AT_MOST;
            }
            break;

        case View.MeasureSpec.UNSPECIFIED:
            if (childDimension >= 0) {
                //表示子View的宽或高不是match_parent，也不是wrap_content而是一个具体的数值，比如100px
                //子View的size就是childDimension，子View的mode也为MeasureSpec.EXACTLY
                resultSize = childDimension;
                resultMode = View.MeasureSpec.EXACTLY;
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                //子View的size为0，子View的mode为MeasureSpec.UNSPECIFIED
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = View.MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                //子View的size为0，子View的mode为MeasureSpec.UNSPECIFIED
                resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
                resultMode = View.MeasureSpec.UNSPECIFIED;
            }
            break;
    }
    return View.MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

这样就计算出来子view的模式和大小，也同时说明了子view的大小和模式依赖于父view的大小和模式

.	具体大小	match_parent	wrap_content
EXACTLY	childDimension / EXACTLY	size / EXACTLY	size / AT_MOST
AT_MOST	childDimension / EXACTLY	size / AT_MOST	size / AT_MOST
UNSPECIFIED	childDimension / EXACTLY	0 / UNSPECIFIED	0 / UNSPECIFIED

- size是父容器的剩余大小，包括除去padding，margin，父容器使用空间
- childDimension是用户设置layout_height或者layout_width的具体值

下面进入最后一步:

child.measure(childWidthMeasureSpec, childHeightMeasureSpec);

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

onMeasure( )源码流程如下:
(1) 在onMeasure调用setMeasuredDimension( )设置View的宽和高.
(2) 在setMeasuredDimension()中调用getDefaultSize()获取View的宽和高.
(3) 在getDefaultSize()方法中又会调用到getSuggestedMinimumWidth()或者getSuggestedMinimumHeight()获取到View宽和高的最小值.

protected int getSuggestedMinimumWidth() {
    return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}

该方法返回View的宽度的最小值MinimumWidth.

下面看getDefaultSize():

public static int getDefaultSize(int size, int measureSpec) {
     int result = size;//该方法的第一个输入参数size就是调用getSuggestedMinimumWidth()方法获得的View的宽或高的最小值。
     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;
}

该方法用于获取View的宽或者高的大小。所以除去第一种情况也就是极少情况出现的UNSPECIFIED以外在measure阶段View的宽和高由其measureSpec中的specSize决定！！

如果子View在XML布局文件中对于大小的设置采用wrap_content，那么不管父View的specMode是MeasureSpec.AT_MOST还是MeasureSpec.EXACTLY对于子View而言系统给它设置的specMode都是MeasureSpec.AT_MOST，并且其大小都是parentLeftSize即父View目前剩余的可用空间

最后setMeasuredDimension()：

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
     mMeasuredWidth = measuredWidth;
     mMeasuredHeight = measuredHeight;
     mPrivateFlags |= MEASURED_DIMENSION_SET;
}

在此调用setMeasuredDimension( )设置View的宽和高的测量值。

LinearLayout.layout()

    void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
        mTotalLength = 0;//记录内部使用的高度，别被字面意思误导了以为是LinearLayout的高度
        int maxWidth = 0;
        int childState = 0;
        int alternativeMaxWidth = 0;
        int weightedMaxWidth = 0;
        boolean allFillParent = true;
        float totalWeight = 0;//权重值的总和

        final int count = getVirtualChildCount();//子view的数量,都是调用getChildCount()

        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);

        boolean matchWidth = false;
        boolean skippedMeasure = false;

        final int baselineChildIndex = mBaselineAlignedChildIndex;
        final boolean useLargestChild = mUseLargestChild;

        int largestChildHeight = Integer.MIN_VALUE;
        int consumedExcessSpace = 0;

        int nonSkippedChildCount = 0;

        // See how tall everyone is. Also remember max width.
        for (int i = 0; i < count; ++i) {
            //获取view
            final View child = getVirtualChildAt(i);
            if (child == null) {
                mTotalLength += measureNullChild(i);
                continue;
            }
            //如果是GONE就过
            if (child.getVisibility() == View.GONE) {
               i += getChildrenSkipCount(child, i);
               continue;
            }

            nonSkippedChildCount++;
            //有分割线就加上
            if (hasDividerBeforeChildAt(i)) {
                mTotalLength += mDividerHeight;
            }
            //有时候我们在代码里面通过Inflater服务，动态加载一个布局，然后去设置他的LayoutParams，
            //如果不引用父容器的LayoutParams就会报一个强转错误，原因就在这个 父容器在add,
            //measure的时候都会把子View的LayoutParams强转成自己的类型
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();

            totalWeight += lp.weight;//计入总权重


            //这里就值得注意下了如果当前的LinearLayout是EXACTLY模式，且子view的高度为0，且权重大于0
            //这个子view只有在LinearLayout高度有剩余的时候，才会根据权重的占比去平分剩余空间
            //上文说的二次测量也就指的这部分
            final boolean useExcessSpace = lp.height == 0 && lp.weight > 0;
            if (heightMode == MeasureSpec.EXACTLY && useExcessSpace) {
                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
                skippedMeasure = true;
            } else {
                if (useExcessSpace) {
                    lp.height = LayoutParams.WRAP_CONTENT;
                }
                final int usedHeight = totalWeight == 0 ? mTotalLength : 0;
                //这里就开始测算子View了,使用measureChildWithMargins
                //如果当前的LinearLayout不是EXACTLY模式，且子View的weight大于0，优先会把当前LinearLayout的全部可用高度用于子View测量
                measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
                        heightMeasureSpec, usedHeight);

                final int childHeight = child.getMeasuredHeight();
                if (useExcessSpace) {
                    // Restore the original height and record how much space
                    // we've allocated to excess-only children so that we can
                    // match the behavior of EXACTLY measurement.
                    lp.height = 0;
                    consumedExcessSpace += childHeight;
                }

                final int totalLength = mTotalLength;
                //加上子View的margin值
                mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
                       lp.bottomMargin + getNextLocationOffset(child));

                if (useLargestChild) {
                    largestChildHeight = Math.max(childHeight, largestChildHeight);
                }
            }

            /**
             * If applicable, compute the additional offset to the child's baseline
             * we'll need later when asked {@link #getBaseline}.
             */
            if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {
               mBaselineChildTop = mTotalLength;
            }

            // if we are trying to use a child index for our baseline, the above
            // book keeping only works if there are no children above it with
            // weight.  fail fast to aid the developer.
            if (i < baselineChildIndex && lp.weight > 0) {
                throw new RuntimeException("A child of LinearLayout with index "
                        + "less than mBaselineAlignedChildIndex has weight > 0, which "
                        + "won't work.  Either remove the weight, or don't set "
                        + "mBaselineAlignedChildIndex.");
            }

            boolean matchWidthLocally = false;
            if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {
                // The width of the linear layout will scale, and at least one
                // child said it wanted to match our width. Set a flag
                // indicating that we need to remeasure at least that view when
                // we know our width.
                matchWidth = true;
                matchWidthLocally = true;
            }

            final int margin = lp.leftMargin + lp.rightMargin;
            final int measuredWidth = child.getMeasuredWidth() + margin;
            maxWidth = Math.max(maxWidth, measuredWidth);
            childState = combineMeasuredStates(childState, child.getMeasuredState());

            allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
            if (lp.weight > 0) {
                /*
                 * Widths of weighted Views are bogus if we end up
                 * remeasuring, so keep them separate.
                 */
                weightedMaxWidth = Math.max(weightedMaxWidth,
                        matchWidthLocally ? margin : measuredWidth);
            } else {
                alternativeMaxWidth = Math.max(alternativeMaxWidth,
                        matchWidthLocally ? margin : measuredWidth);
            }

            i += getChildrenSkipCount(child, i);
        }

        if (nonSkippedChildCount > 0 && hasDividerBeforeChildAt(count)) {
            mTotalLength += mDividerHeight;
        }

        if (useLargestChild &&
                (heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
            mTotalLength = 0;

            for (int i = 0; i < count; ++i) {
                final View child = getVirtualChildAt(i);
                if (child == null) {
                    mTotalLength += measureNullChild(i);
                    continue;
                }

                if (child.getVisibility() == GONE) {
                    i += getChildrenSkipCount(child, i);
                    continue;
                }

                final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)
                        child.getLayoutParams();
                // Account for negative margins
                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
                        lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
            }
        }

        // 当测量完子View的大小后，总高度会再加上padding的高度
        mTotalLength += mPaddingTop + mPaddingBottom;

        int heightSize = mTotalLength;

        //如果设置了minimumheight属性，会根据当前使用高度和最小高度进行比较
        //然后取两者中大的值
        heightSize = Math.max(heightSize, getSuggestedMinimumHeight());

        // Reconcile our calculated size with the heightMeasureSpec
        int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
        heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
        // Either expand children with weight to take up available space or
        // shrink them if they extend beyond our current bounds. If we skipped
        // measurement on any children, we need to measure them now.
        //到了这里，会再对带weight属性的子View进行一次测绘
        //首先计算属于高度
        int remainingExcess = heightSize - mTotalLength
                + (mAllowInconsistentMeasurement ? 0 : consumedExcessSpace);
        if (skippedMeasure || remainingExcess != 0 && totalWeight > 0.0f) {
             //如果设置了weightSum就会使用你设置的weightSum，否则采用当前所有子View的权重和。所以如果要手动设置weightSum的时候
            float remainingWeightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;

            mTotalLength = 0;
            //这里的代码就和第一次测量很像了
            for (int i = 0; i < count; ++i) {
                final View child = getVirtualChildAt(i);
                if (child == null || child.getVisibility() == View.GONE) {
                    continue;
                }

                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                final float childWeight = lp.weight;
                if (childWeight > 0) {
                    final int share = (int) (childWeight * remainingExcess / remainingWeightSum);
                    remainingExcess -= share;
                    remainingWeightSum -= childWeight;

                    final int childHeight;
                    if (mUseLargestChild && heightMode != MeasureSpec.EXACTLY) {
                        childHeight = largestChildHeight;
                    } else if (lp.height == 0 && (!mAllowInconsistentMeasurement
                            || heightMode == MeasureSpec.EXACTLY)) {
                        // This child needs to be laid out from scratch using
                        // only its share of excess space.
                        childHeight = share;
                    } else {
                        // This child had some intrinsic height to which we
                        // need to add its share of excess space.
                        childHeight = child.getMeasuredHeight() + share;
                    }

                    final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                            Math.max(0, childHeight), MeasureSpec.EXACTLY);
                    final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin,
                            lp.width);
                    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);

                    // Child may now not fit in vertical dimension.
                    childState = combineMeasuredStates(childState, child.getMeasuredState()
                            & (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
                }

                final int margin =  lp.leftMargin + lp.rightMargin;
                final int measuredWidth = child.getMeasuredWidth() + margin;
                maxWidth = Math.max(maxWidth, measuredWidth);

                boolean matchWidthLocally = widthMode != MeasureSpec.EXACTLY &&
                        lp.width == LayoutParams.MATCH_PARENT;

                alternativeMaxWidth = Math.max(alternativeMaxWidth,
                        matchWidthLocally ? margin : measuredWidth);

                allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;

                final int totalLength = mTotalLength;
                mTotalLength = Math.max(totalLength, totalLength + child.getMeasuredHeight() +
                        lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
            }

            // Add in our padding
            mTotalLength += mPaddingTop + mPaddingBottom;
            // TODO: Should we recompute the heightSpec based on the new total length?
        } else {
            alternativeMaxWidth = Math.max(alternativeMaxWidth,
                                           weightedMaxWidth);


            // We have no limit, so make all weighted views as tall as the largest child.
            // Children will have already been measured once.
            if (useLargestChild && heightMode != MeasureSpec.EXACTLY) {
                for (int i = 0; i < count; i++) {
                    final View child = getVirtualChildAt(i);
                    if (child == null || child.getVisibility() == View.GONE) {
                        continue;
                    }

                    final LinearLayout.LayoutParams lp =
                            (LinearLayout.LayoutParams) child.getLayoutParams();

                    float childExtra = lp.weight;
                    if (childExtra > 0) {
                        child.measure(
                                MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),
                                        MeasureSpec.EXACTLY),
                                MeasureSpec.makeMeasureSpec(largestChildHeight,
                                        MeasureSpec.EXACTLY));
                    }
                }
            }
        }

        if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
            maxWidth = alternativeMaxWidth;
        }

        maxWidth += mPaddingLeft + mPaddingRight;

        // Check against our minimum width
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                heightSizeAndState);

        if (matchWidth) {
            forceUniformWidth(count, heightMeasureSpec);
        }
    }

遍历每个子View，并对每个子View调用measureChildBeforeLayout()在measureChildBeforeLayout()方法内又会调用measureChildWithMargins()从而测量每个子View的大小。在该过程中mTotalLength保存了LinearLayout的高度，所以每当测量完一个子View该值都会发生变化。最后通过setMeasuredDimension()设置LinearLayout的大小

参考：
https://blog.csdn.net/lfdfhl/article/details/51347818
https://my.oschina.net/u/1777377/blog/415845