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
- MeasureSpec封装了父布局传递给子View的布局要求。
- MeasureSpec可以表示宽和高
- 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;
}
0.png
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
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