在日常开发中我们常常使用LinearLayout作为布局Group，本文从其源码实现出发分析测量流程。大家可以带着问题进入下面的分析流程，看看是否能找到答案。
垂直测量
View的测量入口方法是onmeasure方法。LinearLayout的onMeasure方法根据其方向而做不同的处理。本文我们选择垂直布局测量看下。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
而measureVertical方法整体上分为2个部分。即两次测量。
第一次测量
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == View.GONE) {
//GONE的View不用测量，注意INVISIBLE还是会测量的
i += getChildrenSkipCount(child, i);
continue;
}
nonSkippedChildCount++;
if (hasDividerBeforeChildAt(i)) {
//mTotalLength为总的使用高度，这里加上分割线的高度
mTotalLength += mDividerHeight;
}
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//totalWeight为总的weight，这里主要是统计所有使用weight的view的总weight，为后续测量weight>0的View作准备
totalWeight += lp.weight;
final boolean useExcessSpace = lp.height == 0 && lp.weight > 0;
if (heightMode == MeasureSpec.EXACTLY && useExcessSpace) {
// 优化：不必费心测量仅使用多余空间布局的子视图。如果我们有空间可供分配，这些视图将在稍后进行测量。
//这里是指当LinearLayout为精确模式&&子View使用了weight。那么本次就不用测量该View，而只是将其margin统计到已使用的高度上
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
skippedMeasure = true;
} else {
if (useExcessSpace) {
// heightMode 为 UNSPECIFIED 或 AT_MOST，并且此子项仅使用多余的空间进行布局。使用 WRAP_CONTENT 进行测量以便我们能够找出视图的最佳高度。测量后，我们将恢复原始高度 0
lp.height = LayoutParams.WRAP_CONTENT;
}
// 确定此子项想要有多大。如果此子项或先前的子项已指定weight权重，则我们允许它使用所有可用空间（如果需要，我们稍后会缩小空间）。
final int usedHeight = totalWeight == 0 ? mTotalLength : 0;
//测量子View的宽高
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight);
final int childHeight = child.getMeasuredHeight();
if (useExcessSpace) {
// 恢复原始高度并记录我们为多余的子项分配了多少空间，以便我们能够匹配精确测量的行为。
lp.height = 0;
consumedExcessSpace += childHeight;
}
final int totalLength = mTotalLength;
//更新当前总的高度
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
if (useLargestChild) {
//这个largestChildHeight可以理解成所有子View中的高度最大的View的高度，主要是为了在当前View使用weight>0，但是父View的高度不确定的情况下确定子View的高度，比如LinearLayout是wrap_content,
largestChildHeight = Math.max(childHeight, largestChildHeight);
}
}
if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {
mBaselineChildTop = mTotalLength;
}
boolean matchWidthLocally = false;
if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {
// 线性布局的宽度将缩放，并且至少有一个子视图表示它想要匹配我们的宽度。设置一个标志表示当我们知道我们的宽度时，我们至少需要重新测量该视图。
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) {
/*
* 计算最大宽度
*/
weightedMaxWidth = Math.max(weightedMaxWidth,
matchWidthLocally ? margin : measuredWidth);
} else {
alternativeMaxWidth = Math.max(alternativeMaxWidth,
matchWidthLocally ? margin : measuredWidth);
}
i += getChildrenSkipCount(child, i);
}
从第一测量的源码得知，其第一次测量主要是测量weight==0的子View的宽高，并为第二次测量做准备：统计weight的和、统计已使用的高度(为weight能分配的高度做准备)
统计最大的子View的高度。
第二次测量
//remainingExcess为剩余可以为weight>0的子View分配高度的总和
int remainingExcess = heightSize - mTotalLength
+ (mAllowInconsistentMeasurement ? 0 : consumedExcessSpace);
if (skippedMeasure
|| ((sRemeasureWeightedChildren || remainingExcess != 0) && totalWeight > 0.0f)) {
//如果有weight>0的子View。则走这个分支
float remainingWeightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;
mTotalLength = 0;
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) {
// 按照权重计算每个weight>0的子View的高度
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;
}
//生成子View的高度MeasureSpec，模式为精确
final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
Math.max(0, childHeight), MeasureSpec.EXACTLY);
final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin,
lp.width);
//重新测量子View
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));
}
}
}
}