Android高级进阶——View的工作原理（一）Measure
2018-04-11 本文已影响33人
aKaiC
开篇
本篇主要讲解的是 View 工作原理之 measure 过程，主要是以源码的形式进行分析，源码来源 API 26。
从 ViewRootImpl 的 #performTraversals 方法开始说起
performTraversals 方法是整个工作流程的核心，它里面分别取执行了 measure、layout 以及 draw 三个过程，看一下代码：
private void performTraversals() {
...
if (!mStopped || mReportNextDraw) {
boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
(relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
|| mHeight != host.getMeasuredHeight() || contentInsetsChanged ||
updatedConfiguration) {
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
if (didLayout) {
performLayout(lp, mWidth, mHeight);
...
if (!cancelDraw && !newSurface) {
if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); ++i) {
mPendingTransitions.get(i).startChangingAnimations();
}
mPendingTransitions.clear();
}
performDraw();
...
performTraversals 这个方法的代码相当多，它里面分别去执行了 performMeasure、performLayout 以及 performDraw 三个方法，这三个方法内部分别调用了 View 的 measure、layout 以及 draw 对应的三个方法。
而接下来我们要说的 measure 过程是从 #performMeasure(childWidthMeasureSpec, childHeightMeasureSpec)方法开始的，performMeasure() 方法传递了两个参数到 View 的 measure 方法中，而 childWidthMeasureSpec 和 childHeightMeasureSpec 到底是什么呢？ 还是从源码中找找吧：
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
其实就是调用 MeasureSpec.makeMeasureSpec 生成的一个测量规则，而 MeasureSpec 是干嘛用的，接下来介绍：
理解 MeasureSpec
MeasureSpec 是 View 的一个内部类，官方文档是这么介绍 MeasureSpec，A MeasureSpec encapsulates the layout requirements passed from parent to child. Each MeasureSpec represents a requirement for either the width or the height. A MeasureSpec is comprised of a size and a mode. 大致意思是说，该类封装了从父 View 传递到子 View 的尺寸规格，包括了父 View 的宽和高的信息。说白了就是只要有了当前 View 的 MeasureSpec，也就等于确定了 View 的大小。
MeasureSpec 的作用在于：在 measure 过程中，子 View 的 MeasureSpec 是由 父 View 的 MeasureSpec 以及 子 View 的 LayoutParams 共同决定的（后面会给出证实）
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
/**
* UNSPECIFIED 模式：
* 父节点没有对子节点施加任何约束。它可以是任意大小
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
/**
* EXACTLY 模式
* 父 View 已经确定了子 View 的确切尺寸。也就是父 View 的剩余大小，该模式对应于 match_parent 和 具体数值这两种模式
*/
public static final int EXACTLY = 1 << MODE_SHIFT;
/**
* AT_MOST 模式：
* 最大模式,子 View 的最终大小是父 View 的剩余大小，最大不能超过父 View 剩余空间大小，该模式对应于 wrap_content
*/
public static final int AT_MOST = 2 << MODE_SHIFT
/**
*讲 mode 和 size 打包成一个 32 位的 int 型数值，高 2 位 表示测量模式，低 30 位表示 specSize
*/
public static int makeSafeMeasureSpec(int size, int mode) {
if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {
return 0;
}
return makeMeasureSpec(size, mode);
}
/**
* 将 32 位的 MeasureSpec 解包，返回当前 MeasureSpec 的测量模式
*/
@MeasureSpecMode
public static int getMode(int measureSpec) {
//noinspection ResourceType
return (measureSpec & MODE_MASK);
}
/**
* 讲 32 位 MeasureSpec 解包，获取当前 MeasureSpec 的 测量大小
*/
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
}
说完了 MeasureSpec，再回到 performTraversals 的 performMeasure 方法接收了 childWidthMeasureSpec 和 childHeightMeasureSpec 两个 MeasureSpec，而 这两个 MeasureSpec 其实就是根据屏幕的宽度以及高度生成的 MeasureSpec，既然都有了 MeasureSpec，那就开始测量呗！
performMeasure 方法
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
if (mView == null) {
return;
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
方法很简单，就是直接调用了 mView 的 measure(int widthMeasureSpec, int heightMeasureSpec) 方法， 而 mView 其实就是我们的顶级的 DecorView，也就是 ViewGroup，而 ViewGroup 又继承自 View，所以其实调用的还是 View 的 measure 方法，measure 方法内部：
View 的 measure 过程
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
...
}
}
如此简单，我们直接看 onMeasure 方法就可以了
onMeasure
先看下官方介绍：Measure the view and its content to determine the measured width and the measured height. This method is invoked by {@link #measure(int, int)} and should be overridden by subclasses to provide accurate and efficient measurement of their contents.
CONTRACT: When overriding this method, you must call {@link #setMeasuredDimension(int, int)} to store the measured width and height of this view. Failure to do so will trigger an IllegalStateException thrown by {@link #measure(int, int)}. Calling the superclass' {@link #onMeasure(int, int)} is a valid use.
大致意思是：该方法主要是用来确定 View 测量宽和测量高的，该方法应该被子类进行覆盖，以提供更加准确的测量值，并且当重写此方法时，必须调用 #setMeasuredDimension(int, int) 来存储该视图的测量宽度和高度，而且如果不这样做将会抛出 IllegalStateException 异常，建议我们调用父类的 #onMeasure(int, int) 方法
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
上面已经说过了 setMeasureDimension 方法是用来存储测量宽以及高的，来看一下吧：
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
ViewGroup 的 measure 过程
由于 ViewGroup 是 View 的子类，所以当 performMeasure 方法调用 View 的 onMeasure 方法时，首先会先去执行 DecorView（其实也就是 ViewGroup）的 onMeasure 方法，但是 ViewGroup 本身并没有实现 onMeasure、onLayout、onDraw这三个方法，因为不同的 ViewGroup 有不同的实现，比如 LinearLayout 和 RelativeLayout 测量过程和布局过程肯定是不一样的，所以 ViewGroup 的 onMeasure 其实是在其子类中实现的
我们来看一下 LinearLayout 的 onMeasure 的方法
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
if (mOrientation == VERTICAL) {
measureVertical(widthMeasureSpec, heightMeasureSpec);
} else {
measureHorizontal(widthMeasureSpec, heightMeasureSpec);
}
}
这里我们就只看 measureVertical 方法了 measureHorizontal 是一样的思路，来看下 LinearLayout 的 onMeasure 方法
LinearLayout#onMeasure
该方法代码并不算很长，就300行左右，但是如果仔细看的话会发现逻辑其实是很清晰的，一步一步来分析吧
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
//用来记录已经使用的高度
mTotalLength = 0;
int maxWidth = 0;
int childState = 0;
int alternativeMaxWidth = 0;
int weightedMaxWidth = 0;
boolean allFillParent = true;
//权重值得总和
float totalWeight = 0;
//获取子 View 的个数
final int count = getVirtualChildCount();
//LinearLayout 的 宽度模式以及高度模式
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) {
//取出子 View
final View child = getVirtualChildAt(i);
//如果子View为null，就跳过该View，测量下一个View
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
//如果view是隐藏的，就跳过该 View，测量下一个View
if (child.getVisibility() == View.GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
nonSkippedChildCount++;
//计算每个 view 的分割线的高度
if (hasDividerBeforeChildAt(i)) {
mTotalLength += mDividerHeight;
}
//获取当前子 View 的layoutParams，通过 LayoutParams来获取view的宽度、高度、weight、margin
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//记录总权重
totalWeight += lp.weight;
final boolean useExcessSpace = lp.height == 0 && lp.weight > 0;
//根据当前heightMode是EXACTLY，lp.height==0同时lp.weight>0在二次测量时会对符合条件的view进行二次测量
if (heightMode == MeasureSpec.EXACTLY && useExcessSpace) {
// Optimization: don't bother measuring children who are only
// laid out using excess space. These views will get measured
// later if we have space to distribute.
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
//这个标记在二次测量时会用到
skippedMeasure = true;
} else {
//如果这个判断成立，那么就说明 LinearLayout 的 heightMode一定是 UNSPECIFIED 或者 MOST，然后给 view 的height重新赋值，这样在后面测量时就可以得到view的最佳高度
if (useExcessSpace) {
// The heightMode is either UNSPECIFIED or AT_MOST, and
// this child is only laid out using excess space. Measure
// using WRAP_CONTENT so that we can find out the view's
// optimal height. We'll restore the original height of 0
// after measurement.
lp.height = LayoutParams.WRAP_CONTENT;
}
// Determine how big this child would like to be. If this or
// previous children have given a weight, then we allow it to
// use all available space (and we will shrink things later
// if needed).
//已使用的高度（当前view的toppadding+bottompadding+当前view的高度+分割线的高度 =LinearLayout的宽度
final int usedHeight = totalWeight == 0 ? mTotalLength : 0;
//内部调用ViewGroup的measureChildWithMargins方法测量当前view的大小，后面会单独介绍
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight);
//获取当前view 的高度
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;
//累计已经测量过的view的高度，从而获取剩余高度
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;
//根据子view的宽度计算当前view（LinearLayout的宽度）的宽度=子view的宽度+leftmargin+rightMargin，后面还会加上leftPadding以及rightPadding
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));
}
}
// Add in our padding
//当前View的最终高度
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
// Check against our minimum height
//当前View的最终高度
heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
// Reconcile our calculated size with the heightMeasureSpec
//heightSizeAndState会被传入setMeasuredDimension方法中用于保存，也就是作为当前view（LinearLayout的测量高度）
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.
//当前View的剩余高度
int remainingExcess = heightSize - mTotalLength
+ (mAllowInconsistentMeasurement ? 0 : consumedExcessSpace);
//如果有子view使用了weight属性，必进入该判断
if (skippedMeasure || remainingExcess != 0 && totalWeight > 0.0f) {
//weight总和
float remainingWeightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;
mTotalLength = 0;
//遍历子View
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
//如果子view为null，继续循环
if (child == null || child.getVisibility() == View.GONE) {
continue;
}
//获取子view的LayoutParams
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//获取当前子view的weight值
final float childWeight = lp.weight;
//只测量子view的weight大于0的view
if (childWeight > 0) {
//weight*剩余高度/总weightSum，得到当前子view应占剩余高度的比例值
final int share = (int) (childWeight * remainingExcess / remainingWeightSum);
//减去子view使用后的高度
remainingExcess -= share;
//减去子view使用的weight
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);
//这里测量的仅仅是指定了weight值的子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));
}
}
}
}
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);
}
}
代码中注释的已经很清楚了，接下来看看 measureChildBeforeLayout 方法
void measureChildBeforeLayout(View child, int childIndex,
int widthMeasureSpec, int totalWidth, int heightMeasureSpec,
int totalHeight) {
measureChildWithMargins(child, widthMeasureSpec, totalWidth,
heightMeasureSpec, totalHeight);
}
这个方法就比较简单了，其实就是根据前面获取的宽的MeasureSpec和高的MeasureSpec以及已经使用的宽度和已经使用的高度去计算子view
ViewGroup#measureChildWithMargins
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);
}
也很简单就是调用ViewGroup的getChildMeasureSpec方法
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
//剩余大小
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;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
其实根据这个方法我们可以得出一个结论：前面在介绍 MeasureSpec 时已经说过：
MeasureSpec 的作用在于：在 measure 过程中，子 View 的 MeasureSpec 是由 父 View 的 MeasureSpec 以及 子 View 的 LayoutParams 共同决定的
结论：
- 当子 View 的 LayoutParams 为 LayoutParams.MATCH_PARENT 时，子 View 的 MeasureSpec 模式为 父 View 的 MeasureSpec 模式（父 View 为 EXACTLY，子 View 也为 EXACTLY，父 View 为 AT_MOST，子 View 也为 AT_MOST），而 子 View 的大小就是 父 View 剩余空间的大小
- 当子 View 的 LayoutParams 为 LayoutParams.WRAP_CONTENT 时，不管父 View 的 MeasureSpec 模式是 EXACTLY（精准模式）还是 AT_MOST（最大模式），子 View 的 MeasureSpec 都为 AT_MOST（最大模式），子 View 的大小最大不得超过父 View 剩余空间的大小
到这里为止，也就非常清楚的知道 View 的 measure 过程到底是怎么执行的了
有几个小问题在这里记录一下
- 1、自定义 View 时需要自己处理 margin 以及 padding 么？
- 自定义 View 继承 ViewGroup 时，需要手动处理自身的 Padding 以及子 View 的margin
其实这个问题在上面已经解释的很清楚了，我们的 LinearLayout 就是继承自 ViewGroup，看源码时难道没有注意到其实已经对自身的 Padding 以及子 View 的 margin 做处理操作了么，不然为什么一直在就算剩余空间的大小呢？ 嗯哼 ？？？- 自定义 View 继承 View 时，需要手动处理自身 Padding
这个是必须手动处理的，因为父 View 仅仅处理了它自己的 padding 以及子 View 的 margin，并没有处理子 View 的 padding，如果继承 View 时没有对 padding 做处理操作，那么设置 padding 将没有任何效果，可以看 TextView 的 onMeasure 方法
- 2、自定义 View 时，当手动指定 view 的宽高为 wrap_content 时需要自己做处理么？
这个是必须手动进行处理的，因为父 View 在调用 getChildMeasureSpec 方法时，当 view 的 LayoutParams 为 wrap_content 时，view 的测量大小最大不得超过剩余空间的大小，但是并没有给出具体的大小（只给了一个上限值），如果不对 View 的 wrap_content 做出来，默认是采用 match_parent 的，也就是会占满剩余空间，那么后续的 View 将不能正常显示，具体处理可以参照 TextView 的 onMeasure 方法
完事......
