View的绘制从ViewRootImpl#performTraversals方法开始，这篇主要分析View绘制流程中的measure部分。
measure用来测量View的宽和高，它的流程分为View的measure流程和ViewGroup的measure流程，而ViewGroup除了完成自己的测量过程外，还会遍历地调用子元素的measure方法，各个子元素再递归去执行这个流程。
private void performTraversals() {
...
WindowManager.LayoutParams lp = mWindowAttributes;
...
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);//1
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
performLayout(lp, mWidth, mHeight);
......
performDraw();
...
}
注释1表示获取测量DecorView所需的宽度的MeasureSpec。
查看getRootMeasureSpec方法
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;
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
此方法接收两个参数分别是窗口大小和自身的LayoutParams，从代码实现上可以知道DecorView的MeasureSpec的产生过程。具体来说遵守如下规则，根据它的LayoutParams中的宽高的参数来划分。
- LayoutParams.MATCH_PARENT:精确模式，大小就是窗口的大小
- LayoutParams.WRAP_CONTENT:最大模式，大小不定，但是不能超过窗口的大小
- 固定大小(比如100dp)：精确模式，大小为LayoutParams中指定的大小
回到performTraversals，在获取到DecorView宽高的测量规格后，调用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);
}
}
- 根据传入的宽高测量规格调用mView的measure方法。
- 这里的mView就是DecorView，而DecorView extends FrameLayout extends ViewGroup extends View。
- 此方法是属于view的final方法不可被子类重写；
查看measure方法：
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int oWidth = insets.left + insets.right;
int oHeight = insets.top + insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
// Suppress sign extension for the low bytes
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
/*
若mPrivateFlags中包含PFLAG_FORCE_LAYOUT标记，则强制重新布局
比如调用View.requestLayout()会在mPrivateFlags中加入此标记
*/
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// Optimize layout by avoiding an extra EXACTLY pass when the view is
// already measured as the correct size. In API 23 and below, this
// extra pass is required to make LinearLayout re-distribute weight.
/*
与上次相比测量规格是否改变
*/
final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
|| heightMeasureSpec != mOldHeightMeasureSpec;
final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
&& MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
&& getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
final boolean needsLayout = specChanged
&& (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
/*
需要重新布局
*/
if (forceLayout || needsLayout) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
/*
[如果forceLayout为true(直接忽略缓存) 或者 缓存中不存在都回返回-1] 或者 [sIgnoreMeasureCache为true]，都需要执行onMeasure重新测量
*/
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
/*
缓存命中，直接从缓存中取值即可，不必再测量
*/
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
throw new IllegalStateException("View with id " + getId() + ": "
+ getClass().getName() + "#onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
/*
保存本次View的测量规格用于下次判断
*/
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
measure方法主要作用是是否需要调用onMeasure方法，即是否进行实际的测量工作。
从measure()方法的源码中我们可以知道，只有以下两种情况之一，才会进行实际的测量工作：
- forceLayout为true：这表示强制重新布局，可以通过View.requestLayout()来实现；
- needsLayout为true，这需要specChanged为true，表示前后两次的MeasureSpec不一致，并且以下三个条件之一成立：
- sAlwaysRemeasureExactly为true: 该变量默认为false；
- isSpecExactly为false: 若父元素对子元素提出了精确的宽高约束，则该变量为true，否则为false
- matchesSpecSize为false: 表示父元素的宽高尺寸要求与上次测量的结果不同
前面说了当前视图是DecorView，DecorView对onMeasure进行了重写，查看DecorView#onMeasure
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
final DisplayMetrics metrics = getContext().getResources().getDisplayMetrics();
//是否是竖屏
final boolean isPortrait =
getResources().getConfiguration().orientation == ORIENTATION_PORTRAIT;
/*
获取宽高Mode,这里获取的widthMode和heightMode都是MeasureSpec.EXACTLY。
为什么？
因为从getRootMeasureSpec方法可知道，如果rootDimension是MATCH_PARENT或者具体数值，其得到的specMode是精确模式.
那么传进来的rootDimension到底是什么？
是WindowManager.LayoutParams对象的宽高值，而此对象的实例化后，它的width和height就是MATCH_PARENT，查阅源码发现并未对其进行修改。
*/
final int widthMode = getMode(widthMeasureSpec);
final int heightMode = getMode(heightMeasureSpec);
boolean fixedWidth = false;
mApplyFloatingHorizontalInsets = false;
/*
widthMode是MeasureSpec.EXACTLY，跳过
*/
// TODO: 什么情况下widthMode会等于AT_MOST，并执行相关代码? 待研究
if (widthMode == AT_MOST) {
final TypedValue tvw = isPortrait ? mWindow.mFixedWidthMinor : mWindow.mFixedWidthMajor;
if (tvw != null && tvw.type != TypedValue.TYPE_NULL) {
final int w;
if (tvw.type == TypedValue.TYPE_DIMENSION) {
w = (int) tvw.getDimension(metrics);
} else if (tvw.type == TypedValue.TYPE_FRACTION) {
w = (int) tvw.getFraction(metrics.widthPixels, metrics.widthPixels);
} else {
w = 0;
}
if (DEBUG_MEASURE) Log.d(mLogTag, "Fixed width: " + w);
final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
if (w > 0) {
widthMeasureSpec = MeasureSpec.makeMeasureSpec(
Math.min(w, widthSize), EXACTLY);
fixedWidth = true;
} else {
widthMeasureSpec = MeasureSpec.makeMeasureSpec(
widthSize - mFloatingInsets.left - mFloatingInsets.right,
AT_MOST);
mApplyFloatingHorizontalInsets = true;
}
}
}
mApplyFloatingVerticalInsets = false;
if (heightMode == AT_MOST) {
final TypedValue tvh = isPortrait ? mWindow.mFixedHeightMajor
: mWindow.mFixedHeightMinor;
if (tvh != null && tvh.type != TypedValue.TYPE_NULL) {
final int h;
if (tvh.type == TypedValue.TYPE_DIMENSION) {
h = (int) tvh.getDimension(metrics);
} else if (tvh.type == TypedValue.TYPE_FRACTION) {
h = (int) tvh.getFraction(metrics.heightPixels, metrics.heightPixels);
} else {
h = 0;
}
if (DEBUG_MEASURE) Log.d(mLogTag, "Fixed height: " + h);
final int heightSize = MeasureSpec.getSize(heightMeasureSpec);
if (h > 0) {
heightMeasureSpec = MeasureSpec.makeMeasureSpec(
Math.min(h, heightSize), EXACTLY);
} else if ((mWindow.getAttributes().flags & FLAG_LAYOUT_IN_SCREEN) == 0) {
heightMeasureSpec = MeasureSpec.makeMeasureSpec(
heightSize - mFloatingInsets.top - mFloatingInsets.bottom, AT_MOST);
mApplyFloatingVerticalInsets = true;
}
}
}
/*
获取开端
TODO: 待研究
*/
getOutsets(mOutsets);
if (mOutsets.top > 0 || mOutsets.bottom > 0) {
int mode = MeasureSpec.getMode(heightMeasureSpec);
if (mode != MeasureSpec.UNSPECIFIED) {
int height = MeasureSpec.getSize(heightMeasureSpec);
//重新计算高度测量规格
heightMeasureSpec = MeasureSpec.makeMeasureSpec(
height + mOutsets.top + mOutsets.bottom, mode);
}
}
if (mOutsets.left > 0 || mOutsets.right > 0) {
int mode = MeasureSpec.getMode(widthMeasureSpec);
if (mode != MeasureSpec.UNSPECIFIED) {
int width = MeasureSpec.getSize(widthMeasureSpec);
//重新计算宽度测量规格
widthMeasureSpec = MeasureSpec.makeMeasureSpec(
width + mOutsets.left + mOutsets.right, mode);
}
}
//调用父类的onMeasure方法，也就是FrameLayout的onMeasure
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int width = getMeasuredWidth();
boolean measure = false;
widthMeasureSpec = MeasureSpec.makeMeasureSpec(width, EXACTLY);
//widthMode是MeasureSpec.EXACTLY，跳过
if (!fixedWidth && widthMode == AT_MOST) {
final TypedValue tv = isPortrait ? mWindow.mMinWidthMinor : mWindow.mMinWidthMajor;
if (tv.type != TypedValue.TYPE_NULL) {
final int min;
if (tv.type == TypedValue.TYPE_DIMENSION) {
min = (int)tv.getDimension(metrics);
} else if (tv.type == TypedValue.TYPE_FRACTION) {
min = (int)tv.getFraction(mAvailableWidth, mAvailableWidth);
} else {
min = 0;
}
if (DEBUG_MEASURE) Log.d(mLogTag, "Adjust for min width: " + min + ", value::"
+ tv.coerceToString() + ", mAvailableWidth=" + mAvailableWidth);
if (width < min) {
widthMeasureSpec = MeasureSpec.makeMeasureSpec(min, EXACTLY);
measure = true;
}
}
}
// TODO: Support height?
// measure为false，跳过
if (measure) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
}
接着查看FrameLayout的onMeasure()方法。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//获取子元素的数量
int count = getChildCount();
/*
DecorView的测量模式都是精确模式，所以这里为false;
如果是普通视图，就要看具体请看了。
*/
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
/*
mMeasureAllChildren为true或者子元素不为GONE都会参与测量
通过setMeasureAllChildren可以，让子视图处在GONE状态下也可以被测量宽高，目前只发现在FrameLayout有这个处理。
*/
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//测量子元素的宽高
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
//获取子元素的布局参数
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//得到每个子元素中最大宽度，包括自身宽度加上leftMargin和rightMargin
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
//保存测量状态 TODO: 待研究
childState = combineMeasuredStates(childState, child.getMeasuredState());
//measureMatchParentChildren这里为false
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
//保存那些布局参数声明为MATCH_PARENT的子元素
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
//目前得到的最大宽度是，所有子元素最大宽度的那个
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
//目前得到的最大高度是，所有子元素最大高度的那个
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
//取当前的maxHeight和建议最小高度的最大值作为新的maxHeight
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
//如果设置了Foreground，取它们两得最大值
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
//根据上面计算得到的maxWidth和maxHeight以及测量规格和childState，设置自身的宽高
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
count = mMatchParentChildren.size();
//这块在DecorView流程中不走因为count=0
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
执行流程：
- 调用measureChildWithMargins()方法对所有子元素进行了测量，并计算出所有子元素的最大宽度和最大高度(包括margin)。
- 将得到的最大高度和宽度加上padding，这里的padding包括了父元素的padding和前景区域的padding。
- 检查是否设置了最小宽高，将两者中较大的设为最大宽高
- 检查是否设置了前景，将两者中较大的设为最终宽高
- 得到的最终宽高，表示当前视图用这个尺寸能正常显示其所有子元素，然后需要调用resolveSizeAndState方法来获取最终的测量宽高，并保存到mMeasuredWidth与mMeasuredHeight成员变量中。
总结：
FrameLayout通过measureChildWithMargins()方法对所有子元素进行测量后，才能得到自身的测量结果。
接着查看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);
}
上述方法会对子元素进行measure，在调用子元素的measure方法之前会先通过getChildMeasureSpec方法来得到子元素的MeasureSpec。从代码来看，很显然，子元素的MeasureSpec的创建与父容器的MeasureSpec和子元素的LayoutParams有关，此外还和View的margin及padding有关。
接着查看ViewGroup#getChildMeasureSpec
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
// 现在size的值为父容器相应方向上的可用大小
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
//生成子布局的测量规格
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
上述方法主要作用是根据父容器的MeasureSpec同时结合子元素本身的LayoutParam来确定子元素的MeasureSpec，参数中的padding是指父容器中已占用的空间大小，因此子元素可用的大小为父容器的尺寸减去padding。
ViewGroup.getChildMeasureSpec的工作原理对应的图例说明
在measureChildWithMargins()方法中，通过getChildMeasureSpec得到子元素MeasureSpec后，接下来就要调用child.measure()方法，并把获取到的childMeasureSpec传入。这时便又会调用onMeasure()方法，若此时的子View为ViewGroup的子类，便会调用相应容器类的onMeasure()方法，其他容器View的onMeasure()方法与FrameLayout的onMeasure()方法执行过程相似。
回到FrameLayout#onMeasure，当执行完所有子元素的测量工作后，会调用setMeasuredDimension方法保存 resolveSizeAndState方法的返回值，resolveSizeAndState方法根据前面的结果确定最终对FrameLayout的测量结果。
接着查看View#resolveSizeAndState
public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
final int specMode = MeasureSpec.getMode(measureSpec);
final int specSize = MeasureSpec.getSize(measureSpec);
final int result;
switch (specMode) {
case MeasureSpec.AT_MOST:
if (specSize < size) {
// 父元素给定的最大尺寸小于完全显示内容所需尺寸，
// 则在测量结果上加上MEASURED_STATE_TOO_SMALL
result = specSize | MEASURED_STATE_TOO_SMALL;
} else {
result = size;
}
break;
case MeasureSpec.EXACTLY:
// 若specMode为EXACTLY，则不考虑size，result直接赋值为specSize
result = specSize;
break;
case MeasureSpec.UNSPECIFIED:
default:
result = size;
}
return result | (childMeasuredState & MEASURED_STATE_MASK);
}
对比普通View(非ViewGroup)，会调用View#onMeasure方法来进行实际的测量工作。
接着查看View#onMeasure：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
onMeasure代码很简单，setMeasuredDimension主要作用是保存测量后的宽高。
接着查看View#getDefaultSize方法
此方法很显然根据不同的SpecMode值来返回不同的result值，即SpecSize。对于应用层级开发者来说，只需要看AT_MOST和EXACTLY这两种情况。在这两种情况下，都返回SpecSize，即View在这两种模式下的测量宽高直接取决于SpecSize。而这个SpecSize就是View测量后的大小。（View的最终的大小是在layout阶段确定的，但是在几乎所有情况下View的测量大小和最终大小时相等的）
至于UNSPECIFIED这种情况，一般用户系统内部的测量过程，在这种情况下，View的大小为getDefaultSize的第一个参数size，即宽/高分别为getSuggestedMinimumWidth和getSuggestedMinimumHeight这两个方法的返回值。
其中getSuggestedMinimumWidth源码是
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
分析源码，如果View没有设置背景，那么View的宽度为mMinWidth，即为android:minWidth属性所指定的值。这个属性如果不指定，那么mMinWidth则默认为0；如果View指定了背景，则View的宽度为max(mMinWidth，mBackground.getMininumWidth())。
那么mBackground.getMininumWidth()是什么呢？
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
分析源码，此方法返回的就是Drawable的原始宽度，前提是这个Drawable有原始宽度，否则就返回0。那么Drawable在什么情况下有原始宽度呢？比如，ShapeDrawable无原始宽高，而BitmapDrawable又有原始宽高（图片的尺寸）。
对getSuggestedMinimumWidth的逻辑总结：如果View没有设置背景，那么返回android:minWidth这个属性所指定的值，这个值可以为0；如果View设置了背景，则返回android:minWidth和背景的最小宽度这两者的最大值，getSuggestedMinimumWidth的返回值就是View在UNSPECIFIED情况下的测量宽高。