深入理解MeasureSpec
深入理解MeasureSpec
在View的绘制流程一文中提及到MeasureSpec，但是并没有涉及太多，只是一带而过。MeasureSpec是View测量过程中重要的测量依据，这一篇中阅读MeasureSpec源码理解其作用。
1、MeasureSpec介绍
MeasureSpec完整的名称是measure specification，直译为中文叫测量规格。
1.1、测量模式掩码
在了解View的三种测量模式之前，先弄明白MeasureSpec中两个常量的含义：
private static final int MODE_SHIFT = 30; private static final int MODE_MASK = 0x3 << MODE_SHIFT;
MeasureSpec中定义了一个静态常量MODE_MASK，用来从 int 值中提取出测量模式：mode。十六进制0x3，也就是0011，左移30位。使用位移操作因为位移计算在计算机中是非常高效的。
后面计算还会用到MODE_MASK的反码，用来提取出int中存储的View大小：size。
1.2、三种测量模式
一个View的MeasureSpec由大小和模式组成，View共有三种可能的模式：
①UNSPECIFIED：未确定模式。父View没有对子view施加任何约束，子view可以是任何大小。
/** * Measure specification mode: The parent has not imposed any constraint * on the child. It can be whatever size it wants. */ public static final int UNSPECIFIED = 0 << MODE_SHIFT;
②EXACTLY：精确模式。父View已经为子View确定了确切的尺寸， 不管子View要多大，都会给子View这个值。
/** * Measure specification mode: The parent has determined an exact size * for the child. The child is going to be given those bounds regardless * of how big it wants to be. */ public static final int EXACTLY = 1 << MODE_SHIFT;
③AT_MOST：最大模式。子View可以根据需要的大小而定，最大可以达到MeasureSpec中指定的size大小。
/** * Measure specification mode: The child can be as large as it wants up * to the specified size. */ public static final int AT_MOST = 2 << MODE_SHIFT;
1.3、构造MeasureSpec
MeasureSpec提供了组装MeasureSpec值的静态方法makeMeasureSpec(int size,int mode)，传入测量模式mode和View的大小size，通过该方法将mode和size包装在一个int中。
之所以这样做，最主要的原因还是节省内存，避免内存的频繁分配消耗。因为View的测量过程是及其频繁和复杂的，只用一个int使用非常少的内存即可保存和传递View的大小信息。
public static int makeMeasureSpec(int size,int mode) {
if (sUseBrokenMakeMeasureSpec) {
//早期API ≤ 17
return size + mode;
} else {
//高2位保存mode，低30位保存size
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}对于早期应用内部会通过makeSafeMeasureSpec(int size, int mode)方法计算MeasureSpec。
/**
* Like {@link #makeMeasureSpec(int, int)}, but any spec with a mode of UNSPECIFIED
* will automatically get a size of 0. Older apps expect this.
*
* @hide 仅供内部使用，以与系统小部件和较旧的应用程序兼容
*/
@UnsupportedAppUsage
public static int makeSafeMeasureSpec(int size, int mode) {
if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {
return 0;
}
return makeMeasureSpec(size, mode);
}至于多早呢，从View的源码中可以找到答案：在API 17及以前的View如果传入的mode为UNSPECIFIED，那么View的大小直接默认为0。
// Older apps may need this compatibility hack for measurement. sUseBrokenMakeMeasureSpec = targetSdkVersion <= Build.VERSION_CODES.JELLY_BEAN_MR1;
1.4、提取size和mode
MeasureSpec提供了getMode(int measureSpec)方法从包含size和mode的int值中提取出View的测量模式mode。
@MeasureSpecMode
public static int getMode(int measureSpec) {
//noinspection ResourceType
return (measureSpec & MODE_MASK);
}同样，对int低30位用反码即可获得保存的View的size值。
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}最后，MeasureSpec中还有一个adjust(int measureSpec, int delta)方法，用来对MeasureSpec值进行校准，防止size的大小为负。
static int adjust(int measureSpec, int delta) {
//先提取出mode和size
final int mode = getMode(measureSpec);
int size = getSize(measureSpec);
//如果mode是未指定，则不校准
if (mode == UNSPECIFIED) {
// No need to adjust size for UNSPECIFIED mode.
return makeMeasureSpec(size, UNSPECIFIED);
}
//将size大小加上校准值delta，但是不能为负，最小也只能为0.
size += delta;
if (size < 0) {
size = 0;
}
return makeMeasureSpec(size, mode);
}2、MeasureSpec的初始化
View中的MeasureSpec是如何初始化的呢？回顾一下View的绘制流程一文中1.3节的内容。
2.1、根View的MeasureSpec初始化
在ViewRootImpl的performTraversals()方法中开启View的绘制流程：measure -> layout -> draw。只关注测量相关的代码：
private void performTraversals() {
...
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
}通过getRootMeasureSpec(int windowSize, int rootDimension)方法获取根MeasureSpec的值：
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;
}ViewRootImpl中的mWindowAttributes成员变量默认即是WindowManager.LayoutParams类型，这也是根View所使用的属性。
public final WindowManager.LayoutParams mWindowAttributes = new WindowManager.LayoutParams();
而WindowManager.LayoutParams的构造方法，默认Window的宽高都是MATCH_PARENT：
public LayoutParams() {
super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
type = TYPE_APPLICATION;
format = PixelFormat.OPAQUE;
}回到getRootMeasureSpec(int windowSize, int rootDimension)方法中，走第一个case：
... measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY) ...
所以根View的大小就是Window窗口的大小，而且是精确值。
2.2、子View的MeasureSpec初始化
以最常见的容器FrameLayout的onMeasure()方法为例，从根View或者其它View层层传递过来带有mode和size的MeasureSpec值：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
...
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
...
}
...
}在测量子View之前，先遍历一遍所有子View：通过ViewGroup的measureChildWithMargins(...)方法预先得到子View的MeasureSpec。
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//计算子View宽、高的MeasureSpec
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);
//再测量子View
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}最后通过ViewGroup中的getChildMeasureSpec(spec, padding, childDimension)方法计算出子View的MeasureSpec。完整方法如下：
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
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);
}上面六十多行代码主要是是case、if判断，逻辑简化成下表：
至此，把View测量中的一个重要概念MeasureSpec搞清楚了。
记得以前开发的时候用ScrollView嵌套ListView，列表始终显示不全。当时刚入门，不知道怎么搞(〃'▽'〃)。其实是因为MeasureSpec为UNSPECIFIED，无法确定列表具体高度导致。
相关资料：
Android Developers > Docs > MeasureSpec
安卓开发之ScrollView嵌套ListView的一些问题和解决
