View中的MeasureSpec
View在测量过程中依赖于测量规则 MeasureSpec。
- DecorView 的MeasureSpec 取决于 Window的尺寸与 自身的LayoutParams
- 普通View的 MeasureSpec 取决于 父容器的尺寸 与自身的LayoutParams
MeasureSpec
View 中的静态内部类 MeasureSpec
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
public static final int EXACTLY = 1 << MODE_SHIFT;
public static final int AT_MOST = 2 << MODE_SHIFT;
public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
@MeasureSpecMode int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
@MeasureSpecMode
public static int getMode(int measureSpec) {
//noinspection ResourceType
return (measureSpec & MODE_MASK);
}
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
}
实质上 MeasureSpec 是一个32位int值，高2位代表 SpecMode，低30位代表 SpecSize。
SpecMode：测量模式；SpecSize：代表某种测量模式下的尺寸大小。
MeasureSpec通过将SpecMode 与 SpecSize打包成一个int值来避免过多的对象内存分配，为了方便操作提供的了 打包与解包的方法。
- makeMeasureSpec：打包
- getSize、getMode：解包
SpecMode
-
UNSPECIFIED
不指定测量模式。父容器不对View有任何限制，要多大有多大，这种情况一般用于系统内部，表示一种测量状态。开发过程中，几乎用不到。
-
EXACTLY
精确值模式。对应于LayoutParams中的 match_parent 与 具体的数值。父容器已经检测出View所需要的精确大小，这个时候View的最终大小就是SpecSize所指定的值。
-
AT_MOST
最大值模式。对应于LayoutParams中 wrap_content。父容器指定了一个可用的大小即SpecSize，View的大小不能大于这个值，具体是什么值要看不同View的具体实现。
MeasureSpec 与 LayoutParams 的关系
DecorView
ViewRootImpl#measureHierarchy
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
其中desiredWindowWidth，desiredWindowHeight是屏幕的尺寸。
ViewRootImpl#getRootMeasureSpec
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;
}
很简单，根据DecorView的LayoutParams的宽高参数来划分。
- 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：精确模式，大小为LayoutParams中指定的大小。
普通View
这里的普通View指的是布局中View
ViewGroup#measureChildWithMaigin
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);
}
上述方法最后会对子元素进行 measure，在此之前则会先通过getChildMeasureSpec 得到子元素的 MeasureSpec。很显然这里子元素的MeasureSpec与父容器的MeasureSpec以及自身的LayoutParams有关，此外还与View的margin，padding有关。
关于 getChildMeasureSpec 的源码在下面贴出，这里先写出分析结果。
其代码逻辑主要就是根据父容器的MeasureSpec 同时结合自身的LayoutParams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是父容器的尺寸减去padding，如下代码：
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
这里根据getChildMeasureSpec 的代码做出一个表：
普通View的MeasureSpec创建规则
| childLayoutParams\parentLayoutParams | EXACTLY | AT_MOST | UNSPECIFIED |
|---|---|---|---|
| dp/px | EXACTLY childSize |
EXACTLY childSize |
EXACTLY childSize |
| match_parent | EXACTLY parentSize |
AT_MOST parentSize |
UNSPECIFIED 0 |
| wrap_content | AT_MOST parentSize |
AT_MOST parentSize |
UNSPECIFIED 0 |
把代码逻辑简化成这个表之后，就显得很清晰了。
- View 固定宽高时，不管父容器的MeasureSpec 是什么，View 的MeasureSpec 都是精确模式，并且其大小取决于LayoutParams 中的大小
- View 是match_parent 时，其MeasureSpec与父容器保持一致；精确模式，则尺寸与父容器的剩余空间一致；最大模式，则尺寸最大不能超过父容器的剩余空间；
- View 是wrap_content 时，不管父容器的MeasureSpec，View的MeasureSpec都是最大模式，且最大不能超过父容器的剩余空间。
- UNSPECIFIED 模式，不需要关注此模式，开发中几乎用不到
源码如下：
ViewGroup#getChildMeasureSpec
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);
}
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