前言
MeasureSpec 很大程度上决定了一个View尺寸规格，是因为这个过程还受父容器的影响，因为父容器影响View 的MeasureSpec的创建过程。在测量过程中，系统会将View的LayoutParams 根据父容器所施加的规则转换成对应的MeasureSpec,然后再根据这个MeasureSpec测量出View 的最终宽/高。
MeasureSpec 构建
MeasureSpec 代表了一个32位int值，高2位代表SpecMode(测量模式),低30位代表SpcSize（在某种测量模式下的规格大小）
源码：
public static class MeasureSpec{
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
/** @hide */
@IntDef({UNSPECIFIED, EXACTLY, AT_MOST})
@Retention(RetentionPolicy.SOURCE)
public @interface MeasureSpecMode {}
/**
* 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;
/**
* 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;
/**
* 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;
/**
* Creates a measure specification based on the supplied size and mode.
*
* The mode must always be one of the following:
* <ul>
* <li>{@link android.view.View.MeasureSpec#UNSPECIFIED}</li>
* <li>{@link android.view.View.MeasureSpec#EXACTLY}</li>
* <li>{@link android.view.View.MeasureSpec#AT_MOST}</li>
* </ul>
*
* <p><strong>Note:</strong> On API level 17 and lower, makeMeasureSpec's
* implementation was such that the order of arguments did not matter
* and overflow in either value could impact the resulting MeasureSpec.
* {@link android.widget.RelativeLayout} was affected by this bug.
* Apps targeting API levels greater than 17 will get the fixed, more strict
* behavior.</p>
*
* @param size the size of the measure specification
* @param mode the mode of the measure specification
* @return the measure specification based on size and mode
*/
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);
}
}
/**
* 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 internal use only for compatibility with system widgets and older apps
*/
@UnsupportedAppUsage
public static int makeSafeMeasureSpec(int size, int mode) {
if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {
return 0;
}
return makeMeasureSpec(size, mode);
}
/**
* Extracts the mode from the supplied measure specification.
*
* @param measureSpec the measure specification to extract the mode from
* @return {@link android.view.View.MeasureSpec#UNSPECIFIED},
* {@link android.view.View.MeasureSpec#AT_MOST} or
* {@link android.view.View.MeasureSpec#EXACTLY}
*/
@MeasureSpecMode
public static int getMode(int measureSpec) {
//noinspection ResourceType
return (measureSpec & MODE_MASK);
}
/**
* Extracts the size from the supplied measure specification.
*
* @param measureSpec the measure specification to extract the size from
* @return the size in pixels defined in the supplied measure specification
*/
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
static int adjust(int measureSpec, int delta) {
final int mode = getMode(measureSpec);
int size = getSize(measureSpec);
if (mode == UNSPECIFIED) {
// No need to adjust size for UNSPECIFIED mode.
return makeMeasureSpec(size, UNSPECIFIED);
}
size += delta;
if (size < 0) {
Log.e(VIEW_LOG_TAG, "MeasureSpec.adjust: new size would be negative! (" + size +
") spec: " + toString(measureSpec) + " delta: " + delta);
size = 0;
}
return makeMeasureSpec(size, mode);
}
/**
* Returns a String representation of the specified measure
* specification.
*
* @param measureSpec the measure specification to convert to a String
* @return a String with the following format: "MeasureSpec: MODE SIZE"
*/
public static String toString(int measureSpec) {
int mode = getMode(measureSpec);
int size = getSize(measureSpec);
StringBuilder sb = new StringBuilder("MeasureSpec: ");
if (mode == UNSPECIFIED)
sb.append("UNSPECIFIED ");
else if (mode == EXACTLY)
sb.append("EXACTLY ");
else if (mode == AT_MOST)
sb.append("AT_MOST ");
else
sb.append(mode).append(" ");
sb.append(size);
return sb.toString();
}
}
小结： MeasureSpec 通过将SpecMode 和SpecSize 打包成一个int值 来避免过多的对象内存分配。SpecMode 和SpecSize 是一个int 值，一组SpecMode 和SpecSize 可以打包为一个MeasureSpec,而一个MeasureSpec可以通过解包的形式来得出其原始的SpecMode 和SpecSize。
MeasureSpec 分类
SpecMode 有三类：
- UNSPECIFIED
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
- EXACTLY
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。它对应于LayoutParam 中的match_parent 和具体的数值这2种模式
- AT_MOST
父容器指定了一个可用大小即 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，它对应于LayoutParam 中的wrap_content。
MeasureSpec和LayoutParams 的对应关系
系统内部通过MeasureSpec来进行View 的测量，但是正常情况下使用View 指定 MeasureSpec。在View 测量的时候，系统会将LayoutParams在父容器的约束下转换成对应的MeasureSpec，然后再根据这个MeasureSpec 来确定View 测量后的宽/高。
注意 ：MeasureSpec不是唯一由LayoutParams决定，LayoutParams需要和父容器一起才能决定View的MeasureSpec，从而进一步决定View 的宽/高。
// ViewGroup源码中measureChild 为子View构造MeasureSpec 构建过程
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
......
// 该方法 根据父容器的MeasureSpec 同时结合子View本身的LayoutParams 来确定子元素的MeasureSpec
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
// 父容器大小
int specSize = MeasureSpec.getSize(spec);
// padding 父容器中已占用空间大小, size 为子元素可用空间大小
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);
}
对于*View(即DecorView)和普通View来说，MeasureSpec的转换过程不同
- 对于DecorView,其MeasureSpec由窗口的尺寸和其自身的LayoutParams来共同确定。
- 对于普通View, 其MeasureSpec由父容器的MeasureSpec和自身的LayoutParams来共同决定。
总结
- 当View 采用固定宽/高的时候，不管父容器MeasureSpec是什么，View的MeasureSpec都是精确模式并且大小遵循LayoutParams 中的大小。
- 当View 的宽/高是match_parent时，如果父容器的模式是精准模式，那么View也是精准模式并且其大小是父容器的剩余空间；如果父容器是最大模式，那么View也是最大模式并且其大小不会超过父容器的剩余空间。
- 当View 的宽/高是wrap_content时，不管父容器是的模式是精准还是最大化，View 的模式总是最大化并且大小不会超过父容器的剩余空间。
- UNSPECIFD模式主要用于系统内部多次Measure 的情形。