一、MeasureSpec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
*/
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();
}
}
二、解析
参数
MeasureSpec是View中的内部类，基本都是二进制运算。由于int是32位的，用高两位表示mode，低30位表示size，MODE_SHIFT = 30的作用是移位
MODE_MASK = 0x3 << MODE_SHIFT，其中0x3是十六进制，它的二进制是11，向左移位30，结果是11000.....0000（一共30个0），1与任何数“&运算”均为任何数，0与任何数“&运算”均为0，故它的作用是获取measureSpec的高两位的mode，或者低30位的size（MODE_MASK取返，00111...111）
UNSPECIFIED = 0 << MODE_SHIFT，十进制0的二进制是00，左移30位是00000...000（00+30个0）
EXACTLY = 1 << MODE_SHIFT，十进制1的二进制是01，左移30位是00000...000（01+30个0）
AT_MOST = 2 << MODE_SHIFT，十进制2的二进制是10，左移30位是00000...000（10+30个0）
方法
makeMeasureSpec将mode、size二进制运算，如size=4（二进制100），mode是EXACTLY（01000...000）,运算结果01000...100
getMode获取measureSpec 中的mode，与MODE_MASK进行“&运算”，如：10000...100与MODE_MASK（11000...000），结果是10000...000，获取mode
getSize获取measureSpec中的size，与MODE_MASK（00111...111）进行“&运算”，如：10000...100与MODE_MASK（00111...111），结果是00000...100，获取size
三、3种模式
UNSPECIFIED：不对View大小做限制，如：ListView，ScrollView
EXACTLY：确切的大小，如：100dp或者march_parent
AT_MOST：大小不可超过某数值，如：wrap_content
四、子View的MeasureSpec
子View的MeasureSpec由父View的MeasureSpec和子View本身的LayoutPramas共同决定，在ViewGroup的getChildMeasureSpec方法中实现，具体解析在下面代码中。
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
若父View是EXACTLY，则父View有确切数值或者march_parent
case MeasureSpec.EXACTLY:
若子View的childDimension大于0，则表示有确切数值，则子View大小为其本身且mode是EXACTLY
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
}
若子View的childDimension是MATCH_PARENT，则子View的大小为父View的大小且mode是EXACTLY
else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
}
若子View的childDimension是WRAP_CONTENT，则子View的大小为父View的大小且mode是AT_MOST，表示最大不可超过父View数值
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
若父View是AT_MOST，则父View一般是wrap_content，强给子View最大的值
case MeasureSpec.AT_MOST:
若子View的childDimension大于0，则表示有确切数值，则子View大小为其本身且mode是EXACTLY
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
}
若子View的childDimension是MATCH_PARENT，则子View的大小不超过父View的大小且mode是AT_MOST
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;
}
若子View的childDimension是WRAP_CONTENT，则子View的大小不超过父View的大小且mode是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
若父View是UNSPECIFIED，则父View不限制子View大小
case MeasureSpec.UNSPECIFIED:
若子View的childDimension大于0，则表示有确切数值，则子View大小为其本身且mode是EXACTLY
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
}
若子View的childDimension是MATCH_PARENT，因为父View是UNSPECIFIED，所以子View大小为0
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;
}
若子View的childDimension是WRAP_CONTENT，因为父View是UNSPECIFIED，所以子View大小为0
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);
}
总结一下：
不管父View是何模式，若子View有确切数值，则子View大小就是其本身大小，且mode是EXACTLY
若子View是match_parent，则模式与父View相同，且大小同父View（若父View是UNSPECIFIED，则子View大小为0）
若子View是wrap_content，则模式是AT_MOST，大小同父View，表示不可超过父View大小（若父View是UNSPECIFIED，则子View大小为0）
其实只要弄明白三种模式的含义，子View的模式及大小就明了了。
