一、MeasureSpec源碼
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();
}
}
二盆昙、解析
參數(shù)
MeasureSpec是View中的內(nèi)部類(lèi)，基本都是二進(jìn)制運(yùn)算焊虏。由于int是32位的淡喜，用高兩位表示mode，低30位表示size炕淮，MODE_SHIFT = 30的作用是移位
MODE_MASK = 0x3 << MODE_SHIFT拆火，其中0x3是十六進(jìn)制跳夭，它的二進(jìn)制是11涂圆，向左移位30，結(jié)果是11000.....0000（一共30個(gè)0）币叹，1與任何數(shù)“&運(yùn)算”均為任何數(shù)润歉，0與任何數(shù)“&運(yùn)算”均為0，故它的作用是獲取measureSpec的高兩位的mode颈抚，或者低30位的size（MODE_MASK取返踩衩，00111...111）
UNSPECIFIED = 0 << MODE_SHIFT，十進(jìn)制0的二進(jìn)制是00贩汉，左移30位是00000...000（00+30個(gè)0）
EXACTLY = 1 << MODE_SHIFT驱富，十進(jìn)制1的二進(jìn)制是01，左移30位是00000...000（01+30個(gè)0）
AT_MOST = 2 << MODE_SHIFT匹舞，十進(jìn)制2的二進(jìn)制是10褐鸥，左移30位是00000...000（10+30個(gè)0）
方法
makeMeasureSpec將mode、size二進(jìn)制運(yùn)算赐稽，如size=4（二進(jìn)制100）叫榕，mode是EXACTLY（01000...000）,運(yùn)算結(jié)果01000...100
getMode獲取measureSpec 中的mode浑侥，與MODE_MASK進(jìn)行“&運(yùn)算”，如：10000...100與MODE_MASK（11000...000）晰绎，結(jié)果是10000...000寓落，獲取mode
getSize獲取measureSpec中的size，與MODE_MASK（00111...111）進(jìn)行“&運(yùn)算”荞下，如：10000...100與MODE_MASK（00111...111）伶选，結(jié)果是00000...100，獲取size
三尖昏、3種模式
UNSPECIFIED：不對(duì)View大小做限制考蕾，如：ListView，ScrollView
EXACTLY：確切的大小会宪，如：100dp或者march_parent
AT_MOST：大小不可超過(guò)某數(shù)值肖卧，如：wrap_content
四、子View的MeasureSpec
子View的MeasureSpec由父View的MeasureSpec和子View本身的LayoutPramas共同決定掸鹅，在ViewGroup的getChildMeasureSpec方法中實(shí)現(xiàn)塞帐，具體解析在下面代碼中。
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巍沙，則父View有確切數(shù)值或者march_parent
case MeasureSpec.EXACTLY:
若子View的childDimension大于0葵姥，則表示有確切數(shù)值，則子View大小為其本身且mode是EXACTLY
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
}
若子View的childDimension是MATCH_PARENT句携，則子View的大小為父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榔幸，則子View的大小為父View的大小且mode是AT_MOST，表示最大不可超過(guò)父View數(shù)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矮嫉，則父View一般是wrap_content削咆，強(qiáng)給子View最大的值
case MeasureSpec.AT_MOST:
若子View的childDimension大于0，則表示有確切數(shù)值蠢笋，則子View大小為其本身且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拨齐，則子View的大小不超過(guò)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，則子View的大小不超過(guò)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昨寞，則父View不限制子View大小
case MeasureSpec.UNSPECIFIED:
若子View的childDimension大于0瞻惋，則表示有確切數(shù)值，則子View大小為其本身且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援岩，因?yàn)楦竀iew是UNSPECIFIED歼狼，所以子View大小為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?yàn)楦竀iew是UNSPECIFIED享怀，所以子View大小為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);
}
總結(jié)一下：
不管父View是何模式羽峰，若子View有確切數(shù)值，則子View大小就是其本身大小，且mode是EXACTLY
若子View是match_parent限寞，則模式與父View相同忍啸，且大小同父View（若父View是UNSPECIFIED，則子View大小為0）
若子View是wrap_content履植，則模式是AT_MOST计雌，大小同父View，表示不可超過(guò)父View大忻钓（若父View是UNSPECIFIED凿滤，則子View大小為0）
其實(shí)只要弄明白三種模式的含義，子View的模式及大小就明了了庶近。
