#Android源码#MeasureSpec

分析源码得出

MeasureSpec类是View的一个静态内部类。

MeasureSpec是parent到child的布局要求的封装。每个MeasureSpec代表了一个宽度或高度的请求。MeasureSpec由size和mode组成。

有三种mode：

UNSPECIFIED: parent没有对child施加任何约束，child可以设置任意size

EXACTLY: parent指定child的size

AT_MOST: child的size在parent指定的size范围内

为了减少对象的分配，MeasureSpec使用int值（32位）来实现，前2位表示mode，后30位表示size。

调用

makeMeasureSpec(int size, int mode)

来产生该int值。

将该值代入

getMode(int measureSpec)

中获取mode，代入

getSize(int measureSpec)

中获取size。

代入

adjust(int measureSpec, int delta)

方法，调整size。

源码Copy如下

/**
* A MeasureSpec encapsulates the layout requirements passed from parent to child.
* Each MeasureSpec represents a requirement for either the width or the height.
* A MeasureSpec is comprised of a size and a mode. There are three possible
* modes:
* <dl>
* <dt>UNSPECIFIED</dt>
* <dd>
* The parent has not imposed any constraint on the child. It can be whatever size
* it wants.
* </dd>
*
* <dt>EXACTLY</dt>
* <dd>
* 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.
* </dd>
*
* <dt>AT_MOST</dt>
* <dd>
* The child can be as large as it wants up to the specified size.
* </dd>
* </dl>
*
* MeasureSpecs are implemented as ints to reduce object allocation. This class
* is provided to pack and unpack the <size, mode> tuple into the int.
*/
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

/**
* 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(int size, 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}
*/
public static int getMode(int measureSpec) {
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();
}
}