Android 自定义控件基础：MeasureSpec

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*/@UnsupportedAppUsagepublic 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}*/@MeasureSpecModepublic static int getMode(int measureSpec) {//noinspection ResourceTypereturn (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 ");elsesb.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MeasureSpecpublic 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 uscase 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 uscase MeasureSpec.AT_MOST:if (childDimension >= 0) {// Child wants a specific size... so be itresultSize = 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 becase MeasureSpec.UNSPECIFIED:if (childDimension >= 0) {// Child wants a specific size... let him have itresultSize = childDimension;resultMode = MeasureSpec.EXACTLY;} else if (childDimension == LayoutParams.MATCH_PARENT) {// Child wants to be our size... find out how big it should// beresultSize = 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 beresultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;resultMode = MeasureSpec.UNSPECIFIED;}break;}//noinspection ResourceTypereturn MeasureSpec.makeMeasureSpec(resultSize, resultMode);}

对于顶级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。