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 */ @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。
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