前置知识
- 有Android开发基础
- 了解 View 体系
前言
在青训营系列的文章中，我们用两篇文章讲解了 View体系 的知识。但是碍于时间和篇幅的限制，我们还未将该知识点完全打通，所以在这篇文章中，笔者将给大家继续讲述 View 的知识，本文使用 图+源码 的形式给大家讲解，希望这样子能讲得更加明白，大家也能有所收获。
阅读本系列，你可以学到在Android中， View 三大绘制方法 Measure 、Layout 和 Draw 的原理。
而今天这篇文章，我们讲解的是 Measure 方法的前置知识，View的MeasureSpec类。
何为 MeasureSpec
在学习三大方法之前，我们先来了解一下 View 的内部类，MeasureSpec 的作用和原理。
MeasureSpec 在 View 的测量(measure)过程中，担任着存储 View 的规格尺寸的作用。每次启动 Measure 方法的时候，我们都会需要传入对应的 MeasureSpec 参数才能执行。所以说，在 View 的绘制中，我们必须先获取每个 View 的 MeasureSpec 参数，才能执行 Measure 方法。
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 { } //1 /** * 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(); } } 复制代码
上述是 MeasureSpec 的源码，首先，我们看到了 MeasureSpecMode (标注1处) 这个注解定义，注解中表明了有三个枚举类。从这三个枚举类的常量定义中，我们发现他们使用了位计算，0，1，2 三个数全部左移 30 位，其二进制刚好就是int32位的高两位。可见，该类是想将32位二进制数的前两位赋予 MeasureSpecMode 。这三个常量称之为 specMode(测量规格模式)，占据了 32位int值的高两位；而其余低的30位，称之为 specSize(测量规格大小)。
这三种模式释义如下。
- UNSPECIFIED
未指定模式。View可以任意大，父布局不对其进行约束。多用于系统内部测量。- EXACTLY
精确模式。子View的大小已被父View确定，确定的值为是 specSize ，子View大小是被限制，无法被更改的。对应了match_parent和具体的数值。- AT_MOST
最大模式。子View可以任意大，最终可达到最终指定的specSize值，但是不能大于这个值。对应wrap_content属性。
然后，我们看到有 makeMeasureSpec、getMode 和 getSize 这些方法，他们的作用也正如方法名一样，分别是载入保存模式和大小，获得模式、获得大小。
DecorView 如何生成 MeasureSpec
我们知道，每一个View中都会持有自己的一个 MeasureSpec 信息。那在现在介绍完 MeasureSpec 类的构成之后，我们来看一下，顶层 View (DecorView) 是如何获得自己的 MeasureSpec 信息的。
我们看一下 View 工作流开启的方法 performTraversals() ，我们看到注释1和2处，可以看到是 getRootMeasureSpec 方法生成了对应的childWidthMeasureSpec 和 childHeightMeasureSpec 值，再载入到注释3的 performMeasure 中执行对应的 Measure 方法。
private void performTraversals() { ... // TODO: In the CL "ViewRootImpl: Fix issue with early draw report in // seamless rotation". We moved processing of RELAYOUT_RES_BLAST_SYNC // earlier in the function, potentially triggering a call to // reportNextDraw(). That same CL changed this and the next reference // to wasReportNextDraw, such that this logic would remain undisturbed // (it continues to operate as if the code was never moved). This was // done to achieve a more hermetic fix for S, but it's entirely // possible that checking the most recent value is actually more // correct here. if (!mStopped || mReportNextDraw) { if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight() || dispatchApplyInsets || updatedConfiguration) { int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width, lp.privateFlags);//1 int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height, lp.privateFlags);//2 if (DEBUG_LAYOUT) Log.v(mTag, "Ooops, something changed! mWidth=" + mWidth + " measuredWidth=" + host.getMeasuredWidth() + " mHeight=" + mHeight + " measuredHeight=" + host.getMeasuredHeight() + " dispatchApplyInsets=" + dispatchApplyInsets); // Ask host how big it wants to be performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);//3 ... } if (didLayout) { performLayout(lp, mWidth, mHeight); ... if (!performDraw() && mSyncBufferCallback != null) { mSyncBufferCallback.onBufferReady(null); } ... } } } 复制代码
那么， getRootMeasureSpec 方法又做了上什么呢？我们可以查看一下源码
private static int getRootMeasureSpec(int windowSize, int measurement, int privateFlags) { int measureSpec; final int rootDimension = (privateFlags & PRIVATE_FLAG_LAYOUT_SIZE_EXTENDED_BY_CUTOUT) != 0 ? MATCH_PARENT : measurement;//1 switch (rootDimension) { case ViewGroup.LayoutParams.MATCH_PARENT: // Window can't resize. Force root view to be windowSize. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY); break; case ViewGroup.LayoutParams.WRAP_CONTENT: // Window can resize. Set max size for root view. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST); break; default: // Window wants to be an exact size. Force root view to be that size. measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY); break; } return measureSpec; } 复制代码
我们发现 getRootMeasureSpec 这个函数传入了三个值。第一个是窗口大小，第二个是测量信息，第三个是私有标签。
在这段代码的注释1处，我们可以看到第二、第三个参数最后生成了 rootDimension ，这个其实就是DecorView 的 MeasureSpec 类里面的布局参数。由此可得，DecorView 的 MeasureSpec 类是由 DecorView 自身的布局参数和窗口大小决定的。
出于好奇心，我们可以再看一下 performMeasure 方法是执行了什么？我们可以看到，它是直接执行 View 的 measure 方法了。
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { if (mView == null) { return; } Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure"); try { mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);//执行View的measure方法 } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } } 复制代码
对应执行图
