图+源码，读懂View的MeasureSpec

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读懂 View 三大绘制方法的文章

图+源码，读懂View的MeasureSpec - 掘金 (juejin.cn)

图+源码，读懂View的Measure方法 - 掘金 (juejin.cn)

图+源码，读懂View的Layout方法 - 掘金 (juejin.cn)

图+源码，读懂View的Draw方法 - 掘金 (juejin.cn)

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();
    }
}

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);
                    }
                ...
            }
        }
}

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;
}

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);
    }
}