MeasureSpec是View的内部类，MeasureSpec 封装了从父级传递到子级的测量要求。每个 MeasureSpec 代表对宽度或高度的要求。 MeasureSpec 由大小和模式两部分组成。

更详细的说明请看 Google官方文档

下面是MeasureSpec的源码：（基于Android 12）

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public static class MeasureSpec {
    private static final int MODE_SHIFT = 30;
    private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

    public static final int UNSPECIFIED = 0 << MODE_SHIFT;

    public static final int EXACTLY     = 1 << MODE_SHIFT;

    public static final int AT_MOST     = 2 << MODE_SHIFT;

    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);
        }
    }

    public static int makeSafeMeasureSpec(int size, int mode) {
        if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {
            return 0;
        }
        return makeMeasureSpec(size, mode);
    }

    public static int getMode(int measureSpec) {
        return (measureSpec & MODE_MASK);
    }

    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) {
            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);
    }

    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();
    }
}

从源码可以看出，它代表了32位int值，其中高2位代表mode，低30位代表size。mode指测量模式，size指测量值。mode有3种模式：

  • UNSPECIFIED：未指定模式，父View没有对子View施加任何限制。它可以是任何它想要的大小。

  • EXACTLY：精确模式，父View已经确定了子View的确切尺寸。无论子View想要多大，都将获得这些界限。

  • AT_MOST：最大模式，子View可以根据需要达到指定的最大的大小。

makeMeasureSpec用来保存宽和高的信息，getMode用来获取测量模式，getSize用来获取测量大小。

MeasureSpec受自身LayoutParams和父View的MeasureSpec共同影响。作为顶层View的DecorView来说，其并没有父View，那它的MeasureSpec是如何得来的呢？我们回到ViewRootImpl的performTraversals方法：

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if (!mStopped || wasReportNextDraw) {
boolean focusChangedDueToTouchMode = ensureTouchModeLocally(
        (relayoutResult&WindowManagerGlobal.RELAYOUT_RES_IN_TOUCH_MODE) != 0);
if (focusChangedDueToTouchMode || mWidth != host.getMeasuredWidth()
        || mHeight != host.getMeasuredHeight() || dispatchApplyInsets ||
        updatedConfiguration) {
    int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);//1
    int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);

    if (DEBUG_LAYOUT) Log.v(mTag, "Ooops, something changed!  mWidth="
            + mWidth + " measuredWidth=" + host.getMeasuredWidth()
            + " mHeight=" + mHeight
            + " measuredHeight=" + host.getMeasuredHeight()
            + " dispatchApplyInsets=" + dispatchApplyInsets);

    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);//2

再看注释1处的getRootMeasureSpec方法：

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private static int getRootMeasureSpec(int windowSize, int rootDimension) {
    int measureSpec;
    switch (rootDimension) {

    case ViewGroup.LayoutParams.MATCH_PARENT:
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
        break;
    case ViewGroup.LayoutParams.WRAP_CONTENT:
        measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
        break;
    default:
        measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
        break;
    }
    return measureSpec;
}

第一个参数windowSize指窗口的尺寸，所以对于DecorView来说，它的MeasureSpec由自身的LayoutParams和窗口的尺寸决定，这一点和普通的View是不同的。
再回到上面看注释2处的performMeasure方法：

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private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
    if (mView == null) {
        return;
    }
    Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
    try {
        mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);//1
    } finally {
        Trace.traceEnd(Trace.TRACE_TAG_VIEW);
    }
}

注释1处调用了mView.measure方法，进入了view的measure流程。

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