Android measure过程分析

　　作为一名Android开发人员，我们都知道一个View从无到有，会经历3个阶段：

1. measure/测量阶段，也就是确定某个view大小的过程；

2. layout/布局阶段，也就是确定其左上右下坐标的过程；

3. draw/绘制阶段，也就是按照前面2步计算的结果，将view绘制在屏幕相应的位置上；

今天，我带领大家来看看View系统的measure过程。到现在相信大部分人都知道measure是从ViewRootImpl.measureHierarchy

方法开始的，但归根结底是从performTraversals开始的。

　　为了从一开始就清楚onMeasure(int widthMeasureSpec, int heightMeasureSpec)的这2个参数从哪来的，虽然我们都知道

这2个参数表示parent施加给我们的约束，但可能大部分人不明白程序run起来的时候这些值都是从哪里来的。为了弄清楚这个问题，

我们还得从上面ViewRootImpl的measureHierarchy说起，来看其源码：

    private boolean measureHierarchy(final View host, final WindowManager.LayoutParams lp,
            final Resources res, final int desiredWindowWidth, final int desiredWindowHeight) {
        int childWidthMeasureSpec;
        int childHeightMeasureSpec;
        boolean windowSizeMayChange = false;

        if (DEBUG_ORIENTATION || DEBUG_LAYOUT) Log.v(TAG,
                "Measuring " + host + " in display " + desiredWindowWidth
                + "x" + desiredWindowHeight + "...");

        boolean goodMeasure = false;
        if (lp.width == ViewGroup.LayoutParams.WRAP_CONTENT) { // 在WRAP_CONTENT的情况下，先从一个prefered值开始measure
            // On large screens, we don't want to allow dialogs to just
            // stretch to fill the entire width of the screen to display
            // one line of text.  First try doing the layout at a smaller
            // size to see if it will fit.
            final DisplayMetrics packageMetrics = res.getDisplayMetrics();
            res.getValue(com.android.internal.R.dimen.config_prefDialogWidth, mTmpValue, true);
            int baseSize = 0;
            if (mTmpValue.type == TypedValue.TYPE_DIMENSION) {
                baseSize = (int)mTmpValue.getDimension(packageMetrics);
            }
            if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": baseSize=" + baseSize);
            if (baseSize != 0 && desiredWindowWidth > baseSize) { // 如果baseSize真小的话，用baseSize先measure一遍试试
                childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
                childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
                performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("
                        + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");
                if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
                    goodMeasure = true; // measure的结果合适
                } else {
                    // Didn't fit in that size... try expanding a bit.
                    baseSize = (baseSize+desiredWindowWidth)/2; // 加大baseSize重新执行上述过程
                    if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": next baseSize="
                            + baseSize);
                    childWidthMeasureSpec = getRootMeasureSpec(baseSize, lp.width);
                    performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
                    if (DEBUG_DIALOG) Log.v(TAG, "Window " + mView + ": measured ("
                            + host.getMeasuredWidth() + "," + host.getMeasuredHeight() + ")");
                    if ((host.getMeasuredWidthAndState()&View.MEASURED_STATE_TOO_SMALL) == 0) {
                        if (DEBUG_DIALOG) Log.v(TAG, "Good!");
                        goodMeasure = true;
                    }
                }
            }
        }

        if (!goodMeasure) { // 如果用baseSize measure的结果不合适，则老老实实用提供的参数重新measure一遍
            childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
            childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
            performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
            if (mWidth != host.getMeasuredWidth() || mHeight != host.getMeasuredHeight()) {
                windowSizeMayChange = true;
            }
        }

        if (DBG) {
            System.out.println("======================================");
            System.out.println("performTraversals -- after measure");
            host.debug();
        }

        return windowSizeMayChange;
    }

    /**
     * Figures out the measure spec for the root view in a window based on it's
     * layout params.
     *
     * @param windowSize
     *            The available width or height of the window
     *
     * @param rootDimension
     *            The layout params for one dimension (width or height) of the
     *            window.
     *
     * @return The measure spec to use to measure the root view.
     */
    private static int getRootMeasureSpec(int windowSize, int rootDimension) { // 通过具体的windowSize和提供的spec
        int measureSpec;                                                       // 构造一个合适的Root MeasureSpec
        switch (rootDimension) {  // 这里的windowSize就是设备的宽、高，rootDimension就是xml文件里指定的layoutparam

        case ViewGroup.LayoutParams.MATCH_PARENT:
            // Window can't resize. Force root view to be windowSize. 设置root view就是window这么大
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
            break;
        case ViewGroup.LayoutParams.WRAP_CONTENT:
            // Window can resize. Set max size for root view. 设置root view最多是window这么大
            measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
            break;
        default:
            // Window wants to be an exact size. Force root view to be that size. 某一个具体的大小
            measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
            break;
        }
        return measureSpec;
    }

    private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
        Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
        try {
            // 调用root view的measure方法，从此进入到view层次结构，顺便也把MeasureSpec带了进去。。。
            mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

　　从之前的文章中，我们知道DecorView实际上是继承至FrameLayout，由于它和ViewGroup都没有重载这个方法，实际上也没法重载，

因为这个方法是View的一个final方法，代码如下：

/**
     * <p>
     * This is called to find out how big a view should be. The parent
     * supplies constraint information in the width and height parameters.
     * </p>
     *
     * <p>
     * The actual measurement work of a view is performed in
     * {@link #onMeasure(int, int)}, called by this method. Therefore, only
     * {@link #onMeasure(int, int)} can and must be overridden by subclasses.
     * </p>
     *
     *
     * @param widthMeasureSpec Horizontal space requirements as imposed by the
     *        parent
     * @param heightMeasureSpec Vertical space requirements as imposed by the
     *        parent
     *
     * @see #onMeasure(int, int)
     */
    public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int oWidth  = insets.left + insets.right;
            int oHeight = insets.top  + insets.bottom;
            widthMeasureSpec  = MeasureSpec.adjust(widthMeasureSpec,  optical ? -oWidth  : oWidth);
            heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
        }

        // Suppress sign extension for the low bytes
        long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
        if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);

        if ((mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ||
                widthMeasureSpec != mOldWidthMeasureSpec ||
                heightMeasureSpec != mOldHeightMeasureSpec) {

            // first clears the measured dimension flag
            mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;

            resolveRtlPropertiesIfNeeded();

            int cacheIndex = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT ? -1 :
                    mMeasureCache.indexOfKey(key);
            if (cacheIndex < 0 || sIgnoreMeasureCache) {
                // measure ourselves, this should set the measured dimension flag back
                onMeasure(widthMeasureSpec, heightMeasureSpec); // 注意这个调用，这个方法是本文的重点
                mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            } else {
                long value = mMeasureCache.valueAt(cacheIndex);
                // Casting a long to int drops the high 32 bits, no mask needed
                setMeasuredDimension((int) (value >> 32), (int) value);
                mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
            }

            // flag not set, setMeasuredDimension() was not invoked, we raise
            // an exception to warn the developer
            if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
                throw new IllegalStateException("onMeasure() did not set the"
                        + " measured dimension by calling"
                        + " setMeasuredDimension()");
            }

            mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
        }

        mOldWidthMeasureSpec = widthMeasureSpec;
        mOldHeightMeasureSpec = heightMeasureSpec;

        mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
                (long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
    }

正如方法doc说的那样，真正的measure过程是发生在onMeasure方法中的，所以你可以也应该override这个方法，我们紧接着看看View中

的默认实现，代码如下：

    /**
     * <p>
     * Measure the view and its content to determine the measured width and the
     * measured height. This method is invoked by {@link #measure(int, int)} and
     * should be overriden by subclasses to provide accurate and efficient
     * measurement of their contents.
     * </p>
     *
     * <p>
     * <strong>CONTRACT:</strong> When overriding this method, you
     * <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the
     * measured width and height of this view. Failure to do so will trigger an
     * <code>IllegalStateException</code>, thrown by
     * {@link #measure(int, int)}. Calling the superclass'
     * {@link #onMeasure(int, int)} is a valid use.
     * </p>
     *
     * <p>
     * The base class implementation of measure defaults to the background size,
     * unless a larger size is allowed by the MeasureSpec. Subclasses should
     * override {@link #onMeasure(int, int)} to provide better measurements of
     * their content.
     * </p>
     *
     * <p>
     * If this method is overridden, it is the subclass's responsibility to make
     * sure the measured height and width are at least the view's minimum height
     * and width ({@link #getSuggestedMinimumHeight()} and
     * {@link #getSuggestedMinimumWidth()}).
     * </p>
     *
     * @param widthMeasureSpec horizontal space requirements as imposed by the parent.
     *                         The requirements are encoded with
     *                         {@link android.view.View.MeasureSpec}.
     * @param heightMeasureSpec vertical space requirements as imposed by the parent.
     *                         The requirements are encoded with
     *                         {@link android.view.View.MeasureSpec}.
     *
     * @see #getMeasuredWidth()
     * @see #getMeasuredHeight()
     * @see #setMeasuredDimension(int, int)
     * @see #getSuggestedMinimumHeight()
     * @see #getSuggestedMinimumWidth()
     * @see android.view.View.MeasureSpec#getMode(int)
     * @see android.view.View.MeasureSpec#getSize(int)
     */
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
                getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
    }

    /**
     * <p>This method must be called by {@link #onMeasure(int, int)} to store the
     * measured width and measured height. Failing to do so will trigger an
     * exception at measurement time.</p>
     *
     * @param measuredWidth The measured width of this view.  May be a complex
     * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
     * {@link #MEASURED_STATE_TOO_SMALL}.
     * @param measuredHeight The measured height of this view.  May be a complex
     * bit mask as defined by {@link #MEASURED_SIZE_MASK} and
     * {@link #MEASURED_STATE_TOO_SMALL}.
     */
    protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
        boolean optical = isLayoutModeOptical(this);
        if (optical != isLayoutModeOptical(mParent)) {
            Insets insets = getOpticalInsets();
            int opticalWidth  = insets.left + insets.right;
            int opticalHeight = insets.top  + insets.bottom;

            measuredWidth  += optical ? opticalWidth  : -opticalWidth;
            measuredHeight += optical ? opticalHeight : -opticalHeight;
        }
        mMeasuredWidth = measuredWidth; // 赋值
        mMeasuredHeight = measuredHeight;

        mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET; // 设置标志位
    }

    /**
     * Returns the suggested minimum height that the view should use. This
     * returns the maximum of the view's minimum height
     * and the background's minimum height
     * ({@link android.graphics.drawable.Drawable#getMinimumHeight()}).
     * <p>
     * When being used in {@link #onMeasure(int, int)}, the caller should still
     * ensure the returned height is within the requirements of the parent.
     *
     * @return The suggested minimum height of the view.
     */
    protected int getSuggestedMinimumHeight() {// 找到view的最小大小，没background的时候返回mMinHeight，这个你可以在xml中指定
        return (mBackground == null) ? mMinHeight : max(mMinHeight, mBackground.getMinimumHeight());
                                               // 否则返回mMinHeight和background的最小值里的较大者
    }

    /**
     * Returns the suggested minimum width that the view should use. This
     * returns the maximum of the view's minimum width)
     * and the background's minimum width
     *  ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).
     * <p>
     * When being used in {@link #onMeasure(int, int)}, the caller should still
     * ensure the returned width is within the requirements of the parent.
     *
     * @return The suggested minimum width of the view.
     */
    protected int getSuggestedMinimumWidth() {
        return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
    }

    /**
     * Utility to return a default size. Uses the supplied size if the
     * MeasureSpec imposed no constraints. Will get larger if allowed
     * by the MeasureSpec.
     *
     * @param size Default size for this view
     * @param measureSpec Constraints imposed by the parent
     * @return The size this view should be.
     */
    public static int getDefaultSize(int size, int measureSpec) {
        int result = size;
        int specMode = MeasureSpec.getMode(measureSpec);
        int specSize = MeasureSpec.getSize(measureSpec);

        switch (specMode) {
        case MeasureSpec.UNSPECIFIED: // parent imposed的spec没指定，则用自己的值default size
            result = size;
            break;
        case MeasureSpec.AT_MOST:
        case MeasureSpec.EXACTLY: // 否则不论是精确指定或是至多，都用spec提供的值
            result = specSize;
            break;
        }
        return result;
    }

我们看到View.onMeasure方法只是提供了一个通用的、一般的实现，子类一般需要重载它，自己提供更加合理、高效的实现，最重要的是

符合你的需求。同时我们也看到ViewGroup并没有提供它自己的实现，但是提供了一些在measure过程中很有用的方法，其特定子类如

FrameLayout、LinearLayout等在measure过程中都需要用到的。

　　为了更进一步看看这个过程，我们这里以FrameLayout的onMeasure为例分析下，看其源码：

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();
        // matchparent child标记
        final boolean measureMatchParentChildren =
                MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
                MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
        mMatchParentChildren.clear(); // 清空列表

        int maxHeight = 0;
        int maxWidth = 0;
        int childState = 0;
        // 遍历children，初步找出maxHeight和maxWidth，顺便构造mMatchParentChildren列表
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            // 默认情况下，只measure非GONE的child，但你可以设置mMeasureAllChildren来打破这一限制
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                // 调用parent，ViewGroup提供的方法。。。
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                // 我们平时在xml文件中设置的android:layout_xxx其实就是这里的LayoutParams
                final LayoutParams lp = (LayoutParams) child.getLayoutParams();
                maxWidth = Math.max(maxWidth,
                        child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
                maxHeight = Math.max(maxHeight,
                        child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
                childState = combineMeasuredStates(childState, child.getMeasuredState());
                if (measureMatchParentChildren) {
                    if (lp.width == LayoutParams.MATCH_PARENT ||
                            lp.height == LayoutParams.MATCH_PARENT) {
                        mMatchParentChildren.add(child); // 添加matchparent child
                    }
                }
            }
        }
        // 进一步调整maxWidth、maxHeight的值，考虑foreground padding、minimum height/width，还有foreground的最小值
        // Account for padding too
        maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
        maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();

        // Check against our minimum height and width
        maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
        maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());

        // Check against our foreground's minimum height and width
        final Drawable drawable = getForeground();
        if (drawable != null) {
            maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
            maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
        }
        // 设置FrameLayout自身的measuredWidth、measuredHeight
        setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
                resolveSizeAndState(maxHeight, heightMeasureSpec,
                        childState << MEASURED_HEIGHT_STATE_SHIFT));

        count = mMatchParentChildren.size();
        if (count > 1) { // 如果有matchparent child的话
            for (int i = 0; i < count; i++) {
                final View child = mMatchParentChildren.get(i);

                final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
                int childWidthMeasureSpec;
                int childHeightMeasureSpec;

                if (lp.width == LayoutParams.MATCH_PARENT) {
                    // 如果是MATCH_PARENT的话，由于parent已经measure过了，所以就相当于child指定了确定值，
                    // 所以用的MeasureSpec.EXACTLY。。。
                    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth() -
                            getPaddingLeftWithForeground() - getPaddingRightWithForeground() -
                            lp.leftMargin - lp.rightMargin,// 在算child大小的时候要去掉parent的padding，child自己指定的各种margin
                            MeasureSpec.EXACTLY);
                } else {
                    // 否则，根据parent的measureSpec，已经用掉的大小，child的layoutparam的信息，创建一个合适的MeasureSpec
                    childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                            getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                            lp.leftMargin + lp.rightMargin,
                            lp.width);
                }

                if (lp.height == LayoutParams.MATCH_PARENT) {
                    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredHeight() -
                            getPaddingTopWithForeground() - getPaddingBottomWithForeground() -
                            lp.topMargin - lp.bottomMargin,
                            MeasureSpec.EXACTLY);
                } else {
                    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                            lp.topMargin + lp.bottomMargin,
                            lp.height);
                }
                // 用新的childWidthMeasureSpec、childHeightMeasureSpec再次measure child
                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
            }
        }
    }

　　接下来，我们重点看看涉及到的几个ViewGroup方法，代码如下：

    /**
     * Ask one of the children of this view to measure itself, taking into
     * account both the MeasureSpec requirements for this view and its padding
     * and margins. The child must have MarginLayoutParams The heavy lifting is
     * done in getChildMeasureSpec.
     *
     * @param child The child to measure
     * @param parentWidthMeasureSpec The width requirements for this view
     * @param widthUsed Extra space that has been used up by the parent
     *        horizontally (possibly by other children of the parent)
     * @param parentHeightMeasureSpec The height requirements for this view
     * @param heightUsed Extra space that has been used up by the parent
     *        vertically (possibly by other children of the parent)
     */
    protected void measureChildWithMargins(View child,
            int parentWidthMeasureSpec, int widthUsed,
            int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
                mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin // 考虑上parent的padding和child的margin
                        + widthUsed, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
                mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                        + heightUsed, lp.height);
        // 这里如果child是个ViewGroup类型，则实际会递归下去。。。
        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

    /**
     * Does the hard part of measureChildren: figuring out the MeasureSpec to
     * pass to a particular child. This method figures out the right MeasureSpec
     * for one dimension (height or width) of one child view.
     *
     * The goal is to combine information from our MeasureSpec with the
     * LayoutParams of the child to get the best possible results. For example,
     * if the this view knows its size (because its MeasureSpec has a mode of
     * EXACTLY), and the child has indicated in its LayoutParams that it wants
     * to be the same size as the parent, the parent should ask the child to
     * layout given an exact size.
     *
     * @param spec The requirements for this view
     * @param padding The padding of this view for the current dimension and
     *        margins, if applicable
     * @param childDimension How big the child wants to be in the current
     *        dimension
     * @return a MeasureSpec integer for the child
     */
    public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
        // 这个方法是协商型的，最终结果既可能直接由spec（parent提供的），也可能由childDimension决定
        // 所以我们知道了，一个View的大小不是简单的单方面决定的，而是通过一系列条件协商的结果，
        // 有时会尊重parent的spec，有时会坚持自己的dimension要求
        int specMode = MeasureSpec.getMode(spec);
        int specSize = MeasureSpec.getSize(spec);

        int size = Math.max(0, specSize - padding); // 可用的大小

        int resultSize = 0;
        int resultMode = 0;

        switch (specMode) {
        // Parent has imposed an exact size on us
        case MeasureSpec.EXACTLY: // parent说child你应该是个确定的大小
            if (childDimension >= 0) { // child正好设置了确定的大小
                resultSize = childDimension; // 让child是那个确定的大小
                resultMode = MeasureSpec.EXACTLY; // 设置mode为EXACTLY
            } else if (childDimension == LayoutParams.MATCH_PARENT) {
                // Child wants to be our size. So be it.
                resultSize = size; // 其他情况下都是parent spec中的大小，只是mode不同
                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; // 不能超过size
                resultMode = MeasureSpec.AT_MOST;
            }
            break;

        // Parent has imposed a maximum size on us
        case MeasureSpec.AT_MOST: // parent说child你应该最大是某个值。。。
            if (childDimension >= 0) { // child指定确定值了，则听child的
                // 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: // parent没对child的大小有啥要求
            if (childDimension >= 0) { // child指定了确定的值，听child的
                // 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 = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            } else if (childDimension == LayoutParams.WRAP_CONTENT) {
                // Child wants to determine its own size.... find out how
                // big it should be
                resultSize = 0;
                resultMode = MeasureSpec.UNSPECIFIED;
            }
            break;
        }
        return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
    }

　　至此我们已经将measure过程的相关代码大致分析了下，进一步的理解还需要大家在开发中慢慢体会、细细研读源码，最后附上两篇

不错的同主题文章：

http://www.cnblogs.com/xilinch/archive/2012/10/24/2737178.html。

https://blog.csdn.net/qq_30379689/article/details/54588736。