Android-RelativeLayout、FrameLayo

1、LinearLayout 的测量

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    if (mOrientation == VERTICAL) {//根据布局方向进行测量
        measureVertical(widthMeasureSpec, heightMeasureSpec);
    } else {
        measureHorizontal(widthMeasureSpec, heightMeasureSpec);
    }
}

竖直方向分析

void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
    mTotalLength = 0;
    int maxWidth = 0;
    int childState = 0;
    int alternativeMaxWidth = 0;
    int weightedMaxWidth = 0;
    boolean allFillParent = true;
    float totalWeight = 0;

    final int count = getVirtualChildCount();

    final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    final int heightMode = MeasureSpec.getMode(heightMeasureSpec);

    boolean matchWidth = false;//适配宽度
    boolean skippedMeasure = false;//跳过测量

    final int baselineChildIndex = mBaselineAlignedChildIndex;//基线对其校准，使用此布局内部的子View作为新的基线index
    final boolean useLargestChild = mUseLargestChild;//是否相同权重

    int largestChildHeight = Integer.MIN_VALUE;
    int consumedExcessSpace = 0;//被消耗的剩余空间

    int nonSkippedChildCount = 0;

    // See how tall everyone is. Also remember max width.
    for (int i = 0; i < count; ++i) {
        final View child = getVirtualChildAt(i);
        if (child == null) {
            mTotalLength += measureNullChild(i);
            continue;
        }

        if (child.getVisibility() == View.GONE) {
            i += getChildrenSkipCount(child, i);
            continue;
        }

        nonSkippedChildCount++;
        if (hasDividerBeforeChildAt(i)) {
            mTotalLength += mDividerHeight;
        }

        final LayoutParams lp = (LayoutParams) child.getLayoutParams();

        totalWeight += lp.weight;

        final boolean useExcessSpace = lp.height == 0 && lp.weight > 0;//使用权重weight
        if (heightMode == MeasureSpec.EXACTLY && useExcessSpace) {//使用权重weight，跳过测量
            // Optimization: don't bother measuring children who are only
            // laid out using excess space. These views will get measured
            // later if we have space to distribute.
            final int totalLength = mTotalLength;
            mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
            skippedMeasure = true;
        } else {//Mode 模式 AT_MOST、UNSPECIFIED or 没有使用使用权重Weight
            if (useExcessSpace) {
                // The heightMode is either UNSPECIFIED or AT_MOST, and
                // this child is only laid out using excess space. Measure
                // using WRAP_CONTENT so that we can find out the view's
                // optimal height. We'll restore the original height of 0
                // after measurement.
                lp.height = LayoutParams.WRAP_CONTENT;
            }

            // Determine how big this child would like to be. If this or
            // previous children have given a weight, then we allow it to
            // use all available space (and we will shrink things later
            // if needed).
            final int usedHeight = totalWeight == 0 ? mTotalLength : 0;//计算被使用的高度
            measureChildBeforeLayout(child, i, widthMeasureSpec, 0,//没有权重weight的子View的自测，便于确定LinearLayout的测量值
                    heightMeasureSpec, usedHeight);

            final int childHeight = child.getMeasuredHeight();
            if (useExcessSpace) {
                // Restore the original height and record how much space
                // we've allocated to excess-only children so that we can
                // match the behavior of EXACTLY measurement.
                lp.height = 0;
                consumedExcessSpace += childHeight;
            }

            final int totalLength = mTotalLength;
            mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
                    lp.bottomMargin + getNextLocationOffset(child));

            if (useLargestChild) {
                largestChildHeight = Math.max(childHeight, largestChildHeight);
            }
        }

        /**
            * If applicable, compute the additional offset to the child's baseline
            * we'll need later when asked {@link #getBaseline}.
            */
        if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {
            mBaselineChildTop = mTotalLength;//校准基线以供其他的View更改布局
        }

        // if we are trying to use a child index for our baseline, the above
        // book keeping only works if there are no children above it with
        // weight.  fail fast to aid the developer.
        if (i < baselineChildIndex && lp.weight > 0) {
            throw new RuntimeException("A child of LinearLayout with index "
                    + "less than mBaselineAlignedChildIndex has weight > 0, which "
                    + "won't work.  Either remove the weight, or don't set "
                    + "mBaselineAlignedChildIndex.");
        }

        boolean matchWidthLocally = false;
        if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {
            // The width of the linear layout will scale, and at least one
            // child said it wanted to match our width. Set a flag
            // indicating that we need to remeasure at least that view when
            // we know our width.
            matchWidth = true;
            matchWidthLocally = true;
        }

        final int margin = lp.leftMargin + lp.rightMargin;
        final int measuredWidth = child.getMeasuredWidth() + margin;
        maxWidth = Math.max(maxWidth, measuredWidth);
        childState = combineMeasuredStates(childState, child.getMeasuredState());

        allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
        if (lp.weight > 0) {
            /*
                * Widths of weighted Views are bogus if we end up
                * remeasuring, so keep them separate.
                */
            weightedMaxWidth = Math.max(weightedMaxWidth,
                    matchWidthLocally ? margin : measuredWidth);
        } else {
            alternativeMaxWidth = Math.max(alternativeMaxWidth,
                    matchWidthLocally ? margin : measuredWidth);
        }

        i += getChildrenSkipCount(child, i);
    }

    if (nonSkippedChildCount > 0 && hasDividerBeforeChildAt(count)) {
        mTotalLength += mDividerHeight;
    }

    if (useLargestChild &&
            (heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
        mTotalLength = 0;

        for (int i = 0; i < count; ++i) {
            final View child = getVirtualChildAt(i);
            if (child == null) {
                mTotalLength += measureNullChild(i);
                continue;
            }

            if (child.getVisibility() == GONE) {
                i += getChildrenSkipCount(child, i);
                continue;
            }

            final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)child.getLayoutParams();
            // Account for negative margins
            final int totalLength = mTotalLength;
            mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
                    lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
        }
    }

    // Add in our padding
    mTotalLength += mPaddingTop + mPaddingBottom;

    int heightSize = mTotalLength;

    // Check against our minimum height
    heightSize = Math.max(heightSize, getSuggestedMinimumHeight());

    // Reconcile our calculated size with the heightMeasureSpec
    int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
    heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
    // Either expand children with weight to take up available space or
    // shrink them if they extend beyond our current bounds. If we skipped
    // measurement on any children, we need to measure them now.
    int remainingExcess = heightSize - mTotalLength
            + (mAllowInconsistentMeasurement ? 0 : consumedExcessSpace);
    if (skippedMeasure || remainingExcess != 0 && totalWeight > 0.0f) {//测量之前设置权重的
        float remainingWeightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;

        mTotalLength = 0;

        for (int i = 0; i < count; ++i) {
            final View child = getVirtualChildAt(i);
            if (child == null || child.getVisibility() == View.GONE) {
                continue;
            }

            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            final float childWeight = lp.weight;
            if (childWeight > 0) {
                final int share = (int) (childWeight * remainingExcess / remainingWeightSum);//计算使用权重后应该得到的额外空间
                remainingExcess -= share;
                remainingWeightSum -= childWeight;

                final int childHeight;
                if (mUseLargestChild && heightMode != MeasureSpec.EXACTLY) {
                    childHeight = largestChildHeight;
                } else if (lp.height == 0 && (!mAllowInconsistentMeasurement
                        || heightMode == MeasureSpec.EXACTLY)) {
                    // This child needs to be laid out from scratch using
                    // only its share of excess space.
                    childHeight = share;
                } else {
                    // This child had some intrinsic height to which we
                    // need to add its share of excess space.
                    childHeight = child.getMeasuredHeight() + share;
                }

                final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                        Math.max(0, childHeight), MeasureSpec.EXACTLY);
                final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                        mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin,
                        lp.width);
                child.measure(childWidthMeasureSpec, childHeightMeasureSpec);//拥有权重的子View进行测量，即完成左右的子View的测量

                // Child may now not fit in vertical dimension.
                childState = combineMeasuredStates(childState, child.getMeasuredState()
                        & (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
            }

            final int margin =  lp.leftMargin + lp.rightMargin;
            final int measuredWidth = child.getMeasuredWidth() + margin;
            maxWidth = Math.max(maxWidth, measuredWidth);

            boolean matchWidthLocally = widthMode != MeasureSpec.EXACTLY &&
                    lp.width == LayoutParams.MATCH_PARENT;

            alternativeMaxWidth = Math.max(alternativeMaxWidth,
                    matchWidthLocally ? margin : measuredWidth);

            allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;

            final int totalLength = mTotalLength;
            mTotalLength = Math.max(totalLength, totalLength + child.getMeasuredHeight() +
                    lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
        }

        // Add in our padding
        mTotalLength += mPaddingTop + mPaddingBottom;
        // TODO: Should we recompute the heightSpec based on the new total length?
    } else {
        alternativeMaxWidth = Math.max(alternativeMaxWidth,
                                        weightedMaxWidth);


        // We have no limit, so make all weighted views as tall as the largest child.
        // Children will have already been measured once.
        if (useLargestChild && heightMode != MeasureSpec.EXACTLY) {
            for (int i = 0; i < count; i++) {
                final View child = getVirtualChildAt(i);
                if (child == null || child.getVisibility() == View.GONE) {
                    continue;
                }

                final LinearLayout.LayoutParams lp =
                        (LinearLayout.LayoutParams) child.getLayoutParams();

                float childExtra = lp.weight;
                if (childExtra > 0) {
                    child.measure(
                            MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),
                                    MeasureSpec.EXACTLY),
                            MeasureSpec.makeMeasureSpec(largestChildHeight,
                                    MeasureSpec.EXACTLY));
                }
            }
        }
    }

    if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
        maxWidth = alternativeMaxWidth;
    }

    maxWidth += mPaddingLeft + mPaddingRight;

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

    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
            heightSizeAndState);

    if (matchWidth) {
        forceUniformWidth(count, heightMeasureSpec);
    }
}

LinearLayout在进行measure操作的过程中，使用变量mTotalLength保存已经measure过的child所占用的高度，该变量刚开始时是0。
在for循环中调用measureChildBeforeLayout（）对每一个不设置weight的child进行测量，该函数实际上仅仅是调用了measureChildWithMargins()；
每次for循环对child测量完毕后，调用child.getMeasuredHeight()获取该子视图最终的高度，并将这个高度添加到mTotalLength中。
在本步骤中，暂时避开了lp.weight>0的子视图，即暂时先不测量这些子视图，因为后面将把父视图剩余的高度按照weight值的大小平均分配给相应的子视图。
源码中使用了一个局部变量totalWeight累计所有子视图的weight值。处理lp.weight>0的情况需要注意，如果变量heightMode是EXACTLY，
那么，当其他子视图占满父视图的高度后，weight>0的子视图可能分配不到布局空间，从而不被显示，只有当heightMode是AT_MOST或者UNSPECIFIED时，weight>0的视图才能优先获得布局高度。
最后我们的结论是：如果不使用weight属性，LinearLayout会在当前方向上进行一次measure的过程，如果使用weight属性，LinearLayout会避开设置过weight属性的view做第一次measure，
完了再对设置过weight属性的view做第二次measure。由此可见，weight属性对性能是有影响的。

2、RelativeLayout 的测量

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    if (mDirtyHierarchy) {
        mDirtyHierarchy = false;
        sortChildren();//对View进行依赖性排序，如C依赖B，B依赖A进行位置的确定，则在以来表中的顺序 A->B->C，以便之后的测量
    }

    int myWidth = -1;
    int myHeight = -1;

    int width = 0;
    int height = 0;

    final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
    final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
    final int heightSize = MeasureSpec.getSize(heightMeasureSpec);

    // Record our dimensions if they are known;
    if (widthMode != MeasureSpec.UNSPECIFIED) {
        myWidth = widthSize;
    }

    if (heightMode != MeasureSpec.UNSPECIFIED) {
        myHeight = heightSize;
    }

    if (widthMode == MeasureSpec.EXACTLY) {
        width = myWidth;
    }

    if (heightMode == MeasureSpec.EXACTLY) {
        height = myHeight;
    }

    View ignore = null;
    int gravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
    final boolean horizontalGravity = gravity != Gravity.START && gravity != 0;
    gravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
    final boolean verticalGravity = gravity != Gravity.TOP && gravity != 0;

    int left = Integer.MAX_VALUE;
    int top = Integer.MAX_VALUE;
    int right = Integer.MIN_VALUE;
    int bottom = Integer.MIN_VALUE;

    boolean offsetHorizontalAxis = false;
    boolean offsetVerticalAxis = false;

    if ((horizontalGravity || verticalGravity) && mIgnoreGravity != View.NO_ID) {
        ignore = findViewById(mIgnoreGravity);
    }

    final boolean isWrapContentWidth = widthMode != MeasureSpec.EXACTLY;
    final boolean isWrapContentHeight = heightMode != MeasureSpec.EXACTLY;

    // We need to know our size for doing the correct computation of children positioning in RTL
    // mode but there is no practical way to get it instead of running the code below.
    // So, instead of running the code twice, we just set the width to a "default display width"
    // before the computation and then, as a last pass, we will update their real position with
    // an offset equals to "DEFAULT_WIDTH - width".
    final int layoutDirection = getLayoutDirection();
    if (isLayoutRtl() && myWidth == -1) {
        myWidth = DEFAULT_WIDTH;
    }

    View[] views = mSortedHorizontalChildren;//排序后的View数组
    int count = views.length;

    for (int i = 0; i < count; i++) {
        View child = views[i];
        if (child.getVisibility() != GONE) {
            LayoutParams params = (LayoutParams) child.getLayoutParams();
            int[] rules = params.getRules(layoutDirection);

            applyHorizontalSizeRules(params, myWidth, rules);//根据水平依赖规则确定Params，通过RelativeLayout的图表树结构
            measureChildHorizontal(child, params, myWidth, myHeight);//第一次，水平测量

            if (positionChildHorizontal(child, params, myWidth, isWrapContentWidth)) {
                offsetHorizontalAxis = true;
            }
        }
    }

    views = mSortedVerticalChildren;
    count = views.length;
    final int targetSdkVersion = getContext().getApplicationInfo().targetSdkVersion;

    for (int i = 0; i < count; i++) {
        final View child = views[i];
        if (child.getVisibility() != GONE) {
            final LayoutParams params = (LayoutParams) child.getLayoutParams();

            applyVerticalSizeRules(params, myHeight, child.getBaseline());//根据竖直依赖规则确定Params
            measureChild(child, params, myWidth, myHeight);//第二次测量
            if (positionChildVertical(child, params, myHeight, isWrapContentHeight)) {
                offsetVerticalAxis = true;
            }

            if (isWrapContentWidth) {
                if (isLayoutRtl()) {
                    if (targetSdkVersion < Build.VERSION_CODES.KITKAT) {
                        width = Math.max(width, myWidth - params.mLeft);
                    } else {
                        width = Math.max(width, myWidth - params.mLeft + params.leftMargin);
                    }
                } else {
                    if (targetSdkVersion < Build.VERSION_CODES.KITKAT) {
                        width = Math.max(width, params.mRight);
                    } else {
                        width = Math.max(width, params.mRight + params.rightMargin);
                    }
                }
            }

            if (isWrapContentHeight) {
                if (targetSdkVersion < Build.VERSION_CODES.KITKAT) {
                    height = Math.max(height, params.mBottom);
                } else {
                    height = Math.max(height, params.mBottom + params.bottomMargin);
                }
            }

            if (child != ignore || verticalGravity) {
                left = Math.min(left, params.mLeft - params.leftMargin);
                top = Math.min(top, params.mTop - params.topMargin);
            }

            if (child != ignore || horizontalGravity) {
                right = Math.max(right, params.mRight + params.rightMargin);
                bottom = Math.max(bottom, params.mBottom + params.bottomMargin);
            }
        }
    }

    // Use the top-start-most laid out view as the baseline. RTL offsets are
    // applied later, so we can use the left-most edge as the starting edge.
    View baselineView = null;
    LayoutParams baselineParams = null;
    for (int i = 0; i < count; i++) {
        final View child = views[i];
        if (child.getVisibility() != GONE) {
            final LayoutParams childParams = (LayoutParams) child.getLayoutParams();
            if (baselineView == null || baselineParams == null
                    || compareLayoutPosition(childParams, baselineParams) < 0) {
                baselineView = child;
                baselineParams = childParams;
            }
        }
    }
    mBaselineView = baselineView;

    if (isWrapContentWidth) {
        // Width already has left padding in it since it was calculated by looking at
        // the right of each child view
        width += mPaddingRight;

        if (mLayoutParams != null && mLayoutParams.width >= 0) {
            width = Math.max(width, mLayoutParams.width);
        }

        width = Math.max(width, getSuggestedMinimumWidth());
        width = resolveSize(width, widthMeasureSpec);

        if (offsetHorizontalAxis) {//通过循环设置水平的偏移量
            for (int i = 0; i < count; i++) {
                final View child = views[i];
                if (child.getVisibility() != GONE) {
                    final LayoutParams params = (LayoutParams) child.getLayoutParams();
                    final int[] rules = params.getRules(layoutDirection);
                    if (rules[CENTER_IN_PARENT] != 0 || rules[CENTER_HORIZONTAL] != 0) {
                        centerHorizontal(child, params, width);
                    } else if (rules[ALIGN_PARENT_RIGHT] != 0) {
                        final int childWidth = child.getMeasuredWidth();
                        params.mLeft = width - mPaddingRight - childWidth;
                        params.mRight = params.mLeft + childWidth;
                    }
                }
            }
        }
    }

    if (isWrapContentHeight) {
        // Height already has top padding in it since it was calculated by looking at
        // the bottom of each child view
        height += mPaddingBottom;

        if (mLayoutParams != null && mLayoutParams.height >= 0) {
            height = Math.max(height, mLayoutParams.height);
        }

        height = Math.max(height, getSuggestedMinimumHeight());
        height = resolveSize(height, heightMeasureSpec);

        if (offsetVerticalAxis) {
            for (int i = 0; i < count; i++) {
                final View child = views[i];
                if (child.getVisibility() != GONE) {
                    final LayoutParams params = (LayoutParams) child.getLayoutParams();
                    final int[] rules = params.getRules(layoutDirection);
                    if (rules[CENTER_IN_PARENT] != 0 || rules[CENTER_VERTICAL] != 0) {
                        centerVertical(child, params, height);
                    } else if (rules[ALIGN_PARENT_BOTTOM] != 0) {
                        final int childHeight = child.getMeasuredHeight();
                        params.mTop = height - mPaddingBottom - childHeight;
                        params.mBottom = params.mTop + childHeight;
                    }
                }
            }
        }
    }

    if (horizontalGravity || verticalGravity) {
        final Rect selfBounds = mSelfBounds;
        selfBounds.set(mPaddingLeft, mPaddingTop, width - mPaddingRight,
                height - mPaddingBottom);

        final Rect contentBounds = mContentBounds;
        Gravity.apply(mGravity, right - left, bottom - top, selfBounds, contentBounds,
                layoutDirection);

        final int horizontalOffset = contentBounds.left - left;
        final int verticalOffset = contentBounds.top - top;
        if (horizontalOffset != 0 || verticalOffset != 0) {
            for (int i = 0; i < count; i++) {
                final View child = views[i];
                if (child.getVisibility() != GONE && child != ignore) {
                    final LayoutParams params = (LayoutParams) child.getLayoutParams();
                    if (horizontalGravity) {
                        params.mLeft += horizontalOffset;
                        params.mRight += horizontalOffset;
                    }
                    if (verticalGravity) {
                        params.mTop += verticalOffset;
                        params.mBottom += verticalOffset;
                    }
                }
            }
        }
    }

    if (isLayoutRtl()) {
        final int offsetWidth = myWidth - width;
        for (int i = 0; i < count; i++) {
            final View child = views[i];
            if (child.getVisibility() != GONE) {
                final LayoutParams params = (LayoutParams) child.getLayoutParams();
                params.mLeft -= offsetWidth;
                params.mRight -= offsetWidth;
            }
        }
    }

    setMeasuredDimension(width, height);//设置自己的测量值
}

RelativeLayout会进行两次测量，因为RelativeLayout有水平和竖直的以来关系，需要先对这种这种依赖关系进行排序，然后在进行测量来确定这种相对位置，
第二次在进行一次测量，来最终确定位置。依赖关系如：C通过A来进行对齐，A依赖B进行对其，这样排序后的顺序：B->A->C，方便以后的测量，通过完成子类的测量最终完成自己的测量

3、FrameLayout的测量

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int count = getChildCount();

    final boolean measureMatchParentChildren =
            MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
            MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
    mMatchParentChildren.clear();

    int maxHeight = 0;
    int maxWidth = 0;
    int childState = 0;

    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (mMeasureAllChildren || child.getVisibility() != GONE) {
            measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);//子View测量，来确定FrameLayout的测量值
            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);
                }
            }
        }
    }

    // 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());
    }

    setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
            resolveSizeAndState(maxHeight, heightMeasureSpec,
                    childState << MEASURED_HEIGHT_STATE_SHIFT));

    count = mMatchParentChildren.size();
    if (count > 1) {
        for (int i = 0; i < count; i++) {
            final View child = mMatchParentChildren.get(i);
            final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

            final int childWidthMeasureSpec;
            if (lp.width == LayoutParams.MATCH_PARENT) {
                final int width = Math.max(0, getMeasuredWidth()
                        - getPaddingLeftWithForeground() - getPaddingRightWithForeground()
                        - lp.leftMargin - lp.rightMargin);
                childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
                        width, MeasureSpec.EXACTLY);
            } else {
                childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
                        getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
                        lp.leftMargin + lp.rightMargin,
                        lp.width);
            }

            final int childHeightMeasureSpec;
            if (lp.height == LayoutParams.MATCH_PARENT) {
                final int height = Math.max(0, getMeasuredHeight()
                        - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
                        - lp.topMargin - lp.bottomMargin);
                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                        height, MeasureSpec.EXACTLY);
            } else {
                childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
                        getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
                        lp.topMargin + lp.bottomMargin,
                        lp.height);
            }

            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);//完成子View的测量
        }
    }
}

FrameLayout也需要测量两边，但是它的子View之间没有依赖性关系。第一次子View的测量确定FrameLayout的测量值，第二次进行子View的测量

RelativeLayout，FrameLayout，LinearLayout性能总结

RelativeLayout：测量两次，而且需要根据关联关系进行排序，确定Params来测量，性能最低
FrameLayout：测量两次，子View没有关联关系，性能中等
LinearLayout：测量一次，子View之间根据水平或竖直进行排版，性能最高布局和绘制基本上向能相似