FrameLayout#onMeasure

 @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int count = getChildCount();//获取子view数量

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

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

        //对于每一个子View进行遍历，获取最大Height和最大Width
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                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);
                        //mMatchParentChildren类型是ArrayList<View>
                    }
                }
            }
        }

        // 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));//①

        count = mMatchParentChildren.size();
        if (count > 1) {
            for (int i = 0; i < count; i++) {
                //遍历所有match_parent的子View
                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);
            }
        }
    }

① View#resolveSizeAndState

public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) {
        final int specMode = MeasureSpec.getMode(measureSpec);
        final int specSize = MeasureSpec.getSize(measureSpec);
        final int result;
        switch (specMode) {
            case MeasureSpec.AT_MOST:
                if (specSize < size) {
                    result = specSize | MEASURED_STATE_TOO_SMALL;
                } else {
                    result = size;
                }
                break;
            case MeasureSpec.EXACTLY:
                result = specSize;
                break;
            case MeasureSpec.UNSPECIFIED:
            default:
                result = size;
        }
        return result | (childMeasuredState & MEASURED_STATE_MASK);
    }

RelativeLayout#onMeasure

 @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        if (mDirtyHierarchy) {
            mDirtyHierarchy = false;
            sortChildren();
        }

        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;//①
        }

        View[] views = mSortedHorizontalChildren;
        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);
                measureChildHorizontal(child, params, myWidth, myHeight);

                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());
                measureChild(child, params, myWidth, myHeight);
                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) {
                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);
    }

最终决定RelativeLayout尺寸的是 setMeasuredDimension(width, height)，是width和height两个参数。
myWidth在AT_MOST和EXACTLY模式下取widthSize，UNSPECIFIED为-1，在①处设为DEFAULT_WIDTH；
myHeight在AT_MOST和EXACTLY模式下取heightSize，UNSPECIFIED为-1；
width在AT_MOST模式下取myWidth，否则为0；
height在AT_MOST模式下取myHeight，否则为0；

mSortedHorizontalChildren
mSortedVerticalChildren

RelativeLayout#measureChildHorizontal

private void measureChildHorizontal(
            View child, LayoutParams params, int myWidth, int myHeight) {
        final int childWidthMeasureSpec = getChildMeasureSpec(params.mLeft, params.mRight,
                params.width, params.leftMargin, params.rightMargin, mPaddingLeft, mPaddingRight,
                myWidth);//根据给出的值测量出子View的规格

        final int childHeightMeasureSpec;
        if (myHeight < 0 && !mAllowBrokenMeasureSpecs) {
            if (params.height >= 0) {
                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                        params.height, MeasureSpec.EXACTLY);
            } else {
                // Negative values in a mySize/myWidth/myWidth value in
                // RelativeLayout measurement is code for, "we got an
                // unspecified mode in the RelativeLayout's measure spec."
                // Carry it forward.
                childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
            }
        } else {
            final int maxHeight;
            if (mMeasureVerticalWithPaddingMargin) {
                maxHeight = Math.max(0, myHeight - mPaddingTop - mPaddingBottom
                        - params.topMargin - params.bottomMargin);
            } else {
                maxHeight = Math.max(0, myHeight);
            }

            final int heightMode;
            if (params.height == LayoutParams.MATCH_PARENT) {
                heightMode = MeasureSpec.EXACTLY;
            } else {
                heightMode = MeasureSpec.AT_MOST;
            }
            childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(maxHeight, heightMode);
        }

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

高度模式为UNSPECIFIED时，设其高度为params.height或0，一般可以不用管；
高度模式指定时，根据模式和myHeight合成高度规格；
调用子View的measure方法