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

        // 記錄我們已知的尺寸；
        /**
            View的measure方法裡對繪製過程做了一個優化，如果我們或者我們的子View沒有要求強制重新整理，
            而父View給子View的傳入值也沒有變化（也就是說子View的位置沒變化），就不會做無謂的measure。

            RelativeLayout要做兩次measure，而在做橫向的測量時，縱向的測量結果尚未完成，只好暫時使用myHeight傳入子View系統

            假如子View的Height不等於（設定了margin）myHeight的高度，
            那麼measure中上面程式碼所做得優化將不起作用，這一過程將進一步影響RelativeLayout的繪製效能。
            優化
             如果可以，儘量使用padding代替margin。
        **/
 

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

        int width = 0;
        int height = 0;

        //得到模式
        final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
        final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
        //得到寬高
        final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
        final
 
 int heightSize = MeasureSpec.getSize(heightMeasureSpec);

        // Record our dimensions if they are known;
        // 記錄我們已知的尺寸；
        if (widthMode != MeasureSpec.UNSPECIFIED) {
            myWidth = widthSize;
        }

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

        //模式
 

        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".
        /***
             我們需要知道我們的尺寸，以便正確計算兒童的位置,在 RTL（從右到左）的佈局鏡面反射模式

             但是沒有實際的方法來獲得它，而不是執行下面的程式碼。

             因此，我們不是將程式碼執行兩次，而是將寬度設定為“預設顯示寬度”。在計算之前，然後作為最後一個通過，我們會更新他們的真實位置

             一個偏移量等於  "DEFAULT_WIDTH - width".
        **/
        final int layoutDirection = getLayoutDirection();
        if (isLayoutRtl() && myWidth == -1) {
            myWidth = DEFAULT_WIDTH;
        }

        //水平方向的測量 孩子
        View[] views = mSortedHorizontalChildren;
        int count = views.length;

        for (int i = 0; i < count; i++) {
            View child = views[i];
            if (child.getVisibility() != GONE) {//只測量非gone的

                //孩子的引數
                LayoutParams params = (LayoutParams) child.getLayoutParams();
                //孩子引數規則
                int[] rules = params.getRules(layoutDirection);

                /**
                儘可能的根據水平方向上的rules給child的childParams的mLeft和mRight設定值
                根據理由:
                    (1)view隔壁是否有其他view
                    (2)view是否對齊 alignWithParent 

                根據孩子自己的佈局引數，得到rules，給child的childParams的mLeft和mRight設定值
                如果，值為VALUE_NOT_SET，則表示這個方向上一個rule都沒有，並且值需要根據其他約束推斷。
                （在positionChildVertical()方法中被推斷）

                計算得到的數值 後續的步驟中還有可能被調整！

                注意的是child的屬性優先順序的問題。在applyHorizontalSizeRules()
                (注意！這裡限定在applyHorizontalSizeRules()方法中!)如果同一方向上有多個rules，
                那麼在applyHorizontalSizeRules()方法中最後判斷的那個rule生效。

                **/
                applyHorizontalSizeRules(params, myWidth, rules);

                //測量寬高
                measureChildHorizontal(child, params, myWidth, myHeight);

                //推斷並設定mTop,mBottom中被設定為VALUE_NOT_SET的變數。
                if (positionChildHorizontal(child, params, myWidth, isWrapContentWidth)) {
                    offsetHorizontalAxis = true;
                }
            }
        }

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

        //豎直方向 測量
        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);
    }

/***
        儘可能的根據水平方向上的rules給child的childParams的mLeft和mRight設定值。

    **/
    private void applyHorizontalSizeRules(LayoutParams childParams, int myWidth, int[] rules) {
        RelativeLayout.LayoutParams anchorParams;

        // VALUE_NOT_SET indicates a "soft requirement" in that direction. For example:
        // left=10, right=VALUE_NOT_SET means the view must start at 10, but can go as far as it
        // wants to the right
        // left=VALUE_NOT_SET, right=10 means the view must end at 10, but can go as far as it
        // wants to the left
        // left=10, right=20 means the left and right ends are both fixed
        /**
            value_not_set表示“軟要求”在那個方向

            left=10, right=VALUE_NOT_SET 的意思，控制元件開始於10，可以往右邊拓展延伸

        **/
        childParams.mLeft = VALUE_NOT_SET;
        childParams.mRight = VALUE_NOT_SET;

        //右邊是否存在view
        anchorParams = getRelatedViewParams(rules, LEFT_OF);
        if (anchorParams != null) {//當前view 有related view；anchorParams 是相關view的引數
            //計算得到本view的 mRight
            childParams.mRight = anchorParams.mLeft - (anchorParams.leftMargin +
                    childParams.rightMargin);
        } else if (childParams.alignWithParent && rules[LEFT_OF] != 0) {//右邊不存在view
            //根據父親相關引數，得到 mRight
            if (myWidth >= 0) {
                childParams.mRight = myWidth - mPaddingRight - childParams.rightMargin;
            }
        }

        //同理 左邊是否存在控制元件
        anchorParams = getRelatedViewParams(rules, RIGHT_OF);
        if (anchorParams != null) {
            childParams.mLeft = anchorParams.mRight + (anchorParams.rightMargin +
                    childParams.leftMargin);
        } else if (childParams.alignWithParent && rules[RIGHT_OF] != 0) {
            childParams.mLeft = mPaddingLeft + childParams.leftMargin;
        }

        anchorParams = getRelatedViewParams(rules, ALIGN_LEFT);
        if (anchorParams != null) {
            childParams.mLeft = anchorParams.mLeft + childParams.leftMargin;
        } else if (childParams.alignWithParent && rules[ALIGN_LEFT] != 0) {
            childParams.mLeft = mPaddingLeft + childParams.leftMargin;
        }

        anchorParams = getRelatedViewParams(rules, ALIGN_RIGHT);
        if (anchorParams != null) {
            childParams.mRight = anchorParams.mRight - childParams.rightMargin;
        } else if (childParams.alignWithParent && rules[ALIGN_RIGHT] != 0) {
            if (myWidth >= 0) {
                childParams.mRight = myWidth - mPaddingRight - childParams.rightMargin;
            }
        }

        if (0 != rules[ALIGN_PARENT_LEFT]) {
            childParams.mLeft = mPaddingLeft + childParams.leftMargin;
        }

        if (0 != rules[ALIGN_PARENT_RIGHT]) {
            if (myWidth >= 0) {
                childParams.mRight = myWidth - mPaddingRight - childParams.rightMargin;
            }
        }
    }

    private void applyVerticalSizeRules(LayoutParams childParams, int myHeight, int myBaseline) {
        final int[] rules = childParams.getRules();

        // Baseline alignment overrides any explicitly specified top or bottom.
        int baselineOffset = getRelatedViewBaselineOffset(rules);
        if (baselineOffset != -1) {
            if (myBaseline != -1) {
                baselineOffset -= myBaseline;
            }
            childParams.mTop = baselineOffset;
            childParams.mBottom = VALUE_NOT_SET;
            return;
        }

        RelativeLayout.LayoutParams anchorParams;

        childParams.mTop = VALUE_NOT_SET;
        childParams.mBottom = VALUE_NOT_SET;

        anchorParams = getRelatedViewParams(rules, ABOVE);
        if (anchorParams != null) {
            childParams.mBottom = anchorParams.mTop - (anchorParams.topMargin +
                    childParams.bottomMargin);
        } else if (childParams.alignWithParent && rules[ABOVE] != 0) {
            if (myHeight >= 0) {
                childParams.mBottom = myHeight - mPaddingBottom - childParams.bottomMargin;
            }
        }

        anchorParams = getRelatedViewParams(rules, BELOW);
        if (anchorParams != null) {
            childParams.mTop = anchorParams.mBottom + (anchorParams.bottomMargin +
                    childParams.topMargin);
        } else if (childParams.alignWithParent && rules[BELOW] != 0) {
            childParams.mTop = mPaddingTop + childParams.topMargin;
        }

        anchorParams = getRelatedViewParams(rules, ALIGN_TOP);
        if (anchorParams != null) {
            childParams.mTop = anchorParams.mTop + childParams.topMargin;
        } else if (childParams.alignWithParent && rules[ALIGN_TOP] != 0) {
            childParams.mTop = mPaddingTop + childParams.topMargin;
        }

        anchorParams = getRelatedViewParams(rules, ALIGN_BOTTOM);
        if (anchorParams != null) {
            childParams.mBottom = anchorParams.mBottom - childParams.bottomMargin;
        } else if (childParams.alignWithParent && rules[ALIGN_BOTTOM] != 0) {
            if (myHeight >= 0) {
                childParams.mBottom = myHeight - mPaddingBottom - childParams.bottomMargin;
            }
        }

        if (0 != rules[ALIGN_PARENT_TOP]) {
            childParams.mTop = mPaddingTop + childParams.topMargin;
        }

        if (0 != rules[ALIGN_PARENT_BOTTOM]) {
            if (myHeight >= 0) {
                childParams.mBottom = myHeight - mPaddingBottom - childParams.bottomMargin;
            }
        }
    }

【view的測量優化依據】