Android RelativeLayout(7.1) 源码解析

这节看一下RelativeLayout是如何实现的，它继承ViewGroup，下面主要分析它的
1、onMeasure
2、onLayout
Note：
1、onMeasure时，有三种测量模式：UNSPECIFIED、EXACTLY、AT_MOST
2、UNSPECIFIED 发生在创建的View没有明确layout_width和layout_height
3、EXACTLY发生在明确layout_width和layout_height具体大小，或者设置value为match_parent
4、AT_MOST发生在 layout_width和layout_height的value为wrap_content
5、下面定义 水平方向例外规则为:(LEFT_OF, RIGHT_OF, ALIGN_LEFT, ALIGN_RIGHT, START_OF, END_OF, ALIGN_START, ALIGN_END)
6、下面定义垂直方向的依赖关系为：(ABOVE,BELOW, ALIGN_BASELINE, ALIGN_TOP, ALIGN_BOTTOM)

下面是onMeasure的全部code

RelativeLayout.java
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;
        }

        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);
    }

接下来，我们一点一点的分析上面的code

onMeasure
  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;
        }

1、当view执行requestLayout时，mDirtyHierarchy会变为true，此时就会执行sortChildren()方法，该方法 主要是将RelativeLayout的sub child进行两种规则的排序，一种是按照水平依赖关系(LEFT_OF、RIGHT_OF,etc)进行排序用数组mSortedHorizontalChildren保存结果，另一种是按照垂直依赖关系(ABOVE、BELOW,etc)进行排序用数组mSortedVerticalChildren保存结果
2、MeasureSpec.getMode获得开始时的measure mode
3、MeasureSpec.getSize或则开始时的width 和height

final int layoutDirection = getLayoutDirection();

这里我们默认的layoutDirection是LTR：水平方向从左往右排版，
而RTL正好相反，是从右向左排版,如下图

onMeasure
 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;
                }
            }
        }

1、applyHorizontalSizeRules方法通过水平方向的依赖规则对params的right和left进行计算赋值
2、measureChildHorizontal 方法里面主要是调用child.mesuare方法
3、positionChildHorizontal 这个方法判断改child view是否设置在 parent view里面为水平居中

onMeasure
 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);
                }
            }
        }

这里的code logic不做多少讲解

onMeasure
  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;

这几行code是为了找出在最left和最top的 View,用mBaselineView保存符合条件的view

onMeasure
 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;
                        }
                    }
                }
            }
        }

这里code很简单，centerHorizontal 这个方法是设置params的left和right的值

onMeasure
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;
                        }
                    }
                }
            }
        }

这里的code不做多少解析，继续往下看

onMeasure
 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;
                        }
                    }
                }
            }
        }

这里code主要功能还是对设置在parent view中水平居中或者垂直居中的处理

onMeasure
 setMeasuredDimension(width, height);

最后一行 设置RelativeLayout本身测量的width和height
以上是onMeasure的大概执行过程，下面我们看一下onMeasure里面调用的几个method

onMeasure -> sortChildren
private void sortChildren() {
        final int count = getChildCount();
        if (mSortedVerticalChildren == null || mSortedVerticalChildren.length != count) {
            mSortedVerticalChildren = new View[count];
        }

        if (mSortedHorizontalChildren == null || mSortedHorizontalChildren.length != count) {
            mSortedHorizontalChildren = new View[count];
        }

        final DependencyGraph graph = mGraph;
        graph.clear();

        for (int i = 0; i < count; i++) {
            graph.add(getChildAt(i));
        }

        graph.getSortedViews(mSortedVerticalChildren, RULES_VERTICAL);
        graph.getSortedViews(mSortedHorizontalChildren, RULES_HORIZONTAL);
    }

code很简单，我们看一下graph.getSortedViews方法

void getSortedViews(View[] sorted, int... rules) {
            final ArrayDeque roots = findRoots(rules);
            int index = 0;

            Node node;
            while ((node = roots.pollLast()) != null) {
                final View view = node.view;
                final int key = view.getId();

                sorted[index++] = view;

                final ArrayMap dependents = node.dependents;
                final int count = dependents.size();
                for (int i = 0; i < count; i++) {
                    final Node dependent = dependents.keyAt(i);
                    final SparseArray dependencies = dependent.dependencies;

                    dependencies.remove(key);
                    if (dependencies.size() == 0) {
                        roots.add(dependent);
                    }
                }
            }

            if (index < sorted.length) {
                throw new IllegalStateException("Circular dependencies cannot exist"
                        + " in RelativeLayout");
            }
        }

1、这里面一开始调用了findRoots方法，该方法是找出所有完全不依赖其它view的RelativeLayout的child view
2、这里用到一个类Note，其中Node.dependents保存的是布局上其它所有依赖此Node的view信息，Node.dependencies保存的是此Node布局上所依赖的所有的View信息

onMeasure -> applyHorizontalSizeRules

private void applyHorizontalSizeRules(LayoutParams childParams, int myWidth, int[] rules) {
        RelativeLayout.LayoutParams anchorParams;    
        childParams.mLeft = VALUE_NOT_SET;
        childParams.mRight = VALUE_NOT_SET;

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

        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;
            }
        }
    }

该方法根据水平方向的依赖规则去计算view params的right和left的值

在onMeasure其它的一些方法例如measureChildHorizontal、positionChildHorizontal等这样的Method都是一些logic的处理，感兴趣的可以自己查看一下source code

下面我们看一下onLayout的具体实现：

 protected void onLayout(boolean changed, int l, int t, int r, int b) {
        //  The layout has actually already been performed and the positions
        //  cached.  Apply the cached values to the children.
        final int count = getChildCount();

        for (int i = 0; i < count; i++) {
            View child = getChildAt(i);
            if (child.getVisibility() != GONE) {
                RelativeLayout.LayoutParams st =
                        (RelativeLayout.LayoutParams) child.getLayoutParams();
                child.layout(st.mLeft, st.mTop, st.mRight, st.mBottom);
            }
        }
    }

从code上来看，onLayout的实现很简单，原因是onMeasure里面已经处理好了每一个view的left 、top、right、bottom的值，所以这里只需要简单遍历所有的view并执行child.layout就ok了

summary：
RelativeLayout利用位置上相对依赖的关系，计算出每一个view的上下左右的值，我们可以从读code中来学习 编写自定义控件的ideal