Android(安卓)RelativeLayout(7.1) 源码解析
16lz
2021-01-25
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.javaprotected 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
onMeasureif (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 -> sortChildrenprivate 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 -> applyHorizontalSizeRulesprivate 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
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