RelativeLayout源码详解
需要关注的点
1:RelativeLayout进行两次测量
源码分析
一般所有控件类的源码,都会从 measure, layout和draw3个方法入手,查看他们的回调函数onMeasure, onLayout和onDraw 只要明白这3个流程,一般控件的整个实现也就明白了 LinearLayout作为一个ViewGroup的子类,主要作为一个布局容器出现,所以我们需要重点查看onMeasure方法,
if (mDirtyHierarchy) {
mDirtyHierarchy = false;
sortChildren();
}
首先根据mDirtyHierarchy来判断是否需要将子控件重新进行排序
public void requestLayout() {
super.requestLayout();
mDirtyHierarchy = true;
}
mDirtyHierarchy只有在requestLayout方法下进行修改,当mDirtyHierarchy为true时,调用sortChildren方式将childview进行重新排序
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);
}
根据类名DependencyGraph,这是一个依赖关系图,主要是add方法和getSortedViews
先看看add方法
void add(View view) {
// 因为是图,根据view来生成节点,
final int id = view.getId();
final Node node = Node.acquire(view);
// 如果是有效ID,则将该节点添加到list中
if (id != View.NO_ID) {
mKeyNodes.put(id, node);
}
mNodes.add(node);
}
再看看getSortedViews方法
void getSortedViews(View[] sorted, int... rules) {
// 当view找不到其他可依赖的view时,作为root节点
final ArrayDeque<Node> roots = findRoots(rules);
int index = 0;
Node node;
// 读取root的下一个node
while ((node = roots.pollLast()) != null) {
final View view = node.view;
final int key = view.getId();
// 将符合规则的view加入sorted中
sorted[index++] = view;
// dependents 依赖该node的node (A C依赖B 则B的dependents中存A C)
final ArrayMap<Node, DependencyGraph> dependents = node.dependents;
final int count = dependents.size();
// 遍历所有依赖自己的node
for (int i = 0; i < count; i++) {
final Node dependent = dependents.keyAt(i);
//dependencies 是被依赖的的node的规则和node(A 依赖 B D 则dependencies存有B D )
final SparseArray<Node> dependencies = dependent.dependencies;
//移除当前node和dependencies的依赖关系
dependencies.remove(key);
if (dependencies.size() == 0) {
//如果解除依赖后没有其它依赖 则将该node也视为rootNode
roots.add(dependent);
}
}
}
if (index < sorted.length) {
throw new IllegalStateException("Circular dependencies cannot exist"
+ " in RelativeLayout");
}
}
举个简单的例子,A依赖B B依赖C 首先存入C 因为C不依赖任何其它的,在遍历依赖于C的的node时,dependencies中只有B,在将B的该依赖删掉后,B不再依赖其他任何的node,被添加到roots中,然后再遍历依赖于B的node时,dependencies中只有A,在将A的该依赖删掉后,A不再依赖其他任何的node,被添加到roots中,最后遍历依赖于A的node时,A没有依赖与被依赖,循环结果,sorted中的顺序为C B A
private ArrayDeque<Node> findRoots(int[] rulesFilter) {
//keyNodes为nodelist
final SparseArray<Node> keyNodes = mKeyNodes;
final ArrayList<Node> nodes = mNodes;
final int count = nodes.size();
// Find roots can be invoked several times, so make sure to clear
// all dependents and dependencies before running the algorithm
//Find root可以多次调用,因此请确保清除
//运行算法之前的所有依赖项和依赖项
for (int i = 0; i < count; i++) {
final Node node = nodes.get(i);
node.dependents.clear();
node.dependencies.clear();
}
// Builds up the dependents and dependencies for each node of the graph
// 为图形的每个节点建立依赖项和依赖项
for (int i = 0; i < count; i++) {
final Node node = nodes.get(i);
final LayoutParams layoutParams = (LayoutParams) node.view.getLayoutParams();
final int[] rules = layoutParams.mRules;
final int rulesCount = rulesFilter.length;
// Look only the the rules passed in parameter, this way we build only the
// dependencies for a specific set of rules
//只查看参数中传递的规则,这样我们只构建
//特定规则集的依赖项
//遍历所有node 存入当前view和他所依赖的关系
for (int j = 0; j < rulesCount; j++) {
// rule对应被依赖view的id
final int rule = rules[rulesFilter[j]];
if (rule > 0 || ResourceId.isValid(rule)) {
// The node this node depends on
// 此节点所依赖的节点
final Node dependency = keyNodes.get(rule);
// Skip unknowns and self dependencies
// 跳过未知和自我依赖
if (dependency == null || dependency == node) {
continue;
}
// Add the current node as a dependent
// 将当前节点添加为从属节点
dependency.dependents.put(node, this);
// Add a dependency to the current node
// 向当前节点添加依赖项
node.dependencies.put(rule, dependency);
}
}
}
final ArrayDeque<Node> roots = mRoots;
roots.clear();
// Finds all the roots in the graph: all nodes with no dependencies
// 查找图中的所有根:没有依赖关系的所有节点
// 再次遍历 如果该node的依赖关系为0 即该view不依赖任何view 则视为rootView
for (int i = 0; i < count; i++) {
final Node node = nodes.get(i);
if (node.dependencies.size() == 0) roots.addLast(node);
}
return roots;
}
整个排序过程是一个拓扑关系图,但是我对拓扑关系图没有什么研究,在此不做过多展开
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;
// 记录我们的尺寸,如果他们是已知的
// 如果不是UNSPECIFIED模式 则将widthSize赋值于myWidth
if (widthMode != MeasureSpec.UNSPECIFIED) {
myWidth = widthSize;
}
// 如果不是UNSPECIFIED模式 则将heightSize赋值于myHeight
if (heightMode != MeasureSpec.UNSPECIFIED) {
myHeight = heightSize;
}
//如果是EXACTLY模式 则将myWidth和myHeight记录
if (widthMode == MeasureSpec.EXACTLY) {
width = myWidth;
}
if (heightMode == MeasureSpec.EXACTLY) {
height = myHeight;
}
View ignore = null;
//判断是否为Start 和 top 确定左上角坐标
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;
// 记录ignore的view
if ((horizontalGravity || verticalGravity) && mIgnoreGravity != View.NO_ID) {
ignore = findViewById(mIgnoreGravity);
}
//宽度和高度是否为warp模式
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".
//我们需要知道我们的大小做正确的计算儿童定位在RTL
//模式,但没有实际的方法来获得它,而不是运行下面的代码。
//因此,我们没有运行两次代码,而是将宽度设置为“默认显示宽度”
//在计算之前,作为最后一步,我们将使用
//偏移等于“DEFAULT_WIDTH-WIDTH”。
//在计算和分配的子View的坐标的时候 需要用到父VIew的尺寸 但是暂时无法拿到准确值(待完成下面操作)
//先使用默认值代替 在计算后 用偏移量更新真是坐标
final int layoutDirection = getLayoutDirection();
if (isLayoutRtl() && myWidth == -1) {
myWidth = DEFAULT_WIDTH;
}
进行一些参数的初始化
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();
// 根据方向获得子View中设置的规则
int[] rules = params.getRules(layoutDirection);
// 将左右方向规则转换为左右的坐标
applyHorizontalSizeRules(params, myWidth, rules);
// 测算水平方向的子View的尺寸
measureChildHorizontal(child, params, myWidth, myHeight);
//确定水平方向子View的位置
if (positionChildHorizontal(child, params, myWidth, isWrapContentWidth)) {
offsetHorizontalAxis = true;
}
}
}
遍历水平关系的view
先看看applyHorizontalSizeRules方法
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
//值VALUE_NOT_SETT表示该方向的“软需求”。例如:
//left=10,right=VALUE_NOT_SETT表示视图必须从10开始,但可以走到最远
//想去右边
//left=VALUE_NOT_SETT,right=10表示视图必须在10处结束,但可以走到最远
//想往左边走
//左=10,右=20表示左右两端都是固定的
childParams.mLeft = VALUE_NOT_SET;
childParams.mRight = VALUE_NOT_SET;
//得到当前子View的layout_toLeftOf属性对应的View
anchorParams = getRelatedViewParams(rules, LEFT_OF);
if (anchorParams != null) {
//如果这个属性存在 则当前子View的右坐标是layout_toLeftOf对应的view的左坐标减去对应view的marginLeft的值和自身marginRight的值
childParams.mRight = anchorParams.mLeft - (anchorParams.leftMargin +
childParams.rightMargin);
} else if (childParams.alignWithParent && rules[LEFT_OF] != 0) {
//如果alignWithParent为true alignWithParent取alignWithParentIfMissing
//如果layout_toLeftOf的view为空 或者gone 则将RelativeLayout当做被依赖的对象
//如果父容器RelativeLayout的宽度大于0
//则子View的右坐标为 父RelativeLayout的宽度减去 mPaddingRight 和自身的marginRight
if (myWidth >= 0) {
childParams.mRight = myWidth - mPaddingRight - childParams.rightMargin;
}
}
//类似的方法 得到左坐标(通过参数RIGHT_OF)
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;
}
//类似的方法 得到左坐标(通过参数ALIGN_LEFT)
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;
}
//类似的方法 得到左坐标(通过参数ALIGN_RIGHT)
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;
}
}
//根据ALIGN_PARENT_LEFT 将自己放到父RelativeLayout的左边
if (0 != rules[ALIGN_PARENT_LEFT]) {
childParams.mLeft = mPaddingLeft + childParams.leftMargin;
}
//根据ALIGN_PARENT_RIGHT 将自己放到父RelativeLayout的右边
if (0 != rules[ALIGN_PARENT_RIGHT]) {
if (myWidth >= 0) {
childParams.mRight = myWidth - mPaddingRight - childParams.rightMargin;
}
}
}
applyHorizontalSizeRules这个方法主要是通过水平上的依赖规则对params的right和left进行计算赋值
在看看measureChildHorizontal方法
private void measureChildHorizontal(
View child, LayoutParams params, int myWidth, int myHeight) {
// 获得child的宽度MeasureSpec
final int childWidthMeasureSpec = getChildMeasureSpec(params.mLeft, params.mRight,
params.width, params.leftMargin, params.rightMargin, mPaddingLeft, mPaddingRight,
myWidth);
final int childHeightMeasureSpec;
// 在低于4.2的时候 mAllowBrokenMeasureSpecs为true
//当myHeight < 0 时 则根据父RelativeLayout设置其MeasureSpec模式
if (myHeight < 0 && !mAllowBrokenMeasureSpecs) {
// 如果父RelativeLayout的height大于0 则 设置子view的MeasureSpec模式为EXACTLY
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.
// 反之 如果其小于0 则设置子View的MeasureSpec为UNSPECIFIED
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
}
} else {
// 当前myHeight >= 0
// 判断当前高度是否与父RelativeLayout高度相同 设置heightMode
// 根据maxHeight 和heightMode设置子View的MeasureSpec模式
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);
}
//获得了子View的WidthMeasureSpec和HeightMeasureSpec
//子View可以通过measure方法获取自身的size
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
measureChildHorizontal 方法里面主要是获取到WidthMeasureSpec和HeightMeasureSpec后调用child.mesuare方法来测量水平方向上面的childview的尺寸
再看看positionChildHorizontal方法
private boolean positionChildHorizontal(View child, LayoutParams params, int myWidth,
boolean wrapContent) {
// 获取RelativeLayout的布局方向
final int layoutDirection = getLayoutDirection();
int[] rules = params.getRules(layoutDirection);
if (params.mLeft == VALUE_NOT_SET && params.mRight != VALUE_NOT_SET) {
// Right is fixed, but left varies
// 右边是固定的,左边是变化的
// 如果右边界有效 左边界无效 根据右边界计算出左边界
params.mLeft = params.mRight - child.getMeasuredWidth();
} else if (params.mLeft != VALUE_NOT_SET && params.mRight == VALUE_NOT_SET) {
// Left is fixed, but right varies
// 左边是固定的,右边是变化的
// 如果左边界有效 右边界无效 根据左边界计算右左边界
params.mRight = params.mLeft + child.getMeasuredWidth();
} else if (params.mLeft == VALUE_NOT_SET && params.mRight == VALUE_NOT_SET) {
// Both left and right vary
// 左右边界都无效
// 设置了CENTER_IN_PARENT或者 CENTER_HORIZONTAL的情况下
if (rules[CENTER_IN_PARENT] != 0 || rules[CENTER_HORIZONTAL] != 0) {
if (!wrapContent) {
//非wrap情况下
//把子View水平中心固定在RelativeLayout的中心
centerHorizontal(child, params, myWidth);
} else {
// 这个方法比较简单,就不单独说了,这里是根据RLT判断
// 如果isLayoutRtl为true,那就先算右边距,右边距为myWidth - mPaddingRight - params.rightMargin
// 左边距为右边距加上测量宽度,如果isLayoutRtl为false,那就先算左边距,左
// 边距为mPaddingRight + params.rightMargin,右边距为左边距加上测量宽度
positionAtEdge(child, params, myWidth);
}
return true;
} else {
// This is the default case. For RTL we start from the right and for LTR we start
// from the left. This will give LEFT/TOP for LTR and RIGHT/TOP for RTL.
//这是默认情况。对于RTL,我们从右边开始,对于LTR,我们从右边开始
//从左边。这将为LTR提供LEFT/TOP,为RTL提供RIGHT/TOP。
具体描述,上面有写
positionAtEdge(child, params, myWidth);
}
}
// 当为CENTER_IN_PARENT CENTER_HORIZONTAL ALIGN_PARENT_END三种情况之一时返回True
return rules[ALIGN_PARENT_END] != 0;
}
positionChildHorizontal 方法确定水平方向子View的位置,最后返回childview是否水平居中
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();
// 将竖直方向规则转换为坐标
applyVerticalSizeRules(params, myHeight, child.getBaseline());
//测量子View
measureChild(child, params, myWidth, myHeight);
//确定竖直方向子View的位置
if (positionChildVertical(child, params, myHeight, isWrapContentHeight)) {
offsetVerticalAxis = true;
}
//首先判断是否为wrap模式
if (isWrapContentWidth) {
if (isLayoutRtl()) {
//根据RTL或者LTR和版本进行区分
//Build.VERSION_CODES.KITKAT = 19
//主要对margin进行处理
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);
}
}
}
垂直方向上面与水平方向上面大体相识,不再重复,有区别的在于,垂直方向上面再确定位置之后,还需要对margin进行一定的处理
// 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.
//使用最顶层的布局视图作为基线。RTL偏移为
//稍后应用,因此我们可以使用最左边的边作为起始边。
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;
baseline计算
//如果是wrap模式
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);
//在得到最后的width之后 对依赖RelativeLayout的子View添上偏移量
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);
// 对CENTER_IN_PARENT或者CENTER_HORIZONTAL的子View重测
if (rules[CENTER_IN_PARENT] != 0 || rules[CENTER_HORIZONTAL] != 0) {
centerHorizontal(child, params, width);
} else if (rules[ALIGN_PARENT_RIGHT] != 0) {
// 对ALIGN_PARENT_RIGHT重测
final int childWidth = child.getMeasuredWidth();
params.mLeft = width - mPaddingRight - childWidth;
params.mRight = params.mLeft + childWidth;
}
}
}
}
}
// 同上
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;
}
}
}
}
}
// 根据gravity再次修正
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;
}
}
}
}
}
//如果是RTL(右到左显示)则再次修改
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);
小结
- RelativeLayout的两次测量是因为在水平方向和垂直方向上面都要进行一次测量
- RelativeLayout更加关注子View的left right top bottom值 并且优先级高于width和height