Android 源码的责任链模式
责任链模式介绍
责任链模式是行为型设计模式之一。多个节点首尾相连接所构成的模型称为链。对于链式结构，每个节点都可以被拆开再连接，因此，链式结构也具有很好的灵活性。将这样一种结构应用于编程领域，将每一个节点看作是一个对象，每一个对象拥有不同的处理逻辑，将一个请求从链式的首端发出，沿着链的路径依次传递给每一个节点对象，直至有对象处理这个请求为止。我们将这种模式称为责任链模式。
责任链模式的定义
使多个对象都有机会处理请求，从而避免了请求的发送者和接收者之间的耦合关系。将这些对象连成一条链，并沿着这条链传递该请求，只到有对象处理它为止。
ViewGroup 的 dispatchTouchEvent
责任链模式在 Android 源码中比较类似的实现是 View 的 MotionEvent 事件分发。每当用户接触屏幕的时候，Android 都会将对应的事件包装成一个事件对象从 ViewTree 的顶部自上而下地分发传递。
ViewGroup 的 dispatchTouchEvent 方法如下：
public boolean dispatchTouchEvent(MotionEvent ev) {
...
// 对于辅助功能的事件处理
if (ev.isTargetAccessibilityFocus() && isAccessibilityFocusedViewOrHost()) {
ev.setTargetAccessibilityFocus(false);
}
boolean handled = false;
if (onFilterTouchEventForSecurity(ev)) {
final int action = ev.getAction();
final int actionMasked = action & MotionEvent.ACTION_MASK;
// 处理原始的 ACTION_DOWN 事件
if (actionMasked == MotionEvent.ACTION_DOWN) {
// 这里主要是在新事件开始时，处理完上一个事件
cancelAndClearTouchTargets(ev);
resetTouchState();
}
// 检查事件拦截
final boolean intercepted;
if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) {
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
if (!disallowIntercept) {
intercepted = onInterceptTouchEvent(ev);
// 恢复事件防止其改变
ev.setAction(action); // restore action in case it was changed
} else {
intercepted = false;
}
} else {
// There are no touch targets and this action is not an initial down
// so this view group continues to intercept touches.
intercepted = true;
}
// 如果事件被拦截了，则进行正常的事件分发
// If intercepted, start normal event dispatch. Also if there is already
// a view that is handling the gesture, do normal event dispatch.
if (intercepted || mFirstTouchTarget != null) {
ev.setTargetAccessibilityFocus(false);
}
// 检查事件是否取消
// Check for cancelation.
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL;
// 如果有必要的话，为 DOWN 事件检查所有的目标对象
// Update list of touch targets for pointer down, if needed.
final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
TouchTarget newTouchTarget = null;
boolean alreadyDispatchedToNewTouchTarget = false;
// 如果事件未被取消并未被拦截
if (!canceled && !intercepted) {
// 如果有辅助功能的参与，则直接将事件投递到对应的 View，否则将事件分发给所有的子 View
// If the event is targeting accessibility focus we give it to the
// view that has accessibility focus and if it does not handle it
// we clear the flag and dispatch the event to all children as usual.
// We are looking up the accessibility focused host to avoid keeping
// state since these events are very rare.
View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
? findChildWithAccessibilityFocus() : null;
// 如果事件为起始事件
if (actionMasked == MotionEvent.ACTION_DOWN
|| (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
final int actionIndex = ev.getActionIndex(); // always 0 for down
final int idBitsToAssign = split ? 1 << ev.getPointerId(actionIndex)
: TouchTarget.ALL_POINTER_IDS;
// Clean up earlier touch targets for this pointer id in case they
// have become out of sync.
removePointersFromTouchTargets(idBitsToAssign);
final int childrenCount = mChildrenCount;
// 如果 TouchTarget 为空并且子元素不为 0
if (newTouchTarget == null && childrenCount != 0) {
final float x = ev.getX(actionIndex);
final float y = ev.getY(actionIndex);
// 由上至下去寻找一个可以接收该事件的子 View
// Find a child that can receive the event.
// Scan children from front to back.
final ArrayList<View> preorderedList = buildTouchDispatchChildList();
final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled();
final View[] children = mChildren;
// 遍历子元素
for (int i = childrenCount - 1; i >= 0; i--) {
final int childIndex = getAndVerifyPreorderedIndex(
childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(
preorderedList, children, childIndex);
// If there is a view that has accessibility focus we want it
// to get the event first and if not handled we will perform a
// normal dispatch. We may do a double iteration but this is
// safer given the timeframe.
if (childWithAccessibilityFocus != null) {
if (childWithAccessibilityFocus != child) {
continue;
}
childWithAccessibilityFocus = null;
i = childrenCount - 1;
}
// 如果这个子元素无法接收 Pointer Event 或这个事件点压根就没有落在子元素的边界范围内，那么跳出该次循环继续遍历
if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) {
ev.setTargetAccessibilityFocus(false);
continue;
}
// 找到 Event 改由哪个子元素持有
newTouchTarget = getTouchTarget(child);
if (newTouchTarget != null) {
// Child is already receiving touch within its bounds.
// Give it the new pointer in addition to the ones it is handling.
newTouchTarget.pointerIdBits |= idBitsToAssign;
break;
}
resetCancelNextUpFlag(child);
// 投递事件执行触摸操作
// 如果子元素还是一个 ViewGroup，则递归调用重复此过程
// 如果子元素是一个 View，那么则会调用 View 的 dispatchTouchEvent，并最终由 onTouchEvent 处理
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
// 子 View 在其边界范围内接收事件
// Child wants to receive touch within its bounds.
mLastTouchDownTime = ev.getDownTime();
if (preorderedList != null) {
// childIndex points into presorted list, find original index
for (int j = 0; j < childrenCount; j++) {
if (children[childIndex] == mChildren[j]) {
mLastTouchDownIndex = j;
break;
}
}
} else {
mLastTouchDownIndex = childIndex;
}
mLastTouchDownX = ev.getX();
mLastTouchDownY = ev.getY();
newTouchTarget = addTouchTarget(child, idBitsToAssign);
alreadyDispatchedToNewTouchTarget = true;
break;
}
// The accessibility focus didn't handle the event, so clear
// the flag and do a normal dispatch to all children.
ev.setTargetAccessibilityFocus(false);
}
if (preorderedList != null) preorderedList.clear();
}
// 如果发现没有子元素可以持有该次事件
if (newTouchTarget == null && mFirstTouchTarget != null) {
// Did not find a child to receive the event.
// Assign the pointer to the least recently added target.
newTouchTarget = mFirstTouchTarget;
while (newTouchTarget.next != null) {
newTouchTarget = newTouchTarget.next;
}
newTouchTarget.pointerIdBits |= idBitsToAssign;
}
}
}
// Dispatch to touch targets.
if (mFirstTouchTarget == null) {
// No touch targets so treat this as an ordinary view.
handled = dispatchTransformedTouchEvent(ev, canceled, null,
TouchTarget.ALL_POINTER_IDS);
} else {
// Dispatch to touch targets, excluding the new touch target if we already
// dispatched to it. Cancel touch targets if necessary.
TouchTarget predecessor = null;
TouchTarget target = mFirstTouchTarget;
while (target != null) {
final TouchTarget next = target.next;
if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
handled = true;
} else {
final boolean cancelChild = resetCancelNextUpFlag(target.child)
|| intercepted;
if (dispatchTransformedTouchEvent(ev, cancelChild,
target.child, target.pointerIdBits)) {
handled = true;
}
if (cancelChild) {
if (predecessor == null) {
mFirstTouchTarget = next;
} else {
predecessor.next = next;
}
target.recycle();
target = next;
continue;
}
}
predecessor = target;
target = next;
}
}
// Update list of touch targets for pointer up or cancel, if needed.
if (canceled
|| actionMasked == MotionEvent.ACTION_UP
|| actionMasked == MotionEvent.ACTION_HOVER_MOVE) {
resetTouchState();
} else if (split && actionMasked == MotionEvent.ACTION_POINTER_UP) {
final int actionIndex = ev.getActionIndex();
final int idBitsToRemove = 1 << ev.getPointerId(actionIndex);
removePointersFromTouchTargets(idBitsToRemove);
}
}
if (!handled && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(ev, 1);
}
return handled;
}
ViewGroup 的 dispatchTransformedTouchEvent
这里我们主要看看 dispatchTransformedTouchEvent 方法是如何调度子元素 dispatchTouchEvent 方法的：
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) {
final boolean handled;
// Canceling motions is a special case. We don't need to perform any transformations
// or filtering. The important part is the action, not the contents.
final int oldAction = event.getAction();
// 如果事件被取消
if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL);
// 如果没有子元素
if (child == null) {
// 那么就直接调用父类的 dispatchTouchEvent，注意，这里的父类终会为 View 类
handled = super.dispatchTouchEvent(event);
} else {
// 如果有子元素则传递 cancel 事件
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction);
return handled;
}
// 计算即将被传递的点的数量
// Calculate the number of pointers to deliver.
final int oldPointerIdBits = event.getPointerIdBits();
final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;
// 如果事件没有相应的点，那么就丢弃该事件
// If for some reason we ended up in an inconsistent state where it looks like we
// might produce a motion event with no pointers in it, then drop the event.
if (newPointerIdBits == 0) {
return false;
}
// 声明临时变量保存坐标转换后的 MotionEvent
// If the number of pointers is the same and we don't need to perform any fancy
// irreversible transformations, then we can reuse the motion event for this
// dispatch as long as we are careful to revert any changes we make.
// Otherwise we need to make a copy.
final MotionEvent transformedEvent;
// 如果事件点的数量一致
if (newPointerIdBits == oldPointerIdBits) {
// 子元素为空或子元素有一个单位矩阵
if (child == null || child.hasIdentityMatrix()) {
// 再次区分子元素为空的情况
if (child == null) {
// 为空则调用父类 dispatchTouchEvent
handled = super.dispatchTouchEvent(event);
} else {
// 否则尝试获取 xy 方向上的偏移量（如果通过 scrollTo 或者 scrollBy 对子视图进行滚动的话）
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
// 将 MotionEvent 进行坐标变换
event.offsetLocation(offsetX, offsetY);
// 再将变换后的 MotionEvent 传递给子元素
handled = child.dispatchTouchEvent(event);
// 复位 MotionEvent 以便之后再次使用
event.offsetLocation(-offsetX, -offsetY);
}
// 如果通过以上的逻辑判断，当前事件被持有则可以直接返回
return handled;
}
transformedEvent = MotionEvent.obtain(event);
} else {
transformedEvent = event.split(newPointerIdBits);
}
// 下述是一样的不再赘述
// Perform any necessary transformations and dispatch.
if (child == null) {
handled = super.dispatchTouchEvent(transformedEvent);
} else {
final float offsetX = mScrollX - child.mLeft;
final float offsetY = mScrollY - child.mTop;
transformedEvent.offsetLocation(offsetX, offsetY);
if (! child.hasIdentityMatrix()) {
transformedEvent.transform(child.getInverseMatrix());
}
handled = child.dispatchTouchEvent(transformedEvent);
}
// Done.
transformedEvent.recycle();
return handled;
}
ViewGroup 事件投递的递归调用就类似于一条责任链，一旦其寻找到责任者，那么将由责任者持有并消费掉该次事件，具体地体现再 View 地 onTouchEvent 方法中返回值的设置，如果 onTouchEvent 返回 false，那么意味着当前 View 不会是该次事件的责任人，将不会对其持有；如果为 true 则相反，此时 View 会持有该事件并不再向外传递。
