1、职责链模式概述：
职责链模式(Chain of Responsibility Pattern)：避免请求发送者与接收者耦合在一起，让多个对象都有可能接收请求，将这些对象连接成一条链，并且沿着这条链传递请求，直到有对象处理它为止。职责链模式是一种对象行为型模式。《设计模式的艺术》
使用场景：
有企业OA系统开发经验的同学，会对职责链模式有比较深刻的理解。在多数OA系统中都有请假、物资申请等各种审批流程，这些流程比较复杂，一个请求对应着多个处理者，而且不同角色的处理者对应的层级不同。因此大多数设计人员都会采用职责链模式来进行流程设计，降低系统耦合性，提升扩展性。
2、策略模式UML类图：
image.png
Handler(抽象处理类)：作为所有处理者的父类，定义处理者的公共接口，并且持有一个后继节点处理者Handler的对象引用；
ConcreteHandler(具体处理类)：继承抽象处理类，实现处理请求的方法。
Android事件分发剖析
image.png
上图展示了一个事件从输入到最终处理的一个基本流程，其中省略了window这一层，事实上事件的分发顺序为：Activity--->PhoneWindow--->DecorView---->具体布局。图中出现了3个方法：dispatchTouchEvent、onInterceptTouchEvent、onTouchEvent，这三个方法负责完成一个事件的分发。Android系统通过职责链模式实现事件分发功能，自顶向下遍历所有的处理者，直至找到一个处理者处理事件，或者都不处理。Activity、Window、ViewGroup、叶子View存在层次，事件会按照上述层次进行传递。当然，View体系也运用了组合模式进行设计，本篇文章只分析事件分发机制。下面分析上述3个方法的源码：
View类：
public boolean dispatchTouchEvent(MotionEvent event) { // If the event should be handled by accessibility focus first.
if (event.isTargetAccessibilityFocus()) { // We don't have focus or no virtual descendant has it, do not handle the event.
if (!isAccessibilityFocusedViewOrHost()) { return false;
} // We have focus and got the event, then use normal event dispatch.
event.setTargetAccessibilityFocus(false);
} boolean result = false; if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(event, 0);
} final int actionMasked = event.getActionMasked(); if (actionMasked == MotionEvent.ACTION_DOWN) { // Defensive cleanup for new gesture
stopNestedScroll();
} if (onFilterTouchEventForSecurity(event)) { //noinspection SimplifiableIfStatement
ListenerInfo li = mListenerInfo; if (li != null && li.mOnTouchListener != null
&& (mViewFlags & ENABLED_MASK) == ENABLED
&& li.mOnTouchListener.onTouch(this, event)) {
result = true;
} //View的dispatchTouchEvent方法若是正常流程一定会调用onTouchEvent方法
if (!result && onTouchEvent(event)) {
result = true;
}
} if (!result && mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
} // Clean up after nested scrolls if this is the end of a gesture;
// also cancel it if we tried an ACTION_DOWN but we didn't want the rest
// of the gesture.
if (actionMasked == MotionEvent.ACTION_UP ||
actionMasked == MotionEvent.ACTION_CANCEL ||
(actionMasked == MotionEvent.ACTION_DOWN && !result)) {
stopNestedScroll();
} return result;
}ViewGroup类：
public boolean dispatchTouchEvent(MotionEvent ev) { if (mInputEventConsistencyVerifier != null) {
mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
} // If the event targets the accessibility focused view and this is it, start
// normal event dispatch. Maybe a descendant is what will handle the click.
//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; // Handle an initial down.
if (actionMasked == MotionEvent.ACTION_DOWN) { // Throw away all previous state when starting a new touch gesture.
// The framework may have dropped the up or cancel event for the previous gesture
// due to an app switch, ANR, or some other state change.
cancelAndClearTouchTargets(ev);
resetTouchState();
} // Check for interception.
final boolean intercepted; if (actionMasked == MotionEvent.ACTION_DOWN
|| mFirstTouchTarget != null) { //down事件处理
final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { //disallowIntercept默认是false，意味着肯定会调用onInterceptTouchEvent方法
intercepted = onInterceptTouchEvent(ev);
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;
} // 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);
} // Check for cancelation.(检查是否被取消)
final boolean canceled = resetCancelNextUpFlag(this)
|| actionMasked == MotionEvent.ACTION_CANCEL; // 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; //事件未被取消、拦截
if (!canceled && !intercepted) { // If the event is targeting accessiiblity 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; 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; //newTouchTarget为空，并且子View个数不为0
if (newTouchTarget == null && childrenCount != 0) { final float x = ev.getX(actionIndex); final float y = ev.getY(actionIndex); // Find a child that can receive the event.
// Scan children from front to back.（由上至下扫描所有能接收该事件的子View）
final ArrayList<View> preorderedList = buildOrderedChildList(); final boolean customOrder = preorderedList == null
&& isChildrenDrawingOrderEnabled(); final View[] children = mChildren; //遍历所有的子View，寻找事件处理者
for (int i = childrenCount - 1; i >= 0; i--) { final int childIndex = customOrder
? getChildDrawingOrder(childrenCount, i) : i; final View child = (preorderedList == null)
? children[childIndex] : preorderedList.get(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;
} if (!canViewReceivePointerEvents(child)
|| !isTransformedTouchPointInView(x, y, child, null)) { //该子View无法处理，跳出本次循环继续遍历
ev.setTargetAccessibilityFocus(false); continue;
}
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.
//找到事件对应的处理者，推出循环
newTouchTarget.pointerIdBits |= idBitsToAssign; break;
}
resetCancelNextUpFlag(child); //真正分发事件的方法，若子View为ViewGroup则继续则递归调用，重复此过程
//若子View就是叶子View那么就调用该View的dispatchTouchEvent
if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) { // 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;
}private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
View child, int desiredPointerIdBits) { final boolean handled; //事件被取消
final int oldAction = event.getAction(); if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
event.setAction(MotionEvent.ACTION_CANCEL); //没有子元素
if (child == null) { //调用父类方法
handled = super.dispatchTouchEvent(event);
} else {
handled = child.dispatchTouchEvent(event);
}
event.setAction(oldAction); return handled;
} // Calculate the number of pointers to deliver.（计算即将传递点的数量）
final int oldPointerIdBits = event.getPointerIdBits(); final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits; // 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.
//由于某些原因，事件没有相应的点，那么丢弃该事件
if (newPointerIdBits == 0) { return false;
} // 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; if (newPointerIdBits == oldPointerIdBits) { if (child == null || child.hasIdentityMatrix()) { if (child == null) { //子元素为空，调用父类方法
handled = super.dispatchTouchEvent(event);
} else { final float offsetX = mScrollX - child.mLeft; final float offsetY = mScrollY - child.mTop;
event.offsetLocation(offsetX, offsetY);
handled = child.dispatchTouchEvent(event);
event.offsetLocation(-offsetX, -offsetY);
} //通过上述判断，若事件被持有则可以直接返回
return handled;
}
transformedEvent = MotionEvent.obtain(event);
} else {
transformedEvent = event.split(newPointerIdBits);
} // 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;
}上述源码可以看出，View事件分发机制，对于View而言，onTouch、onTouchEvent、onClick优先级依次递减。
4、优缺点分析：
优点：
1）将请求与处理者分离，请求事件无须知道具体由哪个处理者进行处理，降低系统耦合；
2）当系统需要增加新的处理者类时，无须需改原系统代码，符合开闭原则；
缺点：
1）由于请求者并不知道职责链配置规则，有可能出现请求不被任何处理者处理；
2）职责链建立不当，容易出现死循环；
结束语
职责链模式在软件设计中经常被用到，其使用场景特征比较明显。在使用职责链模式时，要划分好各个处理类的条件，切记不要出现死循环。
作者：Tifkingsly
链接：https://www.jianshu.com/p/88fc33c2807c