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  • Android 事件分发与责任链模式

    一、责任链模式

    责任链模式是一种行为模式,为请求创建一个接收者的对象链.这样就避免,一个请求链接多个接收者的情况.进行外部解耦.类似于单向链表结构。

    优点:

    1. 降低耦合度。它将请求的发送者和接收者解耦。

    2. 简化了对象。使得对象不需要知道链的结构。

    3. 增强给对象指派职责的灵活性。通过改变链内的成员或者调动它们的次 序,允许动态地新增或者删除责任。

    4. 增加新的请求处理类很方便。

    缺点:

    1. 不能保证请求一定被接收。

    2. 系统性能将受到一定影响,而且在进行代码调试时不太方便,可能会造成循环调用。

    3. 可能不容易观察运行时的特征,有碍于除错。

    在Android事件分发机制是责任链模式最典型的应用:

    dispatchTouchEvent的,就是责任链中的将事件交给下一级处理的.

    onInterceptTouchEvent ,就是责任链中,处理自己处理事务的方法.

    onTouchEvent 是责任链中 事件上报的事件链。

    下面我们来通过Android事件分发机制来感悟一下责任链模式在Android中的应用。

    二、Android 事件分发传递机制

    1. View事件传递分发层级结构

     a). 事件收集之后最先传递给 Activity, 然后依次向下传递,大致如下:

    Activity -> PhoneWindow -> DecorView -> ViewGroup -> ... -> View

    这样的事件分发机制逻辑非常清晰,可是,你是否注意到一个问题?如果最后分发到View,如果这个View也没有处理事件怎么办,就这样让事件浪费掉?当然不会啦。

     b). 如果没有任何View消费掉事件,那么这个事件会按照反方向回传,最终传回给Activity,如果最后 Activity 也没有处理,本次事件才会被抛弃:

    Activity <- PhoneWindow <- DecorView <- ViewGroup <- ... <- View

    可以看到,这是一个非常经典的责任链模式,如果我能处理就拦截下来自己干,如果自己不能处理或者不确定就交给责任链中下一个对象。 这种设计是非常精巧的,上层View既可以直接拦截该事件,自己处理,也可以先询问(分发给)子View,如果子View需要就交给子View处理,如果子View不需要还能继续交给上层View处理。既保证了事件的有序性,又非常的灵活。

    View点击事件分发有三个关键流程方法:

    1.dispatchTouchEvent:事件下发 --- View和ViewGroup都有的方法

    2.onInterceptTouchEvent:拦截下发的事件,并交给自己OnTouchEvent处理处理 ---ViewGroup才有的方法

    3.onTouchEvent:事件上报 --- View和ViewGroup都有的方法

    以下是不同层级对事件的分发、拦截和消费的功能表:

    可以看到 Activity 和 View 都是没有事件拦截的:

    a). Activity 作为原始的事件分发者,如果 Activity 拦截了事件会导致整个屏幕都无法响应事件,这肯定不是我们想要的效果。

    b). View最为事件传递的最末端,要么消费掉事件,要么不处理进行回传,根本没必要进行事件拦截。

    下图是点击View,事件传递但是都没有被处理,生成的一个完整的事件分发流程图:

    如果事件被View处理了,那么事件分发流程图应该如下:

    如果事件被ViewGroup拦截处理了, 那么事件分发流程图应该如下:

    从上面的流程,我们可以概括Android的事件分发机制为:责任链模式,事件层层传递,直到被消费。

    三、Q&A

    上面我们讲解了一下Android的事件分发机制,可能很多人会有疑惑,下面我们针对部分疑惑进行分析和说明:

    1. 为什么 View 会有 dispatchTouchEvent ?

    答:我们知道 View 可以注册很多事件监听器,例如:单击事件(onClick)、长按事件(onLongClick)、触摸事件(onTouch),并且View自身也有 onTouchEvent 方法,那么问题来了,这么多与事件相关的方法应该由谁管理?毋庸置疑就是 dispatchTouchEvent,所以 View 也会有事件分发。

    View的dispatchTouchEvent源码:

    /**
         * Pass the touch screen motion event down to the target view, or this
         * view if it is the target.
         *
         * @param event The motion event to be dispatched.
         * @return True if the event was handled by the view, false otherwise.
         */
        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)) {
                if ((mViewFlags & ENABLED_MASK) == ENABLED && handleScrollBarDragging(event)) {
                    result = true;
                }
                //noinspection SimplifiableIfStatement
                ListenerInfo li = mListenerInfo;
                if (li != null && li.mOnTouchListener != null
                        && (mViewFlags & ENABLED_MASK) == ENABLED
                        && li.mOnTouchListener.onTouch(this, event)) {
                    result = true;
                }
    
                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;
        }
    View Code

    2. View事件分发时各个方法调用顺序是怎样的?

    a). 单击事件(onClickListener) 需要两个两个事件(ACTION_DOWN 和 ACTION_UP )才能触发,如果先分配给onClick判断,等它判断完再交由其他相应时间显然是不合理的,会造成 View 无法响应其他事件,应该最后调用。(所以此调用顺序最后)

    b). 长按事件(onLongClickListener) 同理,也是需要长时间等待才能出结果,肯定不能排到前面,但因为不需要ACTION_UP,应该排在 onClick 前面。(onLongClickListener > onClickListener)

    c). 触摸事件(onTouchListener) 如果用户注册了触摸事件,说明用户要自己处理触摸事件了,这个应该排在最前面。(最前)

    d). View自身处理(onTouchEvent) 提供了一种默认的处理方式,如果用户已经处理好了,也就不需要了,所以应该排在 onClickListener 后面。(onTouchListener > onClickListener)

    所以事件的调度顺序应该是 onTouchListener > onTouchEvent > onLongClickListener > onClickListener

    3. ViewGroup 的事件分发流程又是如何的呢?

    在默认的情况下 ViewGroup 事件分发流程是这样的。

    a). 判断自身是否需要(询问 onInterceptTouchEvent 是否拦截),如果需要,调用自己的 onTouchEvent。

    b). 自身不需要或者不确定,则询问 ChildView ,一般来说是调用手指触摸位置的 ChildView。

    c). 如果子 ChildView 不需要则调用自身的 onTouchEvent。

    ViewGroup的dispatchTouchEvent源码:

    @Override
    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.
            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) {
                    final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                    if (!disallowIntercept) {
                        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.
                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;
                if (!canceled && !intercepted) {
    
                    // 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;
    
                    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;
                        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.
                            final ArrayList<View> preorderedList = buildTouchDispatchChildList();
                            final boolean customOrder = preorderedList == null
                                    && isChildrenDrawingOrderEnabled();
                            final View[] children = mChildren;
                            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;
                                }
    
                                if (!canViewReceivePointerEvents(child)
                                        || !isTransformedTouchPointInView(x, y, child, null)) {
                                    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.
                                    newTouchTarget.pointerIdBits |= idBitsToAssign;
                                    break;
                                }
    
                                resetCancelNextUpFlag(child);
                                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();
                        }
    
                        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;
        }
    View Code

    可能看这么长的代码,我们会比较懵,下面我们用伪代码来表示一下:

    public boolean dispatchTouchEvent(MotionEvent ev) {
         boolean consume = false;
         if (onInterceptTouchEvent(ev)) {
              consume = onTouchEvent(ev);
         } else {
              consume = child.dispatchTouchEvent(ev)
         }
         return consume;
    }

    这样我们就能更直白的看懂View事件的传递机制了。

    4. ViewGroup将事件分发给ChildView的机制

    • ViewGroup分发事件时会遍历 ChildView,如果手指触摸的点在 ChildView 区域内就分发给这个View。当 ChildView 重叠时,一般会分配给显示在最上面的 ChildView。
    • ViewGroup判断是否需要拦截,主要是根据onInterruptTouchEvent的返回值进行判断。
    • 在Down事件中将touch事件分发给ChildView,如果有ChildView捕获消费了Down事件,就会对mFirstTouchTarget进行赋值。mFirstTouchTarget的作用就是记录消费事件的View。
    • 在ViewGroup的dispatchTouchEvent方法中,会根据mFirstTouchTarget 是否为 null,决定是自己处理 touch 事件,还是分发给子 View。
    • Down事件是touch事件序列的起点,决定了后续的事件由谁来消费处理。Cancel事件的触发场景为:父View先不拦截,但在MOVE事件中又重新拦截,此时子View会收到一个Cancel事件,

    5. ViewGroup 和 ChildView 同时注册了事件监听器(onClick等),哪个会执行?

    事件优先给 ChildView,会被 ChildView消费掉,ViewGroup 不会响应。

    四、参考资料

    1. Android事件传递机制分析

    2. Android 事件分发机制详解

    3. 安卓自定义View进阶-事件分发机制原理

    4. 必问的事件分发,你答得上来吗

     

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  • 原文地址:https://www.cnblogs.com/renhui/p/12127680.html
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