Android事件分發與責任鏈模式

1、責任鏈模式

責任鏈模式是一種行爲模式,爲請求建立一個接收者的對象鏈.這樣就避免,一個請求連接多個接收者的狀況.進行外部解耦.相似於單向鏈表結構。

優勢：

1. 下降耦合度。它將請求的發送者和接收者解耦。

2. 簡化了對象。使得對象不須要知道鏈的結構。

3. 加強給對象指派職責的靈活性。經過改變鏈內的成員或者調動它們的次 序，容許動態地新增或者刪除責任。

4. 增長新的請求處理類很方便。

缺點：

1. 不能保證請求必定被接收。

2. 系統性能將受到必定影響，並且在進行代碼調試時不太方便，可能會形成循環調用。

3. 可能不容易觀察運行時的特徵，有礙於除錯。

責任鏈的模式在事件分發場景方面的原理：

通常咱們理解的事件分發的模式以下（傳統模式）：

使用責任鏈模式直接將message丟到鏈中,讓他們本身匹配.

2、Android 事件分發傳遞機制

1. View事件傳遞分發層級結構

 a). 事件收集以後最早傳遞給 Activity， 而後依次向下傳遞，大體以下：

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

這樣的事件分發機制邏輯很是清晰，但是，你是否注意到一個問題？若是最後分發到View，若是這個View也沒有處理事件怎麼辦，就這樣讓事件浪費掉？固然不會啦。

 b). 若是沒有任何View消費掉事件，那麼這個事件會按照反方向回傳，最終傳回給Activity，若是最後 Activity 也沒有處理，本次事件纔會被拋棄:

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

能夠看到，這是一個很是經典的責任鏈模式，若是我能處理就攔截下來本身幹，若是本身不能處理或者不肯定就交給責任鏈中下一個對象。 這種設計是很是精巧的，上層View既能夠直接攔截該事件，本身處理，也能夠先詢問(分發給)子View，若是子View須要就交給子View處理，若是子View不須要還能繼續交給上層View處理。既保證了事件的有序性，又很是的靈活。

View點擊事件分發有三個關鍵流程方法:

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

2.onInterceptTouchEvent：攔截下發的事件,並交給本身OnTouchEvent處理處理 ---ViewGroup纔有的方法

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

如下是不一樣層級對事件的分發、攔截和消費的功能表：

能夠看到 Activity 和 View 都是沒有事件攔截的：

a). Activity 做爲原始的事件分發者，若是 Activity 攔截了事件會致使整個屏幕都沒法響應事件，這確定不是咱們想要的效果。

b). View最爲事件傳遞的最末端，要麼消費掉事件，要麼不處理進行回傳，根本不必進行事件攔截。

下圖是點擊View，事件傳遞可是都沒有被處理，生成的一個完整的事件分發流程圖：

若是事件被View處理了，那麼事件分發流程圖應該以下：

若是事件被ViewGroup攔截處理了， 那麼事件分發流程圖應該以下：

從上面的流程，咱們能夠歸納Android的事件分發機制爲：責任鏈模式，事件層層傳遞，直到被消費。

3、Q&A

上面咱們講解了一下Android的事件分發機制，可能不少人會有疑惑，下面咱們針對部分疑惑進行分析和說明：

1. 爲何 View 會有 dispatchTouchEvent ?

答：咱們知道 View 能夠註冊不少事件監聽器，例如：單擊事件(onClick)、長按事件(onLongClick)、觸摸事件(onTouch)，而且View自身也有 onTouchEvent 方法，那麼問題來了，這麼多與事件相關的方法應該由誰管理？毋庸置疑就是 dispatchTouchEvent，因此 View 也會有事件分發。

View的dispatchTouchEvent源碼：

/**
     * 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事件分發時各個方法調用順序是怎樣的？

a). 單擊事件(onClickListener) 須要兩個兩個事件(ACTION_DOWN 和 ACTION_UP )才能觸發，若是先分配給onClick判斷，等它判斷完再交由其餘相應時間顯然是不合理的，會形成 View 沒法響應其餘事件，應該最後調用。(因此此調用順序最後)

b). 長按事件(onLongClickListener) 同理，也是須要長時間等待才能出結果，確定不能排到前面，但由於不須要ACTION_UP，應該排在 onClick 前面。(onLongClickListener > onClickListener)

c). 觸摸事件(onTouchListener) 若是用戶註冊了觸摸事件，說明用戶要本身處理觸摸事件了，這個應該排在最前面。(最前)

d). View自身處理(onTouchEvent) 提供了一種默認的處理方式，若是用戶已經處理好了，也就不須要了，因此應該排在 onClickListener 後面。(onTouchListener > onClickListener)

因此事件的調度順序應該是 onTouchListener > onTouchEvent > onLongClickListener > onClickListener。

3. ViewGroup 的事件分發流程又是如何的呢？

在默認的狀況下 ViewGroup 事件分發流程是這樣的。

a). 判斷自身是否須要(詢問 onInterceptTouchEvent 是否攔截)，若是須要，調用本身的 onTouchEvent。

b). 自身不須要或者不肯定，則詢問 ChildView ，通常來講是調用手指觸摸位置的 ChildView。

c). 若是子 ChildView 不須要則調用自身的 onTouchEvent。

ViewGroup的dispatchTouchEvent源碼：

@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

4. ViewGroup將事件分發給ChildView的機制


ViewGroup分發事件時會遍歷 ChildView，若是手指觸摸的點在 ChildView 區域內就分發給這個View。當 ChildView 重疊時，通常會分配給顯示在最上面的 ChildView。

5. ViewGroup 和 ChildView 同時註冊了事件監聽器(onClick等)，哪一個會執行?

事件優先給 ChildView，會被 ChildView消費掉，ViewGroup 不會響應。

 

附：參考資料：

1. Android事件傳遞機制分析

2. Android 事件分發機制詳解

3. 安卓自定義View進階-事件分發機制原理