Chain 責任鏈模式 | 解說實現 | Android Framework View 事件傳遞

● LinkedItem 有三個責任

A. 對外開放一個請求處理入口（ handleRequest ）

B. 設定鏈的方式 (setNext)

C. 具體的請求者的抽象方法

● 抽象定義能處理的級別

● 抽象定義每個子類處理的方式

● 這裡的重點是使用 依賴倒置 概念，將 請求、處理 抽象化

● 處理者不用知道請求的全貌，只須關注部份（關注自己能不能處理、處理方式）

● 請求者也無須知道是哪個類處理的，只須關注結果即可

可以 在 setNext 時設置一個閥值，避免無意識的破壞調系統性能（不好查）

// ViewGroup.java

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {

    ...

    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) {

            // 在新事件開始時，先處理上一個事件 (透過 Cancel、Clear)
            cancelAndClearTouchTargets(ev);

            resetTouchState();
        }

        ...
    }

    ...
}

● cancelAndClearTouchTargets 方法：透過 TouchTarget Linked 將 cancel 事件傳遞給所有 view

// ViewGroup.java

private void cancelAndClearTouchTargets(MotionEvent event) {
    if (mFirstTouchTarget != null) {
        boolean syntheticEvent = false;
        if (event == null) {
            final long now = SystemClock.uptimeMillis();
            event = MotionEvent.obtain(now, now,
                    MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
            event.setSource(InputDevice.SOURCE_TOUCHSCREEN);
            syntheticEvent = true;
        }

        for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) {
            resetCancelNextUpFlag(target.child);

            // 第二個參數 true, 代表 cacel 事件
            // 傳遞所有 view cancel 事件
            dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits);
        }
        clearTouchTargets();

        if (syntheticEvent) {
            event.recycle();
        }
    }
}

● resetTouchState 方法：清除 FLAG_DISALLOW_INTERCEPT FLAG：不同意 ViewGroup 中斷

// ViewGroup.java

private TouchTarget mFirstTouchTarget;

private void resetTouchState() {
    clearTouchTargets();
    resetCancelNextUpFlag(this);
    mGroupFlags &= ~FLAG_DISALLOW_INTERCEPT;
    mNestedScrollAxes = SCROLL_AXIS_NONE;
}

// 透過該方法將 mFirstTouchTarget 設置為 null
private void clearTouchTargets() {
    TouchTarget target = mFirstTouchTarget;
    if (target != null) {
        do {
            TouchTarget next = target.next;
            target.recycle();
            target = next;
        } while (target != null);
        mFirstTouchTarget = null;
    }
}

// ViewGroup.java

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {

    ...

    boolean handled = false;
    if (onFilterTouchEventForSecurity(ev)) {

        ... 清除上一個點擊事件

        // 檢查中斷
        final boolean intercepted;
        if (actionMasked == MotionEvent.ACTION_DOWN    // 成立
                || mFirstTouchTarget != null) {        // 不成立

            // 判斷 viewgroup flag 是否不允許中斷
            final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;

            // 沒有禁止中斷
            if (!disallowIntercept) {
                // 呼叫 onInterceptTouchEvent 方法
                intercepted = onInterceptTouchEvent(ev);
                ev.setAction(action); // restore action in case it was changed
            } else {
                intercepted = false;
            }
        } else {
            // 並非下壓事件 & 有 childView 已經在處理事件
            intercepted = true;
        }

        ...
    }
}     

// ViewGroup.java

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {

    ...

    boolean handled = false;
    if (onFilterTouchEventForSecurity(ev)) {
        ...

        // 檢查事件是否被取消
        final boolean canceled = resetCancelNextUpFlag(this)
                || actionMasked == MotionEvent.ACTION_CANCEL;

        // Update list of touch targets for pointer down, if needed.
        // 如果有需要的話需要對所有 down 事件檢查
        final boolean isMouseEvent = ev.getSource() == InputDevice.SOURCE_MOUSE;
        final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0
                && !isMouseEvent;

這裡的遞迴操作，就是遍歷二元樹的操作

// ViewGroup.java

// ViewGroup 的 ChildView 數量
private int mChildrenCount;    

// ChildView 聚集
private View[] mChildren;

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {

    ...

    boolean handled = false;
    if (onFilterTouchEventForSecurity(ev)) {
        ...

        if (!canceled && !intercepted) {    // 成立

            // 檢查事件是否有需要分配到其他 View 上
            View childWithAccessibilityFocus = ev.isTargetAccessibilityFocus()
                    ? findChildWithAccessibilityFocus() : null;

            if (actionMasked == MotionEvent.ACTION_DOWN    // 成立
                    || (split && actionMasked == MotionEvent.ACTION_POINTER_DOWN)
                    || actionMasked == MotionEvent.ACTION_HOVER_MOVE) { // 多指觸控

                // 如果是 ACTION_DOWN 的話就是 0
                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);

                // 取得目前 ChildeView 的數量
                final int childrenCount = mChildrenCount;
                if (newTouchTarget == null && childrenCount != 0) {// 成立
                    final float x =
                            isMouseEvent ? ev.getXCursorPosition() : ev.getX(actionIndex);
                    final float y =
                            isMouseEvent ? ev.getYCursorPosition() : ev.getY(actionIndex);
                    // 由上到下找尋可以接收該事件的 ChildView
                    final ArrayList<View> preorderedList = buildTouchDispatchChildList();


                    final boolean customOrder = preorderedList == null
                            && isChildrenDrawingOrderEnabled();

                    final View[] children = mChildren;
                    // 跌代所有 ChildView
                    for (int i = childrenCount - 1; i >= 0; i--) {
                        final int childIndex = getAndVerifyPreorderedIndex(
                                childrenCount, i, customOrder);
                        final View child = getAndVerifyPreorderedView(
                                preorderedList, children, childIndex);

                        ...

                        // canReceivePointerEvents，ChildView 無法收到該事件
                        // isTransformedTouchPointInView，ChildView 無法接都到該事件
                        if (!child.canReceivePointerEvents()
                                || !isTransformedTouchPointInView(x, y, child, null)) {
                            ev.setTargetAccessibilityFocus(false);
                            continue;
                        }

                        // 取得 處理事件的 View
                        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);

                        // 往下分發事件，這邊如果 ChildView 
                        // 1. ViewGroup: 遞迴呼叫 dispatchTransformedTouchEvent 方法
                        // 2. View: 調用 View#dispatchTouchEvent，最終由 onTouchEvent 處理
                        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();
                            
                            // 處理完事件後，賦予 newTouchTarget
                            // @ 查看 addTouchTarget 方法
                            newTouchTarget = addTouchTarget(child, idBitsToAssign);
                            // 標註以分發事件完成
                            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);

                    } // end for

                    if (preorderedList != null) preorderedList.clear();
                } // end if

                // 跌代完所有的 ChildView，但沒有 View 處理該事件
                if (newTouchTarget == null && mFirstTouchTarget != null) {

                    // 指定 pointer 給最後添加的 View
                    newTouchTarget = mFirstTouchTarget;
                    while (newTouchTarget.next != null) {
                        newTouchTarget = newTouchTarget.next;
                    }
                    newTouchTarget.pointerIdBits |= idBitsToAssign;
                }
            }
        }
        
    }
}


// 賦予 mFirstTouchTarget
private TouchTarget addTouchTarget(@NonNull View child, int pointerIdBits) {
    final TouchTarget target = TouchTarget.obtain(child, pointerIdBits);
    target.next = mFirstTouchTarget;
    mFirstTouchTarget = target;
    return target;
}

// ViewGroup.java

// ViewGroup 的 ChildView 數量
private int mChildrenCount;    

// ChildView 聚集
private View[] mChildren;

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {

    ...

    boolean handled = false;
    if (onFilterTouchEventForSecurity(ev)) {
        ...

        if (!canceled && !intercepted) {    // 成立

            ... 結束事件分發
        }

        // 目前沒有 View 處理事件
        if (mFirstTouchTarget == null) {
            // 視為普通視圖 (最終會傳遞到當前 ViewGroup 的 View)
            handled = dispatchTransformedTouchEvent(ev, canceled, null,
                    TouchTarget.ALL_POINTER_IDS);
        } else {
            // 分發到點擊的 View，如果已經分發，有需要的排除其他點擊目標
            TouchTarget predecessor = null;
            TouchTarget target = mFirstTouchTarget;
            while (target != null) {
                final TouchTarget next = target.next;
                
                // 判斷是否已經分發事件
                if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                    // 已分發
                    handled = true;
                } else {
                    // 是否取消事件
                    final boolean cancelChild = resetCancelNextUpFlag(target.child)
                            || intercepted;
                    
                    // 分發事件
                    if (dispatchTransformedTouchEvent(ev, cancelChild,
                            target.child, target.pointerIdBits)) {
                        handled = true;
                    }
                    
                    // 如果是 Cacncel 事件則轉移 mFirstTouchTarget
                    if (cancelChild) {
                        if (predecessor == null) {
                            mFirstTouchTarget = next;
                        } else {
                            predecessor.next = next;
                        }
                        target.recycle();
                        target = next;
                        continue;
                    }
                }
                predecessor = target;
                target = next;
            }
        }

        ...
    }

    ...
    return handled;
}

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
        View child, int desiredPointerIdBits) {
    final boolean handled;


    final int oldAction = event.getAction();
    // 判斷事件是否取消
    if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
        event.setAction(MotionEvent.ACTION_CANCEL);
        
        // 沒有目標 View
        if (child == null) {
            // 傳給自身的 ViewGroup 的 View
            handled = super.dispatchTouchEvent(event);
        } else {
            handled = child.dispatchTouchEvent(event);
        }
        event.setAction(oldAction);
        return handled;
    }
    ...

    return handle;
}

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
        View child, int desiredPointerIdBits) {
    final boolean handled;

    ...

    // 計算將被傳遞的 pointer 數量
    final int oldPointerIdBits = event.getPointerIdBits();
    final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

    // 如果沒有點，則直接返回
    if (newPointerIdBits == 0) {
        return false;
    }

    ...
}

● 偏移量

如果有調用 scrollTo or scrollBy 對 ChildView 進行滾動，就會產生 xy 的偏移量

private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel,
        View child, int desiredPointerIdBits) {
    final boolean handled;

    ...

    // 保存座標轉換後的 MotionEvent
    final MotionEvent transformedEvent;

    // 點擊數量一致
    if (newPointerIdBits == oldPointerIdBits) {
        if (child == null || child.hasIdentityMatrix()) {
            if (child == null) {
                // 呼叫 ViewGroup 的 View
                handled = super.dispatchTouchEvent(event);
            } else {
                // 計算 x,y 偏移量
                final float offsetX = mScrollX - child.mLeft;
                final float offsetY = mScrollY - child.mTop;

                // 設定轉換後的 offset 目標
                event.offsetLocation(offsetX, offsetY);

                // 分發事件 到 ChildView
                handled = child.dispatchTouchEvent(event);

                // 復位
                event.offsetLocation(-offsetX, -offsetY);
            }
            return handled;
        }
        transformedEvent = MotionEvent.obtain(event);
    } else {
        transformedEvent = event.split(newPointerIdBits);
    }
    
    ...
}


觸摸數量不相同*：同上，最終都是透過 dispatchTouchEvent 方法分發事件