一、Android touch事件的相關概念

使用者的Touch事件被包裝成MotionEvent

使用者當前的touch事件主要型別有：

ACTION_DOWN: 表示使用者開始觸控.

 ACTION_MOVE: 表示使用者在移動(手指或者其他)

 ACTION_UP:表示使用者擡起了手指 

ACTION_OUTSIDE: 表示使用者觸碰超出了正常的UI邊界.

ACTION_POINTER_DOWN:有一個非主要的手指按下了.

ACTION_POINTER_UP:一個非主要的手指擡起來了

touch的位置

手指的個數

touch事件的時間

一個touch手勢被定義為以ACTION_DOWN開始和以 ACTION_UP結束。

二、Touch事件的處理流程

當用戶觸控式螢幕幕時，觸發Activity呼叫dispatchTouchEvent

事件物件會按自頂向下的順序在View Tree中傳遞

     父View（ViewGroups）會呼叫dispatchTouchEvent將Event傳遞給子View    

    Event在任何時候都可能被攔截

若某個View想處理touch事件，必須先消耗ACTION_DOWN。考慮到效率，後續的事件將不會向下傳遞。

若某個事件未被消耗，最後會被Activity的onTouchEvent()消耗

若任何View或ViewGroup設定了OnTouchListener，touch事件將被攔截。

 /**
     * Called to process touch screen events. You can override this to
     * intercept all touch screen events before they are dispatched to the
     * window. Be sure to call this implementation for touch screen events
     * that should be handled normally.
     *
     * @param ev The touch screen event.
     *
     * @return boolean Return true if this event was consumed.
     */
    public boolean dispatchTouchEvent(MotionEvent ev) {
        if (ev.getAction() == MotionEvent.ACTION_DOWN) {
            onUserInteraction();
        }
        if (getWindow().superDispatchTouchEvent(ev)) {
            return true;
        }
        return onTouchEvent(ev);
    }
 

ViewGroup.dispatchTouchEvent()的原始碼分析如下：

public boolean dispatchTouchEvent(MotionEvent ev) {
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(ev, 1);
        }
        boolean handled = false;
        if (onFilterTouchEventForSecurity(ev)) {
            final int action = ev.getAction();
            final int actionMasked = action & MotionEvent.ACTION_MASK;
            // 處理初始的down事件
            if (actionMasked == MotionEvent.ACTION_DOWN) {
                //當新開始一個touch事件時，拋棄先前的touch狀態
                //當app切換，發生ANR或一些其他的touch狀態發生時，framework會丟棄或取消先前的touch狀態
                cancelAndClearTouchTargets(ev);
                resetTouchState();
            }
            // 檢查是否進行事件攔截
            final boolean intercepted;
            if (actionMasked == MotionEvent.ACTION_DOWN
                    || mFirstTouchTarget != null) {
                final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
                if (!disallowIntercept) {
                    //回撥onInterceptTouchEvent()，返回false表示不攔截touch，否則攔截touch事件。
                    intercepted = onInterceptTouchEvent(ev);
                    ev.setAction(action); // restore action in case it was changed
                } else {
                    intercepted = false;
                }
            } else {
                //沒有touch事件的傳遞物件，同時touch動作不是初始動作down，所以ViewGroup繼續攔截事件
                intercepted = true;
            }
            // 檢查cancel事件
            final boolean canceled = resetCancelNextUpFlag(this)
                    || actionMasked == MotionEvent.ACTION_CANCEL;
            // 如果有第二個手指touch，更新touch目標列表。touch目標列表是一個View陣列
            final boolean split = (mGroupFlags & FLAG_SPLIT_MOTION_EVENTS) != 0;
            TouchTarget newTouchTarget = null;
            boolean alreadyDispatchedToNewTouchTarget = false;
            if (!canceled && !intercepted) {
                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);
                        // 找到一個能接受Event的子View，再對子View的View樹進行遍歷
                        final View[] children = mChildren;
                        final boolean customOrder = isChildrenDrawingOrderEnabled();
                        //判斷每個子View是否是TouchTarget，若是則新增到TouchTarget連結串列中
                        for (int i = childrenCount - 1; i >= 0; i--) {
                            final int childIndex = customOrder ?
                                    getChildDrawingOrder(childrenCount, i) : i;
                            final View child = children[childIndex];
                            if (!canViewReceivePointerEvents(child)
                                    || !isTransformedTouchPointInView(x, y, child, null)) {
                                continue;
                            }
                            newTouchTarget = getTouchTarget(child);
                            if (newTouchTarget != null) {
                                // 若子View處於touch目標中，同時已經接收了touch事件，則為器增加新的touch點
                                newTouchTarget.pointerIdBits |= idBitsToAssign;
                                break;
                            }
                            resetCancelNextUpFlag(child);
                            //把MotionEvent的點座標轉換到子View的座標系中，為ViewGroup建立一個新TouchTarget，TouchTarget包含了子View
                            if (dispatchTransformedTouchEvent(ev, false, child, idBitsToAssign)) {
                                // Child wants to receive touch within its bounds.
                                mLastTouchDownTime = ev.getDownTime();
                                mLastTouchDownIndex = childIndex;
                                mLastTouchDownX = ev.getX();
                                mLastTouchDownY = ev.getY();
                                newTouchTarget = addTouchTarget(child, idBitsToAssign);
                                alreadyDispatchedToNewTouchTarget = true;
                                break;
                            }
                        }
                    }
                    if (newTouchTarget == null && mFirstTouchTarget != null) {
                        // 沒有發現接收event的子View，把Touch點賦給最早新增到TouchTarget鏈中的物件
                        newTouchTarget = mFirstTouchTarget;
                        while (newTouchTarget.next != null) {
                            newTouchTarget = newTouchTarget.next;
                        }
                        newTouchTarget.pointerIdBits |= idBitsToAssign;
                    }
                }
            }
            // 傳遞給touch目標
            if (mFirstTouchTarget == null) {
                // 若沒有Touch目標，則把自己當成一個View，呼叫
                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;
                    //若已被處理，則忽略。
                    if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) {
                        handled = true;
                    } else {
                        final boolean cancelChild = resetCancelNextUpFlag(target.child)
                                || intercepted;
                        //傳遞給子View處理
                        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;
                }
            }
            // 若在觸控點發生了up或cancel，則更新TouchTarget連結串列
            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;
    }

 /**
     * Transforms a motion event into the coordinate space of a particular child view,
     * filters out irrelevant pointer ids, and overrides its action if necessary.
     * If child is null, assumes the MotionEvent will be sent to this ViewGroup instead.
     */
    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.
        // cancel動作是個特列，無需座標轉換或過濾。
        final int oldAction = event.getAction();
        if (cancel || oldAction == MotionEvent.ACTION_CANCEL) {
            event.setAction(MotionEvent.ACTION_CANCEL);
            if (child == null) {
                handled = super.dispatchTouchEvent(event);
            } else {
                handled = child.dispatchTouchEvent(event);
            }
            event.setAction(oldAction);
            return handled;
        }
        // 計算將被傳遞的點的數量。
        final int oldPointerIdBits = event.getPointerIdBits();
        final int newPointerIdBits = oldPointerIdBits & desiredPointerIdBits;

        // Motion事件沒有對應點，則丟棄這個Motion
        if (newPointerIdBits == 0) {
            return false;
        }

        /*若點的數量一致則無需進行不相關的點座標轉換，呼叫子View的dispatchTouchEvent*/
        // 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.
        /*該變數用於儲存座標轉換後的MoetionEvent*/
        final MotionEvent transformedEvent;
        if (newPointerIdBits == oldPointerIdBits) {
            if (child == null || child.hasIdentityMatrix()) {
                if (child == null) {
                    handled = super.dispatchTouchEvent(event);
                } else {
                    final float offsetX = mScrollX - child.mLeft;
                    final float offsetY = mScrollY - child.mTop;    
                    /*直接對MotionEvent進行座標變換，將MotionEvent傳遞下去*/
                    event.offsetLocation(offsetX, offsetY);
                    handled = child.dispatchTouchEvent(event);
                    /*回覆MotionEvent*/
                    event.offsetLocation(-offsetX, -offsetY);
                }
                return handled;
            }
            transformedEvent = MotionEvent.obtain(event);
        } else {
            transformedEvent = event.split(newPointerIdBits);
        }
        // Perform any necessary transformations and dispatch.
        if (child == null) {    
            /*呼叫父類即View的dispatchTouchEvent方法，該方法會呼叫onTouchEvent*/
            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());
            }
            /*傳遞給子View處理*/
            handled = child.dispatchTouchEvent(transformedEvent);
        }
        // Done.
        transformedEvent.recycle();
        return handled;
    }

  /**
     * 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 (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onTouchEvent(event, 0);
        }
        if (onFilterTouchEventForSecurity(event)) {
            //noinspection SimplifiableIfStatement
            ListenerInfo li = mListenerInfo;
            /*先呼叫listener介面*/
            if (li != null && li.mOnTouchListener != null && (mViewFlags & ENABLED_MASK) == ENABLED
                    && li.mOnTouchListener.onTouch(this, event)) {
                return true;
            }
           /*若MotionEvent未被消耗，則呼叫View的onTouchEvent *
            * ViewGroup中沒有定義onTouchEvent，故做後呼叫View中的onTouchEvent*/
            if (onTouchEvent(event)) {
                return true;
            }
        }
        if (mInputEventConsistencyVerifier != null) {
            mInputEventConsistencyVerifier.onUnhandledEvent(event, 0);
        }
        return false;
    }

小結：

onInterceptTouchEvent:

onInterceptTouchEvent是在ViewGroup裡面定義的，被ViewGroup.dispatchTouchEvent()呼叫，用於攔截所有的touch事件。預設返回false，表示不攔截touch事件，ViewGroup.dispatchTouchEvent()會呼叫子View的dispatchTouchEvent,將touch事件傳遞到子View中。若子View的dispatchTouchEvent 返回false，則ViewGroup的onTouchEvent會被呼叫；若子View的dispatchTouchEvent 返回true，表示消耗了手勢事件，ViewGroup的onTouchEvent則不會被呼叫。若ViewGroup.onInterceptTouchEvent()返回true，表示Touch事件被攔截，ViewGroup. dispatchTransformedTouchEvent()函式將被呼叫，該函式會呼叫super.dispatchTouchEvent(event)，即View的dispatchEvent()，該函式首先會呼叫View.OnTouchListener.onTouch().若listener未消耗Touch事件，則會呼叫View.onTouchEvent().  

onTouchEvent:

view中定義的方法onTouchEvent預設返回true,表示消耗了一個touch事件，ViewGroup中定義的onTouchEvent預設返回false，表示不處理Touch手勢事件。手勢事件型別包括ACTION_DOWN,ACTION_MOVE,ACTION_UP,ACTION_CANCEL等事件。