public boolean onTouchEvent(MotionEvent ev) 
    {
        if(OWL_DEBUG) Log.d(OWL, "onTouchEvent  entering..");
　　　　 //........switch (action & MotionEvent.ACTION_MASK) 
        {
　　　　 //.........此處省略ACTION_DOWN和ACTION_MOVE
        case MotionEvent.ACTION_UP:
            if(OWL_DEBUG) Log.d(OWL, "
 
onTouchEvent  ACTION_UP..");
            if (mTouchState == TOUCH_STATE_SCROLLING) 
            {
                if(OWL_DEBUG) Log.d(OWL, "onTouchEvent  ACTION_UP.. TOUCH_STATE_SCROLLING");
                final int activePointerId = mActivePointerId;
                final int pointerIndex = ev.findPointerIndex(activePointerId);
                final 
 
float x = ev.getX(pointerIndex);
                final VelocityTracker velocityTracker = mVelocityTracker;
                velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
                //橫向速率
                int velocityX = (int) velocityTracker.getXVelocity(activePointerId);
                
 
//移動距離
                final int deltaX = (int) (x - mDownMotionX);
                //頁面寬
                final int pageWidth = getScaledMeasuredWidth(getPageAt(mCurrentPage));
                //移動距離超過頁面寬40%
                boolean isSignificantMove = Math.abs(deltaX) > pageWidth *SIGNIFICANT_MOVE_THRESHOLD;
                final int snapVelocity = mSnapVelocity;
                
                mTotalMotionX += Math.abs(mLastMotionX + mLastMotionXRemainder - x);
                //根據滑動距離和速率，判斷是否是滑動
                boolean isFling = mTotalMotionX > MIN_LENGTH_FOR_FLING && Math.abs(velocityX) > snapVelocity;
// 這鐘情況是頁面朝一個方向移動了一段距離，然後又彈回去了。
                //我們使用一個閥值來判斷是進行翻頁還是返回到初始頁面 
                boolean returnToOriginalPage = false;
                //Math.signum 函式將正數轉換為 1.0，將負數轉換為 -1.0，0 仍然是 0。 實際上，它只是提取一個數的符號
                if (Math.abs(deltaX) > pageWidth * RETURN_TO_ORIGINAL_PAGE_THRESHOLD &&
                        Math.signum(velocityX) != Math.signum(deltaX) && isFling) 
                {
                    //返回當前頁面
                    returnToOriginalPage = true;
                }

                int finalPage;//頁面朝左移動 
                if (((isSignificantMove && deltaX > 0 && !isFling) ||
                        (isFling && velocityX > 0)) && mCurrentPage > 0) 
                {
                    if(OWL_DEBUG) Log.d(OWL, "onTouchEvent  ACTION_UP.. TOUCH_STATE_SCROLLING LeftSnap");
                    finalPage = returnToOriginalPage ? mCurrentPage : mCurrentPage - 1;
                    snapToPageWithVelocity(finalPage, velocityX);
                } 
                //頁面朝右移動 
                else if (((isSignificantMove && deltaX < 0 && !isFling) ||
                        (isFling && velocityX < 0)) && mCurrentPage < getChildCount() - 1) 
                {
                    if(OWL_DEBUG) Log.d(OWL, "onTouchEvent  ACTION_UP.. TOUCH_STATE_SCROLLING RightSnap");
                    finalPage = returnToOriginalPage ? mCurrentPage : mCurrentPage + 1;
                    snapToPageWithVelocity(finalPage, velocityX);
                } 
                else 
                {
                    if(OWL_DEBUG) Log.d(OWL, "onTouchEvent  ACTION_UP.. TOUCH_STATE_SCROLLING Back Origation");
                    //尋找離螢幕中心最近的頁面移動 
                    snapToDestination();
                }
            } 
            else if (mTouchState == TOUCH_STATE_PREV_PAGE) 
            {
                if(OWL_DEBUG) Log.d(OWL, "onTouchEvent  ACTION_UP.. TOUCH_STATE_PREV_PAGE");
                //直接切換到上一頁，沒有進行滑動
                int nextPage = Math.max(0, mCurrentPage - 1);
                if (nextPage != mCurrentPage) {
                    snapToPage(nextPage);
                } else {
                    snapToDestination();
                }
            } 
            else if (mTouchState == TOUCH_STATE_NEXT_PAGE) 
            {
                if(OWL_DEBUG) Log.d(OWL, "onTouchEvent  ACTION_UP.. TOUCH_STATE_NEXT_PAGE");
                //直接切換到下一頁
                int nextPage = Math.min(getChildCount() - 1, mCurrentPage + 1);
                if (nextPage != mCurrentPage) {
                    snapToPage(nextPage);
                } else {
                    snapToDestination();
                }
            } 
            else 
            {
                if(OWL_DEBUG) Log.d(OWL, "onTouchEvent  ACTION_UP.. onUnhandledTap()");
　　　　　　　　　//這裡是空回撥，繼承PagedView的方法，會過載這方法處理一些操作。
                onUnhandledTap(ev);
            }
            mTouchState = TOUCH_STATE_REST;
            mActivePointerId = INVALID_POINTER;
            releaseVelocityTracker();
            break;return true;
    }