Android 4.0 Launcher源码分析系列

桌面的左右滑动功能主要是在PagedView类中实现的，而WorkSpace是PagedView类的子类，所以会继承PagedView中的方法。当我们的手指点击WorkSpace时，首先就会触发PageView中的onInterceptTouchEvent()方法，会根据相应的条件来判断是否对Touch事件进行拦截，如果onInterceptTouchEvent()方法返回为true，则会对Touch事件进行拦截，PageView类的onTouch方法会进行响应从而得到调用。如果返回false，就分两钟情况：(1)我们是点击在它的子控键上进行滑动时，比如我们是点击在桌面的图标上进行左右滑动的，workspace则会把Touch事件分发给它的子控件。(2)而如果仅仅是点击到桌面的空白出Touch事件就不会发生响应。 
  
在我们手指第一次触摸到屏幕时，首先会对onInterceptTouchEvent中的事件进行判断，如果是按下事件(MotionEvent.ACTION_DOWN), 则会记录按下时的X坐标、Y坐标等等数据，同时改变现在Workspace的状态为滚动状态(OUCH_STATE_SCROLLING)，这时会返回ture，把事件交给onTouchEvent函数来处理，onTouchEvent中同样会对事件类型进行判断，当事件方法为(otionEvent.ACTION_DOWN)的时候，就可以开始显示滚动的指示条了(就是Hotseat上显示第几屏的屏点)。当我们按着屏幕不放进行滑动的时候，又会在onInterceptTouchEvent进行事件拦截，但是现在的事件类型变为了 MotionEvent.ACTION_MOVE，因为是移动的操作，所以会在拦截的时候取消桌面长按的事件的响应，同时转到onTouchEvent中对ACTION_MOVE事件的响应中，判断我们移动了多少距离，使用scrollBy方法来对桌面进行移动，并刷新屏幕。最后我们放开手后会触发onTouchEvent中的MotionEvent.ACTION_UP事件，这时会根据滑动的情况来判断是朝左滑动还是朝右滑动，如果手指只滑动了屏幕宽度的少一半距离，则会弹回原来的页面，滑动多于屏幕宽度的一半则会进行翻页。同时要注意无论在什么情况下触发了WorkSpace滑动的事件，则系统会不断调用computeScroll()方法，我们重写这个方法同时在这个方法中调用刷新界面等操作。 
  
滑动过程中所要注意的主要方法如下，具体见代码注释。 
  
//对Touch事件进行拦截   主要用于在拦截各种Touch事件时，设置mTouchState的各种状态  
@Override  
public boolean onInterceptTouchEvent(MotionEvent ev) {  
     /*  
      * This method JUST determines whether we want to intercept the motion.  
      * If we return true, onTouchEvent will be called and we do the actual  
      * scrolling there.  
      * 这个方法仅仅决定了我们是否愿意去对滑动事件进行拦截，如果返回为true,则会调用onTouchEvent我们将会在那里进行事件处理  
      */  
     //对滑动的速率进行跟踪。  
   
     acquireVelocityTrackerAndAddMovement(ev);  
   
     // Skip touch handling if there are no pages to swipe  
     // 如果没有页面，则跳过操作。  
     if (getChildCount() <= 0) return super.onInterceptTouchEvent(ev);  
   
     /*  
      * Shortcut the most recurring case: the user is in the dragging  
      * state and he is moving his finger.  We want to intercept this  
      * motion.  
      * shortcut最常见的情况是：用户处于拖动的状态下，同时在移动它的手指，这时候我们需要拦截这个动作。  
      *   
      */  
     final int action = ev.getAction();  
     //如果是在MOVE的情况下，则进行Touch事件拦截  
     if ((action == MotionEvent.ACTION_MOVE) &&                
             (mTouchState == TOUCH_STATE_SCROLLING)) {  
         return true;  
     }  
   
     switch (action & MotionEvent.ACTION_MASK) {  
         case MotionEvent.ACTION_MOVE: {  
             /*  
              * mIsBeingDragged == false, otherwise the shortcut would have caught it. Check  
              * whether the user has moved far enough from his original down touch.  
              * 如果mIsBeingDragged==false ,否则快捷方式应该捕获到该事件，检查一下用户从它点击的地方位移是否足够  
              */  
             if (mActivePointerId != INVALID_POINTER) {  
                 //根据移动的距离判断是翻页还是移动一段位移，同时设置lastMotionX或者mTouchState这些值。同时取消桌面长按事件。  
                 determineScrollingStart(ev);  
                 break;  
             }  
             // if mActivePointerId is INVALID_POINTER, then we must have missed an ACTION_DOWN  
             // event. in that case, treat the first occurence of a move event as a ACTION_DOWN  
             // i.e. fall through to the next case (don't break)  
             // (We sometimes miss ACTION_DOWN events in Workspace because it ignores all events  
             // while it's small- this was causing a crash before we checked for INVALID_POINTER)  
             // 如果mActivePointerId 是 INVALID_POINTER，这时候我们应该已经错过了ACTION_DOWN事件。在这种情况下，把  
             // 第一次发生移动的事件当作ACTION——DOWN事件，直接进入下一个情况下。  
             // 我们有时候会错过workspace中的ACTION_DOWN事件，因为在workspace变小的时候会忽略掉所有的事件。                  
         }  
   
         case MotionEvent.ACTION_DOWN: {  
             final float x = ev.getX();  
             final float y = ev.getY();  
             // Remember location of down touch  
             // 记录按下的位置  
             mDownMotionX = x;  
             mLastMotionX = x;  
             mLastMotionY = y;  
             mLastMotionXRemainder = 0;  
             mTotalMotionX = 0;  
             //Return the pointer identifier associated with a particular pointer data index is this event.   
             //The identifier tells you the actual pointer number associated with the data,   
             //accounting for individual pointers going up and down since the start of the current gesture.  
             //返回和这个事件关联的触点数据id，计算单独点的id会上下浮动，因为手势的起始位置挥发声改变。  
             mActivePointerId = ev.getPointerId(0);  
             mAllowLongPress = true;  
   
             /*  
              * If being flinged and user touches the screen, initiate drag;  
              * otherwise don't.  mScroller.isFinished should be false when  
              * being flinged.  
              * 如果被拖动同时用户触摸到了屏幕，就开始初始化拖动，否则便不会。  
              * 当拖动完成后mScroller.isFinished就应该设置为false.  
              *   
              */  
             final int xDist = Math.abs(mScroller.getFinalX() - mScroller.getCurrX());  
               
             final boolean finishedScrolling = (mScroller.isFinished() || xDist < mTouchSlop);  
             if (finishedScrolling) {  
                 //标记为TOUCH_STATE_REST状态  
                 mTouchState = TOUCH_STATE_REST;  
                 //取消滚动动画  
                 mScroller.abortAnimation();  
             } else {  
                 //状态为TOUCH_STATE_SCROLLING  
                 mTouchState = TOUCH_STATE_SCROLLING;  
             }  
   
             // check if this can be the beginning of a tap on the side of the pages  
             // to scroll the current page  
             // 检测此事件是不是开始于点击页面的边缘来对当前页面进行滚动。                  
             if (mTouchState != TOUCH_STATE_PREV_PAGE && mTouchState != TOUCH_STATE_NEXT_PAGE) {  
                 if (getChildCount() > 0) {  
                     //根据触点的点位来判断是否点击到上一页，从而更新相应的状态  
                     if (hitsPreviousPage(x, y)) {  
                         mTouchState = TOUCH_STATE_PREV_PAGE;  
                     } else if (hitsNextPage(x, y)) {  
                         mTouchState = TOUCH_STATE_NEXT_PAGE;  
                     }  
                 }  
             }  
             break;  
         }  
   
         case MotionEvent.ACTION_UP:  
         case MotionEvent.ACTION_CANCEL:  
             //触点不被相应时，所做的动作  
             mTouchState = TOUCH_STATE_REST;  
             mAllowLongPress = false;  
             mActivePointerId = INVALID_POINTER;  
             //释放速率跟踪  
             releaseVelocityTracker();  
             break;  
   
         case MotionEvent.ACTION_POINTER_UP:  
             onSecondaryPointerUp(ev);  
             releaseVelocityTracker();  
             break;  
     }  
   
     /*  
      * The only time we want to intercept motion events is if we are in the  
      * drag mode.  
      * 我们唯一会去对移动事件进行拦截的情况时我们在拖动模式下  
      */  
     if(DEBUG) Log.d(TAG, "onInterceptTouchEvent "+(mTouchState != TOUCH_STATE_REST));  
     //只要是mTouchState的状态不为TOUCH_STATE_REST,那么就进行事件拦截  
     return mTouchState != TOUCH_STATE_REST;  
}  
onTouchEvent方法，详细见代码注释： 
  
@Override  
public boolean onTouchEvent(MotionEvent ev) {  
     // Skip touch handling if there are no pages to swipe  
     // 如果没有子页面，就直接跳过  
     if (getChildCount() <= 0) return super.onTouchEvent(ev);  
   
     acquireVelocityTrackerAndAddMovement(ev);  
   
     final int action = ev.getAction();  
   
     switch (action & MotionEvent.ACTION_MASK) {  
     case MotionEvent.ACTION_DOWN:  
         /*  
          * If being flinged and user touches, stop the fling. isFinished  
          * will be false if being flinged.  
          * 如果在滑动的过程中下用户又点击桌面，则取消滑动，从而响应当前的点击。  
          * 在滑动的isFinished将返回false.  
          */  
         if (!mScroller.isFinished()) {  
             mScroller.abortAnimation();  
         }  
   
         // Remember where the motion event started  
         mDownMotionX = mLastMotionX = ev.getX();  
         mLastMotionXRemainder = 0;  
         mTotalMotionX = 0;  
         mActivePointerId = ev.getPointerId(0);  
         //主要用来显示滚动条，表明要开始滚动了，这里可以进行调整，滚动条时逐渐显示还是立刻显示。  
         if (mTouchState == TOUCH_STATE_SCROLLING) {  
             pageBeginMoving();  
         }  
         break;  
   
     case MotionEvent.ACTION_MOVE:  
         if (mTouchState == TOUCH_STATE_SCROLLING) {  
             // Scroll to follow the motion event  
             final int pointerIndex = ev.findPointerIndex(mActivePointerId);  
             final float x = ev.getX(pointerIndex);  
             final float deltaX = mLastMotionX + mLastMotionXRemainder - x;  
             //总共移动的距离  
             mTotalMotionX += Math.abs(deltaX);  
   
             // Only scroll and update mLastMotionX if we have moved some discrete amount.  We  
             // keep the remainder because we are actually testing if we've moved from the last  
             // scrolled position (which is discrete).  
             // 如果我们移动了一小段距离，我们则移动和更新mLastMotionX 。我们保存Remainder变量是因为会检测我们  
   
             //是否是从最后的滚动点位移动的。                 
             if (Math.abs(deltaX) >= 1.0f) {  
                 mTouchX += deltaX;  
                 mSmoothingTime = System.nanoTime() / NANOTIME_DIV;  
                 if (!mDeferScrollUpdate) {  
                     scrollBy((int) deltaX, 0);  
                     if (DEBUG) Log.d(TAG, "onTouchEvent().Scrolling: " + deltaX);  
                 } else {  
                     invalidate();  
                 }  
                 mLastMotionX = x;  
                 mLastMotionXRemainder = deltaX - (int) deltaX;  
             } else {  
             //Trigger the scrollbars to draw. When invoked this method starts an animation to fade the   
             //scrollbars out after a default delay. If a subclass provides animated scrolling,   
             //the start delay should equal the duration of the scrolling animation.  
             //触发scrollbar进行绘制。 使用这个方法来启动一个动画来使scrollbars经过一段时间淡出。如果子类提供了滚动的动画，则  
             //延迟的时间等于动画滚动的时间。  
                 awakenScrollBars();  
             }  
         } else {  
             determineScrollingStart(ev);  
         }  
         break;  
   
     case MotionEvent.ACTION_UP:  
         if (mTouchState == 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);  
             int velocityX = (int) velocityTracker.getXVelocity(activePointerId);  
             final int deltaX = (int) (x - mDownMotionX);  
             final int pageWidth = getScaledMeasuredWidth(getPageAt(mCurrentPage));  
             // 屏幕的宽度*0.4f  
             boolean isSignificantMove = Math.abs(deltaX) > pageWidth *  
                     SIGNIFICANT_MOVE_THRESHOLD;  
             final int snapVelocity = mSnapVelocity;  
   
             mTotalMotionX += Math.abs(mLastMotionX + mLastMotionXRemainder - x);  
   
             boolean isFling = mTotalMotionX > MIN_LENGTH_FOR_FLING &&  
  
                     Math.abs(velocityX) > snapVelocity;  
   
             // In the case that the page is moved far to one direction and then is flung  
             // in the opposite direction, we use a threshold to determine whether we should  
             // just return to the starting page, or if we should skip one further.  
             // 这钟情况是页面朝一个方向移动了一段距离，然后又弹回去了。我们使用一个阀值来判断是进行翻页还是返回到初始页面          
             boolean returnToOriginalPage = false;  
             if (Math.abs(deltaX) > pageWidth * RETURN_TO_ORIGINAL_PAGE_THRESHOLD &&  
                     Math.signum(velocityX) != Math.signum(deltaX) && isFling) {  
                 returnToOriginalPage = true;  
             }  
   
             int finalPage;  
             // We give flings precedence over large moves, which is why we short-circuit our  
             // test for a large move if a fling has been registered. That is, a large  
             // move to the left and fling to the right will register as a fling to the right.  
             //朝右移动  
             if (((isSignificantMove && deltaX > 0 && !isFling) ||  
                     (isFling && velocityX > 0)) && mCurrentPage > 0) {  
                 finalPage = returnToOriginalPage ? mCurrentPage : mCurrentPage - 1;  
                 snapToPageWithVelocity(finalPage, velocityX);  
             //朝左移动  
             } else if (((isSignificantMove && deltaX < 0 && !isFling) ||  
                     (isFling && velocityX < 0)) &&  
                     mCurrentPage < getChildCount() - 1) {  
                 finalPage = returnToOriginalPage ? mCurrentPage : mCurrentPage + 1;  
                 snapToPageWithVelocity(finalPage, velocityX);  
             //寻找离屏幕中心最近的页面移动  
             } else {  
                 snapToDestination();  
             }  
         }  
          //直接移动到前一页  
          else if (mTouchState == TOUCH_STATE_PREV_PAGE) {  
             // at this point we have not moved beyond the touch slop  
             // (otherwise mTouchState would be TOUCH_STATE_SCROLLING), so  
             // we can just page  
             int nextPage = Math.max(0, mCurrentPage - 1);  
             if (nextPage != mCurrentPage) {  
                 snapToPage(nextPage);  
             } else {  
                 snapToDestination();  
             }  
         }  
          //直接移动到下一页  
          else if (mTouchState == TOUCH_STATE_NEXT_PAGE) {  
             // at this point we have not moved beyond the touch slop  
             // (otherwise mTouchState would be TOUCH_STATE_SCROLLING), so  
             // we can just page  
             int nextPage = Math.min(getChildCount() - 1, mCurrentPage + 1);  
             if (nextPage != mCurrentPage) {  
                 snapToPage(nextPage);  
             } else {  
                 snapToDestination();  
             }  
         } else {  
             onUnhandledTap(ev);  
         }  
         mTouchState = TOUCH_STATE_REST;  
         mActivePointerId = INVALID_POINTER;  
         releaseVelocityTracker();  
         break;  
      //对事件不响应  
     case MotionEvent.ACTION_CANCEL:  
         if (mTouchState == TOUCH_STATE_SCROLLING) {  
             snapToDestination();  
         }  
         mTouchState = TOUCH_STATE_REST;  
         mActivePointerId = INVALID_POINTER;  
         releaseVelocityTracker();  
         break;  
   
     case MotionEvent.ACTION_POINTER_UP:  
         onSecondaryPointerUp(ev);  
         break;  
     }  
   
     return true;  
}  
最后有个小知识点要搞清楚，不少网友都问到过我。就是scrollTo和scrollBy的区别。我们查看View类的源代码如下所示，mScrollX记录的是当前View针对屏幕坐标在水平方向上的偏移量，而mScrollY则是记录的时当前View针对屏幕在竖值方向上的偏移量。 
  
从以下代码我们可以得知，scrollTo就是把View移动到屏幕的X和Y位置，也就是绝对位置。而scrollBy其实就是调用的 scrollTo，但是参数是当前mScrollX和mScrollY加上X和Y的位置，所以ScrollBy调用的是相对于mScrollX和mScrollY的位置。我们在上面的代码中可以看到当我们手指不放移动屏幕时，就会调用scrollBy来移动一段相对的距离。而当我们手指松开后，会调用 mScroller.startScroll(mUnboundedScrollX, 0, delta, 0, duration); 来产生一段动画来移动到相应的页面，在这个过程中系统回不断调用computeScroll()，我们再使用scrollTo来把View移动到当前Scroller所在的绝对位置。 
  
/**  
    * Set the scrolled position of your view. This will cause a call to  
    * {@link #onScrollChanged(int, int, int, int)} and the view will be  
    * invalidated.  
    * @param x the x position to scroll to  
    * @param y the y position to scroll to  
    */  
   public void scrollTo(int x, int y) {  
       if (mScrollX != x || mScrollY != y) {  
           int oldX = mScrollX;  
           int oldY = mScrollY;  
           mScrollX = x;  
           mScrollY = y;  
           invalidateParentCaches();  
           onScrollChanged(mScrollX, mScrollY, oldX, oldY);  
           if (!awakenScrollBars()) {  
               invalidate(true);  
           }  
       }  
   }  
   /**  
    * Move the scrolled position of your view. This will cause a call to  
    * {@link #onScrollChanged(int, int, int, int)} and the view will be  
    * invalidated.  
    * @param x the amount of pixels to scroll by horizontally  
    * @param y the amount of pixels to scroll by vertically  
    */  
   public void scrollBy(int x, int y) {  
       scrollTo(mScrollX + x, mScrollY + y);  
   }  
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