Android4.0 Launcher 源码分析系列（三）

        这一篇研究一下整个桌面的左右滑动是如何实现的。 首先傻蛋先画了个图来再来阐述一下WorkSpace的结构。如下图：

      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);
    }