本文来自http://blog.csdn.net/chenshaoyang0011 转载请申明文章出处！
通过上一篇文章Android4.0Launcher2源码分析（五）——Workspace的滑动中，已经了解了Launcher的ViewTree中各层所负责的工作，在DragLayer中就负责对快捷图标和AppWidget等组件的拖拽工作。桌面的滑动和图标的拖拽是两项独立的工作，正常情况下我们用手指滑动桌面会触发滑动操作，而当长按一个图标时，则会触发图标的拖拽操作，此时再滑动则会拖拽图标移动而桌面不会滑动。那么这里就分两大部分来探讨：1、拖拽操作的启动。2、拖拽。
一、拖拽操作的启动
那么首先进入Launcher.onCreate()中来探究下如何激活拖拽的状态。
protected void onCreate(Bundle savedInstanceState) {......setupViews();......}private void setupViews() {......mWorkspace.setOnLongClickListener(this);......}public boolean onLongClick(View v) {......if (!(v instanceof CellLayout)) {v = (View) v.getParent().getParent();}resetAddInfo();CellLayout.CellInfo longClickCellInfo = (CellLayout.CellInfo) v.getTag();......// The hotseat touch handling does not go through Workspace, and we always allow long press// on hotseat items.final View itemUnderLongClick = longClickCellInfo.cell;boolean allowLongPress = isHotseatLayout(v) || mWorkspace.allowLongPress();if (allowLongPress && !mDragController.isDragging()) {if (itemUnderLongClick == null) {......} else {if (!(itemUnderLongClick instanceof Folder)) {// User long pressed on an itemmWorkspace.startDrag(longClickCellInfo);}}}return true;}
图标拖拽功能的激活大概可以分为六步，下面就一步一步的探究下其中的实现：
Step1:Workspace.startDrag(CellLayout.CellInfo cellInfo)
void startDrag(CellLayout.CellInfo cellInfo) {View child = cellInfo.cell;......mDragInfo = cellInfo;//使图标从桌面上消失，给人一种被“拖到空中”的感觉child.setVisibility(GONE);......final Canvas canvas = new Canvas();// We need to add extra padding to the bitmap to make room for the glow effectfinal int bitmapPadding = HolographicOutlineHelper.MAX_OUTER_BLUR_RADIUS;// The outline is used to visualize where the item will land if dropped//图标的轮廓，在桌面上的对应的位置绘制图标的轮廓，显示当手松开图标时它在桌面上的落点mDragOutline = createDragOutline(child, canvas, bitmapPadding);beginDragShared(child, this);}
显示图标的轮廓可以从视觉上给用户更加好的体验。接着，进入beginDragShared()
Step2:Workspace.beginDragShared(View child,DragSource source)
public void beginDragShared(View child, DragSource source) {......// The drag bitmap follows the touch point around on the screenfinal Bitmap b = createDragBitmap(child, new Canvas(), bitmapPadding);final int bmpWidth = b.getWidth();//我们将在DragLayer中绘制“拖拽后”的图标，通过DragLayer.getLoactionInDragLayer()//获取在DragLayer中的坐标，并存放在mTempXY中。mLauncher.getDragLayer().getLocationInDragLayer(child, mTempXY);final int dragLayerX = (int) mTempXY[0] + (child.getWidth() - bmpWidth) / 2;int dragLayerY = mTempXY[1] - bitmapPadding / 2;Point dragVisualizeOffset = null;Rect dragRect = null;//无论child是BubbleTextView或者PagedViewIncon或者FolderIcon的实例//定位图标的位置与大小if (child instanceof BubbleTextView || child instanceof PagedViewIcon) {int iconSize = r.getDimensionPixelSize(R.dimen.app_icon_size);int iconPaddingTop = r.getDimensionPixelSize(R.dimen.app_icon_padding_top);int top = child.getPaddingTop();int left = (bmpWidth - iconSize) / 2;int right = left + iconSize;int bottom = top + iconSize;dragLayerY += top;// Note: The drag region is used to calculate drag layer offsets, but the// dragVisualizeOffset in addition to the dragRect (the size) to position the outline.dragVisualizeOffset = new Point(-bitmapPadding / 2, iconPaddingTop - bitmapPadding / 2);dragRect = new Rect(left, top, right, bottom);} else if (child instanceof FolderIcon) {int previewSize = r.getDimensionPixelSize(R.dimen.folder_preview_size);dragRect = new Rect(0, 0, child.getWidth(), previewSize);}......mDragController.startDrag(b, dragLayerX, dragLayerY, source, child.getTag(),DragController.DRAG_ACTION_MOVE, dragVisualizeOffset, dragRect);b.recycle();}Step3:DragController.startDrag(Bitmap b ,int dragLayerX, int dragLayerY,DragSource source, Object dragInfo, int dragAction, Point dragOffset, Rect dragRegion)
/*** Starts a drag.** @param b The bitmap to display as the drag image. It will be re-scaled to the* enlarged size.* @param dragLayerX The x position in the DragLayer of the left-top of the bitmap.* @param dragLayerY The y position in the DragLayer of the left-top of the bitmap.* @param source An object representing where the drag originated* @param dragInfo The data associated with the object that is being dragged* @param dragAction The drag action: either {@link #DRAG_ACTION_MOVE} or* {@link #DRAG_ACTION_COPY}* @param dragRegion Coordinates within the bitmap b for the position of item being dragged.* Makes dragging feel more precise, e.g. you can clip out a transparent border*/public void startDrag(Bitmap b, int dragLayerX, int dragLayerY,DragSource source, Object dragInfo, int dragAction, Point dragOffset, Rect dragRegion) {......for (DragListener listener : mListeners) {listener.onDragStart(source, dragInfo, dragAction);}final int registrationX = mMotionDownX - dragLayerX;final int registrationY = mMotionDownY - dragLayerY;final int dragRegionLeft = dragRegion == null ? 0 : dragRegion.left;final int dragRegionTop = dragRegion == null ? 0 : dragRegion.top;//设置mDragging=true，表示拖拽已经开始//在DragLayer的onInterceptTouchEvent()中根据这个值判断是否拦截MotionEventmDragging = true;//实例化DragObject，表示拖拽的对象//封装了拖拽对象的信息mDragObject = new DropTarget.DragObject();mDragObject.dragComplete = false;mDragObject.xOffset = mMotionDownX - (dragLayerX + dragRegionLeft);mDragObject.yOffset = mMotionDownY - (dragLayerY + dragRegionTop);mDragObject.dragSource = source;mDragObject.dragInfo = dragInfo;......final DragView dragView = mDragObject.dragView = new DragView(mLauncher, b, registrationX,registrationY, 0, 0, b.getWidth(), b.getHeight());......//将拖拽的图标显示在DragLayer中dragView.show(mMotionDownX, mMotionDownY);handleMoveEvent(mMotionDownX, mMotionDownY);}代码中显示通过一个for语句调用了DragListener.onDragStart()方法，通知它们已经开始拖拽了，其中由于Workspace实现了DragListener并且添加到了mListeners中。所以Workspace.onDragStart()被调用。然后又封装了一个DragObject对象，封装DragSource、DragInfo和DragView等信息。接着，将调用DragView.show()将DragView显示在DragLayer中。
Step4:DragView.show(int touchX,int touchY)
/*** Create a window containing this view and show it.** @param touchX the x coordinate the user touched in DragLayer coordinates* @param touchY the y coordinate the user touched in DragLayer coordinates*/public void show(int touchX, int touchY) {//将DragView添加到DragLayer中mDragLayer.addView(this);//设置位置、尺寸等信息DragLayer.LayoutParams lp = new DragLayer.LayoutParams(0, 0);lp.width = mBitmap.getWidth();lp.height = mBitmap.getHeight();lp.x = touchX - mRegistrationX;lp.y = touchY - mRegistrationY;lp.customPosition = true;setLayoutParams(lp);mLayoutParams = lp;mAnim.start();}其中的内容很简单易懂，就是在将DragView添加到了DragLayer中，并且在合适的位置显示了出来。接着应该调用在DragController.startDrag()中调用handleMoveEvent()，这个将在后文将拖拽过程分析时在看。到这一步，拖拽操作的启动过程就完成了。接着就可以拖拽图标了。
二、拖拽
通过了前面文章的分析，已经知道了拖拽过程的实现在DragLayer中，当进行图标的拖拽时，DragLayer.onInterceptTouchEvent()就会对MotionEvent进行拦截。并且
在自身的onTouchEvent()方法中进行操作，从而实现图标的移动。由于onInterceptTouchEvent()拦截了MotionEvent，因此Workspace等UI控件不会接收到事件，从而不会产生
干扰。那么首先进入DragLayer.onInterceptTouchEvent()：
public boolean onInterceptTouchEvent(MotionEvent ev) {......return mDragController.onInterceptTouchEvent(ev);}/*** Call this from a drag source view.*/public boolean onInterceptTouchEvent(MotionEvent ev) {......final int action = ev.getAction();final int[] dragLayerPos = getClampedDragLayerPos(ev.getX(), ev.getY());final int dragLayerX = dragLayerPos[0];final int dragLayerY = dragLayerPos[1];switch (action) {case MotionEvent.ACTION_MOVE:break;case MotionEvent.ACTION_DOWN:// Remember location of down touchmMotionDownX = dragLayerX;mMotionDownY = dragLayerY;mLastDropTarget = null;break;case MotionEvent.ACTION_UP:if (mDragging) {drop(dragLayerX, dragLayerY);}endDrag();break;case MotionEvent.ACTION_CANCEL:cancelDrag();break;}return mDragging;}这里我们关心的是它的返回值。可以看到方法将mDragging作为返回值。当触发了拖拽状态，在的DragController.startDrag()中将mDragging的值改为true。所以这里也将返回true。DragLayer将拦截MotionEvent，并传给自身的onTouchEvent()方法，在onTouchEvent()中对图标进行移动，刷新界面。
/*** Call this from a drag source view.*/public boolean onTouchEvent(MotionEvent ev) {......final int action = ev.getAction();final int[] dragLayerPos = getClampedDragLayerPos(ev.getX(), ev.getY());final int dragLayerX = dragLayerPos[0];final int dragLayerY = dragLayerPos[1];switch (action) {case MotionEvent.ACTION_DOWN:// Remember where the motion event startedmMotionDownX = dragLayerX;mMotionDownY = dragLayerY;//判断当前的触点是否处于屏幕边缘的ScrollZone，当处于这个区域时//状态mScrollState将转变为SCROLL,并且在一定时间的停留之后，屏幕滑动到另一屏。if ((dragLayerX < mScrollZone) || (dragLayerX > mScrollView.getWidth() - mScrollZone)) {mScrollState = SCROLL_WAITING_IN_ZONE;mHandler.postDelayed(mScrollRunnable, SCROLL_DELAY);} else {mScrollState = SCROLL_OUTSIDE_ZONE;}break;case MotionEvent.ACTION_MOVE://调用handleMoveEvent()处理图标移动handleMoveEvent(dragLayerX, dragLayerY);break;case MotionEvent.ACTION_UP:// Ensure that we've processed a move event at the current pointer location.handleMoveEvent(dragLayerX, dragLayerY);mHandler.removeCallbacks(mScrollRunnable);if (mDragging) { //根据目前相对DragLayer的坐标，将图标“降落”到指定的DropTarget上。drop(dragLayerX, dragLayerY);}endDrag();break;case MotionEvent.ACTION_CANCEL:cancelDrag();break;}return true;}onTouchEvent()中处理的事件涉及到不同状态之间的转换，以及每种状态之下对相应的MotionEvent的对策。这里同样，从简单的情况入手：图标拖拽起来后，移动一段距离，在屏幕的另一个位置放下。
首先，当拖拽起图标时，拖拽图标的状态被启动，这就是第一部分所探讨的内容。
然后，移动拖拽的图标。此时触发了MotionEvent.ACTION_MOVE事件，紧接着调用handleMoveEvent()来处理移动。进入handleMoveEvent()来看看图标移动是怎么实现的。
private void handleMoveEvent(int x, int y) {//更新在DragLayer中的位置mDragObject.dragView.move(x, y);// Drop on someone?final int[] coordinates = mCoordinatesTemp;//根据当前的位置寻找DropTarget对象来放置图标DropTarget dropTarget = findDropTarget(x, y, coordinates);mDragObject.x = coordinates[0];mDragObject.y = coordinates[1];if (dropTarget != null) {DropTarget delegate = dropTarget.getDropTargetDelegate(mDragObject);if (delegate != null) {dropTarget = delegate;}if (mLastDropTarget != dropTarget) {if (mLastDropTarget != null) {//从最后一次记录的DropTarget中退出mLastDropTarget.onDragExit(mDragObject);}//进入到当前寻找到的DropTargetdropTarget.onDragEnter(mDragObject);}dropTarget.onDragOver(mDragObject);} else {if (mLastDropTarget != null) {mLastDropTarget.onDragExit(mDragObject);}}mLastDropTarget = dropTarget;// Scroll, maybe, but not if we're in the delete region.boolean inDeleteRegion = false;if (mDeleteRegion != null) {inDeleteRegion = mDeleteRegion.contains(x, y);}// After a scroll, the touch point will still be in the scroll region.// Rather than scrolling immediately, require a bit of twiddling to scroll againfinal int slop = ViewConfiguration.get(mLauncher).getScaledWindowTouchSlop();mDistanceSinceScroll +=Math.sqrt(Math.pow(mLastTouch[0] - x, 2) + Math.pow(mLastTouch[1] - y, 2));mLastTouch[0] = x;mLastTouch[1] = y;//判断当前拖拽的图标是否处于ScrollZone即滑动区域。//并且根据在哪个一个ScrollZone来处理屏幕滑动的方向。if (!inDeleteRegion && x < mScrollZone) {if (mScrollState == SCROLL_OUTSIDE_ZONE && mDistanceSinceScroll > slop) {mScrollState = SCROLL_WAITING_IN_ZONE;if (mDragScroller.onEnterScrollArea(x, y, SCROLL_LEFT)) {mScrollRunnable.setDirection(SCROLL_LEFT);mHandler.postDelayed(mScrollRunnable, SCROLL_DELAY);}}} else if (!inDeleteRegion && x > mScrollView.getWidth() - mScrollZone) {if (mScrollState == SCROLL_OUTSIDE_ZONE && mDistanceSinceScroll > slop) {mScrollState = SCROLL_WAITING_IN_ZONE;if (mDragScroller.onEnterScrollArea(x, y, SCROLL_RIGHT)) {mScrollRunnable.setDirection(SCROLL_RIGHT);mHandler.postDelayed(mScrollRunnable, SCROLL_DELAY);}}} else {if (mScrollState == SCROLL_WAITING_IN_ZONE) {mScrollState = SCROLL_OUTSIDE_ZONE;mScrollRunnable.setDirection(SCROLL_RIGHT);mHandler.removeCallbacks(mScrollRunnable);mDragScroller.onExitScrollArea();}}}handleMoveEvent()主要处理拖拽过程中需要处理的事务。包括：1、在更新图标在屏幕中的位置，并刷新UI。2、判断图标当前所处的位置。包括SCROLL_OUTSIDE_ZONE和SCROLL_WAITING_IN_ZONE，对处于SCROLL_WAITING_IN_ZONE位置时，需要根据具体的位置，向前或向后切换显示的屏幕。再回到上面假设的情况中。则此时只是简单的刷新了位置信息，并重新绘制图标。
最后，当松开拖拽的对象时，触发了MotionEvent.ACTION_UP事件。则进入下面一段代码：
// Ensure that we've processed a move event at the current pointer location.handleMoveEvent(dragLayerX, dragLayerY);mHandler.removeCallbacks(mScrollRunnable);if (mDragging) {drop(dragLayerX, dragLayerY);}endDrag();先调用handleMoveEvent()确保已经完成了位置移动的操作。接下来调用mHandler.removeCallbacks(mScrollRunnalbe)取消可能存放在消息队列中的滑动任务。接着调用drop(dragLayerX,dragLayerY)将拖拽的对象放置合适的DropTarget对象中（如Workspace，Folder）。
private void drop(float x, float y) {......//根据当前的坐标查找适合的DropTarget对象final DropTarget dropTarget = findDropTarget((int) x, (int) y, coordinates);......boolean accepted = false;if (dropTarget != null) {mDragObject.dragComplete = true;dropTarget.onDragExit(mDragObject);if (dropTarget.acceptDrop(mDragObject)) {//将拖拽的对象放置到指定的DropTarget对象中。dropTarget.onDrop(mDragObject);accepted = true;}}mDragObject.dragSource.onDropCompleted((View) dropTarget, mDragObject, accepted);}最后调用endDrag()结束拖拽过程。
private void endDrag() {if (mDragging) {//拖拽结束mDragging = false;for (DragListener listener : mListeners) {//调用回调方法，通知拖拽结束。listener.onDragEnd();}if (mDragObject.dragView != null) {//不需要DragView了，将其删除mDragObject.dragView.remove();mDragObject.dragView = null;}}}
