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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();
......
}接着进入setupViews();
private void setupViews() {
......
mWorkspace.setOnLongClickListener(this);
......
}从这里我们可以看到对Workspace设置了OnLongClickListener，而Launcher又实现了这个接口。接着进入Launcher.onLongClick()
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 item
mWorkspace.startDrag(longClickCellInfo);
}
}
}
return true;
}当用户在一个item上长按时，则itemUnderLongClick != null，再通过调用Workspace.startDrag()来激活item的拖拽。下面先通过时序图来看下拖拽状态激活所经历的过程：
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 effect
final 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);
}在这个方法中，主要的工作就是让图标从桌面上消失，并且显示一个图标的外部轮廓，以表明它将要放置的位置，其显示的效果如下：
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 screen
final 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();
}Workspace.beginSharedDrag()中主要所做的工作就是计算拖拽目标位于DragLayer中的坐标和尺寸大小，接着又调用DragController.startDrag()
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MotionEvent
mDragging = 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);
}代码中省略了与其他功能的不部分代码，最后调用了DragController.onInterceptTouchEvent() ，并取其返回值作为自身方法的返回值。进入DragController.onInterceptTouchEvent()。
/**
* 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 touch
mMotionDownX = 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 started
mMotionDownX = 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DropTarget
dropTarget.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 again
final 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;
}
}
}至此，拖拽的图标的过程就结束了。
