Android 4.0 Launcher2源码分析——桌面快捷图标的拖拽

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();
        ......
    }

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) {
          <span style="white-space:pre">    </span>//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：

<span style="white-space:pre">  </span>    // 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。