Launcher App拖拽流程

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1，App所在位置是源source，拖拽的目的地是：Target；source和target都是接口
Target接口：主要有四個方法要去實現：

/**
 * Handle an object being dropped on the DropTarget
 * 
 * @param source DragSource where the drag started
 * @param x X coordinate of the drop location
 * @param y Y coordinate of the drop location
 * @param xOffset Horizontal offset with the object being dragged where the original
 *          touch happened
 * @param yOffset Vertical offset with the object being dragged where the original
 *          touch happened
 * @param dragView The DragView that's being dragged around on screen.
 * @param dragInfo Data associated with the object being dragged
 *
 */
void onDrop(DragObject dragObject);

void onDragEnter(DragObject dragObject);

void onDragOver(DragObject dragObject);

void onDragExit(DragObject dragObject);

顧名思義：onDrop：就是在放下後做的操作；
onDragEnter：進入之後的操作；
onDragOver：在目標體之上做的操作；
onDragExit：退出之後的操作；

Source接口：主要有一個方法要實現：

/**
* A callback made back to the source after an item from this source has been dropped on a
* DropTarget.
*/
void onDropCompleted(View target, DragObject d, boolean isFlingToDelete, boolean success);
顧名思義：就是放下完成之後要處理的事情；

App在Workspace上面拖動，所以Workspace既是源也是目的，Workspace實現了這兩個接口
public class Workspace extends SmoothPagedView implements DropTarget, DragSource, DragScroller,

當長按一個app是會調用Launcher的onlongclick方法：

 final View itemUnderLongClick = longClickCellInfo.cell;
        boolean allowLongPress = isHotseatLayout(v) || mWorkspace.allowLongPress();
        if (allowLongPress && !mDragController.isDragging()) {
            if (itemUnderLongClick == null) {
                // User long pressed on empty space
                mWorkspace.performHapticFeedback(HapticFeedbackConstants.LONG_PRESS,
                        HapticFeedbackConstants.FLAG_IGNORE_VIEW_SETTING);
                // Disabling reordering until we sort out some issues.
                if (mWorkspace.isInOverviewMode()) {
                    mWorkspace.startReordering(v);
                } else {
                    mWorkspace.enterOverviewMode();
                }
            } else {
                if (!(itemUnderLongClick instanceof Folder)) {
                    // User long pressed on an item
                    mWorkspace.startDrag(longClickCellInfo);
                }
            }
        }

 最終會進入workspace的startDrag方法會把信息傳遞過來；

     void startDrag(CellLayout.CellInfo cellInfo) {
        View child = cellInfo.cell;

        // Make sure the drag was started by a long press as opposed to a long click.
        if (!child.isInTouchMode()) {
            return;
        }

        mDragInfo = cellInfo;
        child.setVisibility(INVISIBLE);
        CellLayout layout = (CellLayout) child.getParent().getParent();
        layout.prepareChildForDrag(child);

        child.clearFocus();
        child.setPressed(false);

        final Canvas canvas = new Canvas();

        // The outline is used to visualize where the item will land if dropped
        mDragOutline = createDragOutline(child, canvas, DRAG_BITMAP_PADDING);
        beginDragShared(child, this);
    }

當開始拖拽的時候：手按下的view會child.setVisibility(INVISIBLE);隱藏掉；  
同時會繪製圖標的邊框線 createDragOutline(child, canvas, DRAG_BITMAP_PADDING); 

    private Bitmap createDragOutline(View v, Canvas canvas, int padding) {
        final int outlineColor = getResources().getColor(R.color.outline_color);
        final Bitmap b = Bitmap.createBitmap( v.getWidth() + padding, v.getHeight() +                         padding, Bitmap.Config.ARGB_8888);

        canvas.setBitmap(b);
        drawDragView(v, canvas, padding, true);
        mOutlineHelper.applyMediumExpensiveOutlineWithBlur(b, canvas, outlineColor, outlineColor);
        canvas.setBitmap(null);
        return b;
    } 

繪製完成之後  beginDragShared(child, this);開始拖拽，手下的view傳遞過去，同時還需要一個源;

    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(), DRAG_BITMAP_PADDING);

        final int bmpWidth = b.getWidth();
        final int bmpHeight = b.getHeight();

        float scale = mLauncher.getDragLayer().getLocationInDragLayer(child, mTempXY);
        int dragLayerX =
                Math.round(mTempXY[0] - (bmpWidth - scale * child.getWidth()) / 2);
        int dragLayerY =
                Math.round(mTempXY[1] - (bmpHeight - scale * bmpHeight) / 2
                        - DRAG_BITMAP_PADDING / 2);

        LauncherAppState app = LauncherAppState.getInstance();
        DeviceProfile grid = app.getDynamicGrid().getDeviceProfile();
        Point dragVisualizeOffset = null;
        Rect dragRect = null;
        if (child instanceof BubbleTextView || child instanceof PagedViewIcon) {
            int iconSize = grid.iconSizePx;
            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(-DRAG_BITMAP_PADDING / 2, DRAG_BITMAP_PADDING / 2);
            dragRect = new Rect(left, top, right, bottom);
        } else if (child instanceof FolderIcon) {
            int previewSize = grid.folderIconSizePx;
            dragRect = new Rect(0, child.getPaddingTop(), child.getWidth(), previewSize);
        }

        // Clear the pressed state if necessary
        if (child instanceof BubbleTextView) {
            BubbleTextView icon = (BubbleTextView) child;
            icon.clearPressedOrFocusedBackground();
        }

        mDragController.startDrag(b, dragLayerX, dragLayerY, source, child.getTag(),
                DragController.DRAG_ACTION_MOVE, dragVisualizeOffset, dragRect, scale);

        if (child.getParent() instanceof ShortcutAndWidgetContainer) {
            mDragSourceInternal = (ShortcutAndWidgetContainer) child.getParent();
        }

        b.recycle();
    }

這個方法就是首先生成一個和隱藏掉的圖標一樣的圖標，將生成的Drawable交給DragController處理，調用 mDragController.startDrag(）;

 /**
     * 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,
            float initialDragViewScale) {
        if (PROFILE_DRAWING_DURING_DRAG) {
            android.os.Debug.startMethodTracing("Launcher");
        }

        // Hide soft keyboard, if visible
        if (mInputMethodManager == null) {
            mInputMethodManager = (InputMethodManager)
                    mLauncher.getSystemService(Context.INPUT_METHOD_SERVICE);
        }
        mInputMethodManager.hideSoftInputFromWindow(mWindowToken, 0);

        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;

        mDragging = true;

        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(), initialDragViewScale);

        if (dragOffset != null) {
            dragView.setDragVisualizeOffset(new Point(dragOffset));
        }
        if (dragRegion != null) {
            dragView.setDragRegion(new Rect(dragRegion));
        }

        mLauncher.getDragLayer().performHapticFeedback(HapticFeedbackConstants.LONG_PRESS);
        dragView.show(mMotionDownX, mMotionDownY);
        handleMoveEvent(mMotionDownX, mMotionDownY);
    }

mDragController.startDrag(）的方法主要是做隱藏鍵盤，註冊DragController的一個接口，生成DragView就是把生成的Drawable轉化成DragView，將DragView的座標傳給 handleMoveEvent(mMotionDownX, mMotionDownY)方法，同過獲得的座標DragView就可以移動 mDragObject.dragView.move(x, y);然後通過傳過來的座標在Target上找位置

    private DropTarget findDropTarget(int x, int y, int[] dropCoordinates) {
        final Rect r = mRectTemp;

        final ArrayList<DropTarget> dropTargets = mDropTargets;
        final int count = dropTargets.size();
        for (int i=count-1; i>=0; i--) {
            DropTarget target = dropTargets.get(i);
            if (!target.isDropEnabled())
                continue;

            target.getHitRectRelativeToDragLayer(r);

            mDragObject.x = x;
            mDragObject.y = y;
            if (r.contains(x, y)) {

                dropCoordinates[0] = x;
                dropCoordinates[1] = y;
                mLauncher.getDragLayer().mapCoordInSelfToDescendent((View) target, dropCoordinates);

                return target;
            }
        }
        return null;

如果站到位置之後會返回一個target，通過Target判斷DropTarget 的狀態是在哪一個狀態四個狀態就是Target接口的四個方法；

private void checkTouchMove(DropTarget dropTarget) {
        if (dropTarget != null) {
            if (mLastDropTarget != dropTarget) {
                if (mLastDropTarget != null) {
                    mLastDropTarget.onDragExit(mDragObject);
                }
                dropTarget.onDragEnter(mDragObject);
            }
            dropTarget.onDragOver(mDragObject);
        } else {
            if (mLastDropTarget != null) {
                mLastDropTarget.onDragExit(mDragObject);
            }
        }
        mLastDropTarget = dropTarget;
    }

如果是這四種狀態，它們會不停的切換狀態，當你放下時會調用mDragController的drop（）方法，

 private void drop(float x, float y) {
        final int[] coordinates = mCoordinatesTemp;
        final DropTarget dropTarget = findDropTarget((int) x, (int) y, coordinates);

        mDragObject.x = coordinates[0];
        mDragObject.y = coordinates[1];
        boolean accepted = false;
        if (dropTarget != null) {
            mDragObject.dragComplete = true;
            dropTarget.onDragExit(mDragObject);
            if (dropTarget.acceptDrop(mDragObject)) {
                dropTarget.onDrop(mDragObject);
                accepted = true;
            }
        }
        mDragObject.dragSource.onDropCompleted((View) dropTarget, mDragObject, false, accepted);
    }

這個方法會通過findDropTarget（）方法得到一個DropTarget，如果DropTarget不等於空mDragObject.dragComplete = true;說明拖拽完成，調用dropTarget.onDragExit(mDragObject);，判斷如果dropTarget.acceptDrop(mDragObject)，Target接不接受這個view。方法如下：

 /**
     * {@inheritDoc}
     */
    public boolean acceptDrop(DragObject d) {
        // If it's an external drop (e.g. from All Apps), check if it should be accepted
        CellLayout dropTargetLayout = mDropToLayout;
        if (d.dragSource != this) {
            // Don't accept the drop if we're not over a screen at time of drop
            if (dropTargetLayout == null) {
                return false;
            }
            if (!transitionStateShouldAllowDrop()) return false;

            mDragViewVisualCenter = getDragViewVisualCenter(d.x, d.y, d.xOffset, d.yOffset,
                    d.dragView, mDragViewVisualCenter);

            // We want the point to be mapped to the dragTarget.
            if (mLauncher.isHotseatLayout(dropTargetLayout)) {
                mapPointFromSelfToHotseatLayout(mLauncher.getHotseat(), mDragViewVisualCenter);
            } else {
                mapPointFromSelfToChild(dropTargetLayout, mDragViewVisualCenter, null);
            }

            int spanX = 1;
            int spanY = 1;
            if (mDragInfo != null) {
                final CellLayout.CellInfo dragCellInfo = mDragInfo;
                spanX = dragCellInfo.spanX;
                spanY = dragCellInfo.spanY;
            } else {
                final ItemInfo dragInfo = (ItemInfo) d.dragInfo;
                spanX = dragInfo.spanX;
                spanY = dragInfo.spanY;
            }

            int minSpanX = spanX;
            int minSpanY = spanY;
            if (d.dragInfo instanceof PendingAddWidgetInfo) {
                minSpanX = ((PendingAddWidgetInfo) d.dragInfo).minSpanX;
                minSpanY = ((PendingAddWidgetInfo) d.dragInfo).minSpanY;
            }

            mTargetCell = findNearestArea((int) mDragViewVisualCenter[0],
                    (int) mDragViewVisualCenter[1], minSpanX, minSpanY, dropTargetLayout,
                    mTargetCell);
            float distance = dropTargetLayout.getDistanceFromCell(mDragViewVisualCenter[0],
                    mDragViewVisualCenter[1], mTargetCell);
            if (willCreateUserFolder((ItemInfo) d.dragInfo, dropTargetLayout,
                    mTargetCell, distance, true)) {
                return true;
            }
            if (willAddToExistingUserFolder((ItemInfo) d.dragInfo, dropTargetLayout,
                    mTargetCell, distance)) {
                return true;
            }

            int[] resultSpan = new int[2];
            mTargetCell = dropTargetLayout.createArea((int) mDragViewVisualCenter[0],
                    (int) mDragViewVisualCenter[1], minSpanX, minSpanY, spanX, spanY,
                    null, mTargetCell, resultSpan, CellLayout.MODE_ACCEPT_DROP);
            boolean foundCell = mTargetCell[0] >= 0 && mTargetCell[1] >= 0;

            // Don't accept the drop if there's no room for the item
            if (!foundCell) {
                // Don't show the message if we are dropping on the AllApps button and the hotseat
                // is full
                boolean isHotseat = mLauncher.isHotseatLayout(dropTargetLayout);
                if (mTargetCell != null && isHotseat) {
                    Hotseat hotseat = mLauncher.getHotseat();
                    if (hotseat.isAllAppsButtonRank(
                            hotseat.getOrderInHotseat(mTargetCell[0], mTargetCell[1]))) {
                        return false;
                    }
                }

                mLauncher.showOutOfSpaceMessage(isHotseat);
                return false;
            }
        }

這個方法是返回一個boolean值，首先會判斷source是不是自己，是自己就直接返回true，不是自己就是繼續做判斷，會去找位置調用

private int[] findNearestArea(int pixelX, int pixelY,
            int spanX, int spanY, CellLayout layout, int[] recycle) {
        return layout.findNearestArea(
                pixelX, pixelY, spanX, spanY, recycle);
    }

找到位置之後 boolean foundCell = mTargetCell[0] >= 0 && mTargetCell[1] >= 0;

會得到一個boolean值foundCell ，

if (!foundCell) {
                // Don't show the message if we are dropping on the AllApps button and the hotseat
                // is full
                boolean isHotseat = mLauncher.isHotseatLayout(dropTargetLayout);
                if (mTargetCell != null && isHotseat) {
                    Hotseat hotseat = mLauncher.getHotseat();
                    if (hotseat.isAllAppsButtonRank(
                            hotseat.getOrderInHotseat(mTargetCell[0], mTargetCell[1]))) {
                        return false;
                    }
                }

                mLauncher.showOutOfSpaceMessage(isHotseat);
                return false;
            }

判斷是否在hotseat位置，是否在Allapp的按鈕上就是進入主頁面的按鈕上；

最終會到 dropTarget的Ondrop方法

if (dropTarget.acceptDrop(mDragObject)) {
                dropTarget.onDrop(mDragObject);
                accepted = true;
 }

到workspace的ondrop方法看看（在桌面拖動，所以會調用workspace的ondrop方法，那個Target就是Workspace），

  public void onDrop(final DragObject d) {
        mDragViewVisualCenter = getDragViewVisualCenter(d.x, d.y, d.xOffset, d.yOffset, d.dragView,
                mDragViewVisualCenter);

        CellLayout dropTargetLayout = mDropToLayout;

        // We want the point to be mapped to the dragTarget.
        if (dropTargetLayout != null) {
            if (mLauncher.isHotseatLayout(dropTargetLayout)) {
                mapPointFromSelfToHotseatLayout(mLauncher.getHotseat(), mDragViewVisualCenter);
            } else {
                mapPointFromSelfToChild(dropTargetLayout, mDragViewVisualCenter, null);
            }
        }

        int snapScreen = -1;
        boolean resizeOnDrop = false;
        if (d.dragSource != this) {
            final int[] touchXY = new int[] { (int) mDragViewVisualCenter[0],
                    (int) mDragViewVisualCenter[1] };
            onDropExternal(touchXY, d.dragInfo, dropTargetLayout, false, d);
        } else if (mDragInfo != null) {
            final View cell = mDragInfo.cell;

            Runnable resizeRunnable = null;
            if (dropTargetLayout != null && !d.cancelled) {
                // Move internally
                boolean hasMovedLayouts = (getParentCellLayoutForView(cell) != dropTargetLayout);
                boolean hasMovedIntoHotseat = mLauncher.isHotseatLayout(dropTargetLayout);
                long container = hasMovedIntoHotseat ?
                        LauncherSettings.Favorites.CONTAINER_HOTSEAT :
                        LauncherSettings.Favorites.CONTAINER_DESKTOP;
                long screenId = (mTargetCell[0] < 0) ?
                        mDragInfo.screenId : getIdForScreen(dropTargetLayout);
                int spanX = mDragInfo != null ? mDragInfo.spanX : 1;
                int spanY = mDragInfo != null ? mDragInfo.spanY : 1;
                // First we find the cell nearest to point at which the item is
                // dropped, without any consideration to whether there is an item there.

                mTargetCell = findNearestArea((int) mDragViewVisualCenter[0], (int)
                        mDragViewVisualCenter[1], spanX, spanY, dropTargetLayout, mTargetCell);
                float distance = dropTargetLayout.getDistanceFromCell(mDragViewVisualCenter[0],
                        mDragViewVisualCenter[1], mTargetCell);

                // If the item being dropped is a shortcut and the nearest drop
                // cell also contains a shortcut, then create a folder with the two shortcuts.
                if (!mInScrollArea && createUserFolderIfNecessary(cell, container,
                        dropTargetLayout, mTargetCell, distance, false, d.dragView, null)) {
                    stripEmptyScreens();
                    return;
                }

                if (addToExistingFolderIfNecessary(cell, dropTargetLayout, mTargetCell,
                        distance, d, false)) {
                    stripEmptyScreens();
                    return;
                }

                // Aside from the special case where we're dropping a shortcut onto a shortcut,
                // we need to find the nearest cell location that is vacant
                ItemInfo item = (ItemInfo) d.dragInfo;
                int minSpanX = item.spanX;
                int minSpanY = item.spanY;
                if (item.minSpanX > 0 && item.minSpanY > 0) {
                    minSpanX = item.minSpanX;
                    minSpanY = item.minSpanY;
                }

                int[] resultSpan = new int[2];
                mTargetCell = dropTargetLayout.createArea((int) mDragViewVisualCenter[0],
                        (int) mDragViewVisualCenter[1], minSpanX, minSpanY, spanX, spanY, cell,
                        mTargetCell, resultSpan, CellLayout.MODE_ON_DROP);

                boolean foundCell = mTargetCell[0] >= 0 && mTargetCell[1] >= 0;

                // if the widget resizes on drop
                if (foundCell && (cell instanceof AppWidgetHostView) &&
                        (resultSpan[0] != item.spanX || resultSpan[1] != item.spanY)) {
                    resizeOnDrop = true;
                    item.spanX = resultSpan[0];
                    item.spanY = resultSpan[1];
                    AppWidgetHostView awhv = (AppWidgetHostView) cell;
                    AppWidgetResizeFrame.updateWidgetSizeRanges(awhv, mLauncher, resultSpan[0],
                            resultSpan[1]);
                }

                if (getScreenIdForPageIndex(mCurrentPage) != screenId && !hasMovedIntoHotseat) {
                    snapScreen = getPageIndexForScreenId(screenId);
                    snapToPage(snapScreen);
                }

                if (foundCell) {
                    final ItemInfo info = (ItemInfo) cell.getTag();
                    if (hasMovedLayouts) {
                        // Reparent the view
                        getParentCellLayoutForView(cell).removeView(cell);
                        addInScreen(cell, container, screenId, mTargetCell[0], mTargetCell[1],
                                info.spanX, info.spanY);
                    }

                    // update the item's position after drop
                    CellLayout.LayoutParams lp = (CellLayout.LayoutParams) cell.getLayoutParams();
                    lp.cellX = lp.tmpCellX = mTargetCell[0];
                    lp.cellY = lp.tmpCellY = mTargetCell[1];
                    lp.cellHSpan = item.spanX;
                    lp.cellVSpan = item.spanY;
                    lp.isLockedToGrid = true;
                    cell.setId(LauncherModel.getCellLayoutChildId(container, mDragInfo.screenId,
                            mTargetCell[0], mTargetCell[1], mDragInfo.spanX, mDragInfo.spanY));

                    if (container != LauncherSettings.Favorites.CONTAINER_HOTSEAT &&
                            cell instanceof LauncherAppWidgetHostView) {
                        final CellLayout cellLayout = dropTargetLayout;
                        // We post this call so that the widget has a chance to be placed
                        // in its final location

                        final LauncherAppWidgetHostView hostView = (LauncherAppWidgetHostView) cell;
                        AppWidgetProviderInfo pinfo = hostView.getAppWidgetInfo();
                        if (pinfo != null &&
                                pinfo.resizeMode != AppWidgetProviderInfo.RESIZE_NONE) {
                            final Runnable addResizeFrame = new Runnable() {
                                public void run() {
                                    DragLayer dragLayer = mLauncher.getDragLayer();
                                    dragLayer.addResizeFrame(info, hostView, cellLayout);
                                }
                            };
                            resizeRunnable = (new Runnable() {
                                public void run() {
                                    if (!isPageMoving()) {
                                        addResizeFrame.run();
                                    } else {
                                        mDelayedResizeRunnable = addResizeFrame;
                                    }
                                }
                            });
                        }
                    }

                    LauncherModel.modifyItemInDatabase(mLauncher, info, container, screenId, lp.cellX,
                            lp.cellY, item.spanX, item.spanY);
                } else {
                    // If we can't find a drop location, we return the item to its original position
                    CellLayout.LayoutParams lp = (CellLayout.LayoutParams) cell.getLayoutParams();
                    mTargetCell[0] = lp.cellX;
                    mTargetCell[1] = lp.cellY;
                    CellLayout layout = (CellLayout) cell.getParent().getParent();
                    layout.markCellsAsOccupiedForView(cell);
                }
            }

            final CellLayout parent = (CellLayout) cell.getParent().getParent();
            final Runnable finalResizeRunnable = resizeRunnable;
            // Prepare it to be animated into its new position
            // This must be called after the view has been re-parented
            final Runnable onCompleteRunnable = new Runnable() {
                @Override
                public void run() {
                    mAnimatingViewIntoPlace = false;
                    updateChildrenLayersEnabled(false);
                    if (finalResizeRunnable != null) {
                        finalResizeRunnable.run();
                    }
                    stripEmptyScreens();
                }
            };
            mAnimatingViewIntoPlace = true;
            if (d.dragView.hasDrawn()) {
                final ItemInfo info = (ItemInfo) cell.getTag();
                if (info.itemType == LauncherSettings.Favorites.ITEM_TYPE_APPWIDGET) {
                    int animationType = resizeOnDrop ? ANIMATE_INTO_POSITION_AND_RESIZE :
                            ANIMATE_INTO_POSITION_AND_DISAPPEAR;
                    animateWidgetDrop(info, parent, d.dragView,
                            onCompleteRunnable, animationType, cell, false);
                } else {
                    int duration = snapScreen < 0 ? -1 : ADJACENT_SCREEN_DROP_DURATION;
                    mLauncher.getDragLayer().animateViewIntoPosition(d.dragView, cell, duration,
                            onCompleteRunnable, this);
                }
            } else {
                d.deferDragViewCleanupPostAnimation = false;
                cell.setVisibility(VISIBLE);
            }
            parent.onDropChild(cell);
        }
    }

方法蠻長的，我們只看重點：

 if (!mInScrollArea && createUserFolderIfNecessary(cell, container,
                        dropTargetLayout, mTargetCell, distance, false, d.dragView, null)) {
                    stripEmptyScreens();
                    return;
  }

createUserFolderIfNecessary顧名思義就是兩個應用是否合成文件夾；進去裏面看看

 boolean createUserFolderIfNecessary(View newView, long container, CellLayout target,
            int[] targetCell, float distance, boolean external, DragView dragView,
            Runnable postAnimationRunnable) {
        if (distance > mMaxDistanceForFolderCreation) return false;
        View v = target.getChildAt(targetCell[0], targetCell[1]);

        boolean hasntMoved = false;
        if (mDragInfo != null) {
            CellLayout cellParent = getParentCellLayoutForView(mDragInfo.cell);
            hasntMoved = (mDragInfo.cellX == targetCell[0] &&
                    mDragInfo.cellY == targetCell[1]) && (cellParent == target);
        }

        if (v == null || hasntMoved || !mCreateUserFolderOnDrop) return false;
        mCreateUserFolderOnDrop = false;
        final long screenId = (targetCell == null) ? mDragInfo.screenId : getIdForScreen(target);

        boolean aboveShortcut = (v.getTag() instanceof ShortcutInfo);
        boolean willBecomeShortcut = (newView.getTag() instanceof ShortcutInfo);

        if (aboveShortcut && willBecomeShortcut) {
            ShortcutInfo sourceInfo = (ShortcutInfo) newView.getTag();
            ShortcutInfo destInfo = (ShortcutInfo) v.getTag();
            // if the drag started here, we need to remove it from the workspace
            if (!external) {
                getParentCellLayoutForView(mDragInfo.cell).removeView(mDragInfo.cell);
            }

            Rect folderLocation = new Rect();
            float scale = mLauncher.getDragLayer().getDescendantRectRelativeToSelf(v, folderLocation);
            target.removeView(v);

            FolderIcon fi =
                mLauncher.addFolder(target, container, screenId, targetCell[0], targetCell[1]);
            destInfo.cellX = -1;
            destInfo.cellY = -1;
            sourceInfo.cellX = -1;
            sourceInfo.cellY = -1;

            // If the dragView is null, we can't animate
            boolean animate = dragView != null;
            if (animate) {
                fi.performCreateAnimation(destInfo, v, sourceInfo, dragView, folderLocation, scale,
                        postAnimationRunnable);
            } else {
                fi.addItem(destInfo);
                fi.addItem(sourceInfo);
            }
            return true;
        }
        return false;
    }

方法有點長，也只看重點 boolean aboveShortcut = (v.getTag() instanceof ShortcutInfo);
boolean willBecomeShortcut = (newView.getTag() instanceof ShortcutInfo);
就是判斷是否是在應用的上面，就是說，你放下的dragview下面是否是ShortcutInfo，而且你手放下的view也是ShortcutInfo，就合成文件夾 FolderIcon fi = mLauncher.addFolder(target, container, screenId, targetCell[0], targetCell[1]);合成文件夾返回true，

繼續回到workspace的ondrop方法如果不是創建文件夾；繼續往下

 if (addToExistingFolderIfNecessary(cell, dropTargetLayout, mTargetCell,
                        distance, d, false)) {
                    stripEmptyScreens();
                    return;
  }

addToExistingFolderIfNecessary 顧名思義：既是加入到已經存在的文件夾

 boolean addToExistingFolderIfNecessary(View newView, CellLayout target, int[] targetCell,
            float distance, DragObject d, boolean external) {
        if (distance > mMaxDistanceForFolderCreation) return false;

        View dropOverView = target.getChildAt(targetCell[0], targetCell[1]);
        if (!mAddToExistingFolderOnDrop) return false;
        mAddToExistingFolderOnDrop = false;

        if (dropOverView instanceof FolderIcon) {
            FolderIcon fi = (FolderIcon) dropOverView;
            if (fi.acceptDrop(d.dragInfo)) {
                fi.onDrop(d);

                // if the drag started here, we need to remove it from the workspace
                if (!external) {
                    getParentCellLayoutForView(mDragInfo.cell).removeView(mDragInfo.cell);
                }
                return true;
            }
        }
        return false;
    }

這個方法就是判斷下面的view是否是文件夾，就進入到文件夾的Ondrop方法進行對addItem(item);
是否接受應用，文件夾是否沒有銷燬，條件成立之後返回true；就加入到文件夾了

繼續ondrop方法：

if (foundCell) {
                    final ItemInfo info = (ItemInfo) cell.getTag();
                    if (hasMovedLayouts) {
                        // Reparent the view
                        getParentCellLayoutForView(cell).removeView(cell);
                        addInScreen(cell, container, screenId, mTargetCell[0], mTargetCell[1],
                                info.spanX, info.spanY);
  }

如果桌面有位置就會 addInScreen(cell, container, screenId, mTargetCell[0], mTargetCell[1], info.spanX, info.spanY);然後對數據的操作 LauncherModel.modifyItemInDatabase(mLauncher, info, container, screenId, lp.cellX,lp.cellY, item.spanX, item.spanY);

放下完成之後會調用source的onDropCompleted方法做一系列拖拽完成的操作；

if (parentCell != null) {
                    parentCell.removeView(mDragInfo.cell);
 }

會把隱藏的view移除掉

拖拽完成!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!