Launcher3/src/com/android/launcher3/Workspace.java

Workspace

开始拖拽时调用到startDrag()方法

    private int mAppStartDragScreenIndex;
    private int mAppStartDragScreenAppCount;
		
    public void startDrag(CellLayout.CellInfo cellInfo, DragOptions options) {
        View child = cellInfo.cell;

        mDragInfo = cellInfo;
        child.setVisibility(INVISIBLE);

        //开始拖拽时当前屏的索引号
        mAppStartDragScreenIndex = cellInfo.screenId;
        //开始拖拽时当前屏的APP数目
        CellLayout cellLayout = mWorkspaceScreens.valueAt(mAppStartDragScreenIndex);
        if(null != cellLayout.getShortcutsAndWidgets()){
            mAppStartDragScreenAppCount = cellLayout.getShortcutsAndWidgets().getChildCount();
        }

        if (options.isAccessibleDrag) {
            mDragController.addDragListener(
                    new AccessibleDragListenerAdapter(this, WorkspaceAccessibilityHelper::new) {
                        @Override
                        protected void enableAccessibleDrag(boolean enable) {
                            super.enableAccessibleDrag(enable);
                            setEnableForLayout(mLauncher.getHotseat(), enable);
                        }
                    });
        }

        beginDragShared(child, this, options);
    }

拖拽到区域放下时触发onDrop()方法回调

if (dropTargetLayout != null && !d.cancelled) {}

如果不满足条件，APP拖拽时放下，图标会自动回到原来的位置，所以这里加上最后一屏条件判断

dropToScreenIndex，拖拽到屏幕的索引号

mAppStartDragScreenIndex ，开始拖拽时当前屏的索引号

mAppStartDragScreenAppCount ，开始拖拽时当前屏的APP数目

    @Override
    public void onDrop(final DragObject d, DragOptions options) {
        mDragViewVisualCenter = d.getVisualCenter(mDragViewVisualCenter);
        CellLayout dropTargetLayout = mDropToLayout;

        // We want the point to be mapped to the dragTarget.
        if (dropTargetLayout != null) {
            mapPointFromDropLayout(dropTargetLayout, mDragViewVisualCenter);
        }

        boolean droppedOnOriginalCell = false;

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

            boolean isCanDrop = true;
            int pageCanDropTotalCount = dropTargetLayout.getCountX()*dropTargetLayout.getCountY();
            int dropToScreenIndex = getIdForScreen(dropTargetLayout);
            //拖拽到空屏，也就是最后一屏的下一个空屏
            if((dropToScreenIndex+1)>=mWorkspaceScreens.size()
                    //mCurAppStartDragScreenIndex开始拖拽时屏幕的索引号，这里指从最后一屏开始拖拽时
                    && (mAppStartDragScreenIndex +1) == mWorkspaceScreens.size()-1 &&
                    mAppStartDragScreenAppCount < (pageCanDropTotalCount - 6)){
                isCanDrop = false;
            }
            if (dropTargetLayout != null && !d.cancelled && isCanDrop) {
                // Move internally
                boolean hasMovedLayouts = (getParentCellLayoutForView(cell) != dropTargetLayout);
                boolean hasMovedIntoHotseat = mLauncher.isHotseatLayout(dropTargetLayout);
                int container = hasMovedIntoHotseat ?
                        LauncherSettings.Favorites.CONTAINER_HOTSEAT :
                        LauncherSettings.Favorites.CONTAINER_DESKTOP;
                int 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.getDistanceFromWorkspaceCellVisualCenter(
                        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 (createUserFolderIfNecessary(cell, container, dropTargetLayout, mTargetCell,
                        distance, false, d)
                        || addToExistingFolderIfNecessary(cell, dropTargetLayout, mTargetCell,
                        distance, d, false)) {
                    mLauncher.getStateManager().goToState(NORMAL, SPRING_LOADED_EXIT_DELAY);
                    return;
                }

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