AQGridView开源库中的示例DEMO：SpringBoard中为什么需要empty cell

这几天在看AQGridView开源库中的示例DEMO：SpringBoard，一直不明白里面为什么会有empty cell，看了几遍代码终于明白了。

先看下面的代码：

case UIGestureRecognizerStateBegan:        {            NSLog(@"UIGestureRecognizerStateBegan");            NSUInteger index = [_gridView indexForItemAtPoint: [recognizer locationInView: _gridView]];            _emptyCellIndex = index;    // we'll put an empty cell here now                        // find the cell at the current point and copy it into our main view, applying some transforms            AQGridViewCell * sourceCell = [_gridView cellForItemAtIndex: index];                        CGRect frame = [self.view convertRect: sourceCell.frame fromView: _gridView];                        _draggingCell = [[SpringBoardIconCell alloc] initWithFrame: frame reuseIdentifier: @""];                        _draggingCell.icon = [_icons objectAtIndex: index];            [self.view addSubview: _draggingCell];                        // grab some info about the origin of this cell            _dragOriginCellOrigin = frame.origin;            _dragOriginIndex = index;                        [UIView beginAnimations: @"" context: NULL];            [UIView setAnimationDuration: 0.2];            [UIView setAnimationCurve: UIViewAnimationCurveEaseOut];                        // transformation-- larger, slightly transparent --> 按住后，原来的图标变大和透明 by jfl            _draggingCell.transform = CGAffineTransformMakeScale( 1.2, 1.2 );            _draggingCell.alpha = 0.7;                        // also make it center on the touch point            _draggingCell.center = [recognizer locationInView: self.view];                        [UIView commitAnimations];            // reload the grid underneath to get the empty cell in place |--> 加载放大之后，下面的那个empty cell的，如果注释掉这段代码，变化之前的icon还在原地 by jfl            NSLog(@"**********");//            [_gridView reloadItemsAtIndices: [NSIndexSet indexSetWithIndex: index]//                              withAnimation: AQGridViewItemAnimationNone];            NSLog(@"----------");                        break;        }

如果我们把

//            [_gridView reloadItemsAtIndices: [NSIndexSet indexSetWithIndex: index]//                              withAnimation: AQGridViewItemAnimationNone];

注释掉。运行程序会发现如下效果：

在我们拖动的过程当中，左边原来的图标会一直显示，这显然是不对的。

如果不注释掉那个代码它就会调用下面的方法：

- (AQGridViewCell *) gridView: (AQGridView *) gridView cellForItemAtIndex: (NSUInteger) index{    NSLog(@"gridView cellForItemAtIndex");    static NSString * EmptyIdentifier = @"EmptyIdentifier";    static NSString * CellIdentifier = @"CellIdentifier";        if ( index == _emptyCellIndex )    {        NSLog( @"Loading empty cell at index %u", index );        AQGridViewCell * hiddenCell = [gridView dequeueReusableCellWithIdentifier: EmptyIdentifier];        if ( hiddenCell == nil )        {            // must be the SAME SIZE AS THE OTHERS            // Yes, this is probably a bug. Sigh. Look at -[AQGridView fixCellsFromAnimation] to fix            hiddenCell = [[AQGridViewCell alloc] initWithFrame: CGRectMake(0.0, 0.0, 72.0, 72.0)                                                reuseIdentifier: EmptyIdentifier];        }//        如果把这行代码注释掉，icon下面会出现个白色的矩形,所以才用EmptyIdentifier | by jfl//        hiddenCell.hidden = YES;        return ( hiddenCell );    }        SpringBoardIconCell * cell = (SpringBoardIconCell *)[gridView dequeueReusableCellWithIdentifier: CellIdentifier];    if ( cell == nil )    {        cell = [[SpringBoardIconCell alloc] initWithFrame: CGRectMake(0.0, 0.0, 72.0, 72.0) reuseIdentifier: CellIdentifier];    }        cell.icon = [_icons objectAtIndex: index];        return ( cell );}

此时，index == _emptyCellIndex，它会返回那个hiddenCell，即empty cell。然后把它隐藏，这样，我们在拖动的过程当中就看不到它了，达到了要实现的效果。

如果我们把

//        hiddenCell.hidden = YES;

注释掉，可以看到下面的效果：

这其实就是拖动过程当中一直有这个empty cell，只不过被隐藏了。这样就达到了拖动的效果。