AQGridView開源庫中的示例DEMO：SpringBoard中爲什麼需要empty cell

這幾天在看AQGridView開源庫中的示例DEMO：SpringBoard，一直不明白裏面爲什麼會有empty cell，看了幾遍代碼終於明白了。

先看下面的代碼：

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 --> 按住後，原來的圖標變大和透明 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 |--> 加載放大之後，下面的那個empty cell的，如果註釋掉這段代碼，變化之前的icon還在原地 by jfl
            NSLog(@"**********");
//            [_gridView reloadItemsAtIndices: [NSIndexSet indexSetWithIndex: index]
//                              withAnimation: AQGridViewItemAnimationNone];
            NSLog(@"----------");
            
            break;
        }

如果我們把

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

註釋掉。運行程序會發現如下效果：

在我們拖動的過程當中，左邊原來的圖標會一直顯示，這顯然是不對的。

如果不註釋掉那個代碼它就會調用下面的方法：

- (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];
        }
//        如果把這行代碼註釋掉，icon下面會出現個白色的矩形,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 );
}

此時，index == _emptyCellIndex，它會返回那個hiddenCell，即empty cell。然後把它隱藏，這樣，我們在拖動的過程當中就看不到它了，達到了要實現的效果。

如果我們把

//        hiddenCell.hidden = YES;

註釋掉，可以看到下面的效果：

這其實就是拖動過程當中一直有這個empty cell，只不過被隱藏了。這樣就達到了拖動的效果。