Subclassing UICollectionViewLayout and assign to U

问题:

I have a CollectionViewController

- (void)viewDidLoad 
{
   [super viewDidLoad];    
   // assign layout (subclassed below)
   self.collectionView.collectionViewLayout = [[CustomCollectionLayout alloc] init];
}

// data source is working, here's what matters:
- (UICollectionViewCell *)collectionView:(UICollectionView *)cv cellForItemAtIndexPath:(NSIndexPath *)indexPath {
   ThumbCell *cell = (ThumbCell *)[cv dequeueReusableCellWithReuseIdentifier:@"ThumbCell" forIndexPath:indexPath];

   return cell;
}

I also have a UICollectionViewLayout subclass: CustomCollectionLayout.m

#pragma mark - Overriden methods
- (CGSize)collectionViewContentSize
{
    return CGSizeMake(320, 480);
}

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    return [[super layoutAttributesForElementsInRect:rect] mutableCopy];
}

- (UICollectionViewLayoutAttributes *)layoutAttributesForItemAtIndexPath:(NSIndexPath *)indexPath
{
    // configure CellAttributes
    cellAttributes = [super layoutAttributesForItemAtIndexPath:indexPath];

    // random position
    int xRand = arc4random() % 320;
    int yRand = arc4random() % 460;
    cellAttributes.frame = CGRectMake(xRand, yRand, 150, 170);

    return cellAttributes;
}

I'm trying to use a new frame for the cell, just one at a random position. The problem is layoutAttributesForItemAtIndexPath is not called.

Thanks in advance for any advice.

回答1:

Edit: I didn't notice that you've solved your case, but I had the same problem and here's what helped in my case. I'm leaving it here, it may be useful for someone given that there's not much on this topic yet.

Upon drawing, UICollectionView does not call the method layoutAttributesForItemAtIndexPath: but layoutAttributesForElementsInRect: to determine which cells should be visible. It's your responsibility to implement this method and from there call layoutAttributesForItemAtIndexPath: for all visible index paths to determine exact locations.

In other words, add this to your layout:

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSMutableArray * array = [NSMutableArray arrayWithCapacity:16];
    // Determine visible index paths
    for(???) {
        [array addObject:[self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForItem:??? inSection:???]]];
    }
    return [array copy];
}

回答2:

Was an issue with Storyboard parameters. Assigned custom UICollectionLayout from Inspector panel.