UICollectionView willDisplayCell delegate method? - iphone

Question

Anyway to hack into UICollectionView to call willDisplayCell delegate method, when displaying cell?
I need this for lazy loading, and I'm doing it nice with UITableView, but officially UICollectionView doesn't have that kind of delegate method.
So, any ideas? Thanks!

Answer 1

FYI, This has been added to the API for iOS 8:
- (void)collectionView:(UICollectionView *)collectionView willDisplayCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath NS_AVAILABLE_IOS(8_0);

Answer 2

I'm doing something like this in cellForItemAtIndexPath: (for lazy loading images):
If my model has the image, just set the cell's image to it
If the model does not have an image, kick of an async load
When the load completes, the check if the original indexPath is still visible
If so, call cellForItemAtIndexPath for the original indexPath. Since the image now exists in the model, the cell's image will now be set.
Looks like this:
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {
PhotoCell * cell = [collectionView dequeueReusableCellWithReuseIdentifier:#"PhotoCell" forIndexPath:indexPath];
id imageItem = [self.photoSet objectAtIndex:indexPath.row];
ImageManager * listingImage = (ImageManager *)imageItem;
if(listingImage.image){
cell.image = listingImage.image;
} else {
[listingImage loadImage:^(UIImage *image) {
[collectionView reloadItemsAtIndexPaths:#[indexPath]];
dispatch_async(dispatch_get_main_queue(), ^{
if ([[collectionView indexPathsForVisibleItems] containsObject:indexPath]) {
[(PhotoCell *)[collectionView cellForItemAtIndexPath:indexPath] setImage:image];
}
});
}];
}
}
return cell;
}

Answer 3

You can make use of SDWebImage https://github.com/rs/SDWebImage for lazy loading. You can use this effectively with UICollectionView.
- (UICollectionViewCell *)collectionView:(UICollectionView *)cv cellForItemAtIndexPath:(NSIndexPath *)indexPath {
/*---------
----Other CollectiveView stuffs------
-----------------*/
if([[NSFileManager defaultManager] fileExistsAtPath:YOUR_FILE_PATH isDirectory:NO])
{
imagView.image=[UIImage imageWithContentsOfFile:YOUR_FILE_PATH];
}
else
{
UIActivityIndicatorView *act=[[UIActivityIndicatorView alloc]initWithActivityIndicatorStyle:UIActivityIndicatorViewStyleGray];
[imagView addSubview:act];
act.center=CGPointMake(imagView.frame.size.width/2, imagView.frame.size.height/2);
[act startAnimating];
__weak typeof(UIImageView) *weakImgView = imagView;
[imagView setImageWithURL:[NSURL URLWithString:REMOTE_FILE_PATH] completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType){
for(UIView *dd in weakImgView.subviews)
{
if([dd isKindOfClass:[UIActivityIndicatorView class]])
{
UIActivityIndicatorView *act=(UIActivityIndicatorView *)dd;
[act stopAnimating];
[act removeFromSuperview];
}
}
NSString *extension=[YOUR_FILE_PATH pathExtension];
[self saveImage:image withFileName:YOUR_FILE_PATH ofType:extension];
}];
}
}

Answer 4

I have a similar situation when I need to know the index path of cell which is going to be displayed. Ended with - (void)collectionView:(UICollectionView *)collectionView didEndDisplayingCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath. Assumably, one is "didEndDisplaying", another is "willDisplayed".