UICollectionView - How to increase the number of cells to be allocated - ios

Question

My UICollectionView has a non-zero contentInset
self.collectionView.contentInset = UIEdgeInsetsMake(self.mainNavigation.bounds.size.height, 0, 0, 0);
MainNavigation is a transparent navigationBar - once the user scrolls down, collectionView can be seen partially through MainNavigation. More cells are initialized because the "on-screen" frame of the collectionView has increased (these new cells are not dequeued).
The initialization of cells is quite expensive and causes the UI to lag.
What I need is collectionView to initially load in more cells into memory, so that the initial scrolling is smoother.
How can I increase the number of cells that is initially loaded?

Answer 1

I don't think you need to load more cells because it's the natural behavior. If your scroll is not smooth maybe it's because you don't load your images in cells properly.
You must apply the lazy loading pattern. For example, you can do something like this (assuming you have set a NSMutableDictionary* imageDic = [[NSMutableDictionary alloc] init];, your cell have the property #property(nonatomic, retain) UIImageView *imageView; and you are using AFNetworking)
- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView_ cellForItemAtIndexPath:(NSIndexPath *)indexPath{
UICollectionViewCell *cell = [collectionView_ dequeueReusableCellWithReuseIdentifier:#"identifier" forIndexPath:indexPath];
NSString* cellKey = [NSString stringWithFormat:#"%d_%d", indexPath.section, indexPath.row];
NSString* imgName = [cellKey stringByAppendingPathExtension:#"jpg"];
NSString *documentsDirectory = [NSSearchPathForDirectoriesInDomains(NSLibraryDirectory, NSUserDomainMask, YES) objectAtIndex:0];
NSString* imagePath = [documentsDirectory stringByAppendingPathComponent:imgName];
BOOL isDirectory = NO;
NSFileManager* fm = [NSFileManager defaultManager];
// set image with image in cache (it will be fast!)
if ([imageDic objectForKey:cellKey])
{
UIImage *img = [imageDic objectForKey:cellKey];
cell.imageView.image = img;
}
// set image with image saved in document directory and put it in cache
else if ([fm fileExistsAtPath:imagePath isDirectory:&isDirectory])
{
UIImage *img = [UIImage imageWithContentsOfFile:imagePath];
cell.imageView.image = img;
[imageDic setObject:img forKey:cellKey];
}
// download image at imageURL, save it and put it in cache too. Until then set image with a placeholder
else
{
__weak UICollectionViewCell* weakCell = cell;
[cell.imageView setImageWithURLRequest:[NSURLRequest requestWithURL:imageURL] placeholderImage:[UIImage imageNamed:#"placeholder"] success:^(NSURLRequest *request, NSHTTPURLResponse *response, UIImage *image) {
weakCell.imageView.image = image;
[weakCell setNeedsLayout];
[imageDic setObject:img forKey:cellKey];
[UIImageJPEGRepresentation(img, 1.f) writeToFile:imagePath atomically:YES];
} failure:^(NSURLRequest *request, NSHTTPURLResponse *response, NSError *error) {
NSLog(#"failed to dl image");
}];
}
return cell;
}

Answer 2

There is no way to do that.
What you can do is:
Optimize cell code and remove heavy parts from initialization, maybe delay some operations until scroll stops.
Use solid backgrounds for subviews and set proper value for view.opaque. It gives a good performance boost when scrolling. This is something you can debug in simulator with "Color Blended Layers" option turned on. You'll see hits and misses rendered in green and red.
If you use custom CALayers, try rasterizing when/if they are static.
Cache data if delegate performs heavy operations before returning cell.