Preload next (currently invisible) cell UICollectionView - ios

Question

I'm working with a UICollectionView and currently have full screen cells and when I swipe to the next cell the image takes a bit to load into the cell (since they're high res). I'm already using PHCachingImageManager to cache the image before hand and it's already cached before I load the cell, but just loading the image into the imageView takes a noticeable spark of time. I was wondering if there's a way to preload the invisible next cell BEFORE it gets to cellForItemAtIndexPath?
Any thoughts?

Answer 1

It apparently takes some time to load an image that has already been cached from the PHCachingImageManager which was the cause of my delay. I created a test function that saved the next image (and will add the previous one too) in a property while setting up the current cell and my flicker is no more. I'll post the working example when I've cleaned it up...
Update:
So I created an _images NSMutableDictionary to store the already loaded images and populate it using this function
- (void)startCachingSurroundingImages:(NSInteger)index {
CGSize targetSize = PHImageManagerMaximumSize;
PHImageRequestOptions *options = [[PHImageRequestOptions alloc] init];
[options setNetworkAccessAllowed:YES];
[options setDeliveryMode:PHImageRequestOptionsDeliveryModeHighQualityFormat];
if (index < _imagesArray.count - 1 && ![_images objectForKey:[NSIndexPath indexPathForItem:0 inSection:index+1]]) {
NSDictionary *assetDictionary = [_imagesArray objectAtIndex:index + 1];
NSString *assetRefID = [assetDictionary objectForKey:#"asset_id"];
PHFetchResult *assetFetchResult = [PHAsset fetchAssetsWithLocalIdentifiers:#[assetRefID] options:[PHFetchOptions new]];
PHAsset *asset = [assetFetchResult firstObject];
[photoManager requestImageForAsset:asset targetSize:targetSize contentMode:PHImageContentModeAspectFit options:options resultHandler:^(UIImage *result, NSDictionary *info) {
if (result) {
[_images setObject:result forKey:[NSIndexPath indexPathForItem:0 inSection:index+1]];
}
}];
}
if (index - 1 >= 0 && ![_images objectForKey:[NSIndexPath indexPathForItem:0 inSection:index-1]]) {
NSDictionary *leftAssetDictionary = [_imagesArray objectAtIndex:index - 1];
NSString *leftAssetRefID = [leftAssetDictionary objectForKey:#"asset_id"];
PHFetchResult *leftAssetFetchResult = [PHAsset fetchAssetsWithLocalIdentifiers:#[leftAssetRefID] options:[PHFetchOptions new]];
PHAsset *leftAsset = [leftAssetFetchResult firstObject];
[photoManager requestImageForAsset:leftAsset targetSize:targetSize contentMode:PHImageContentModeAspectFit options:options resultHandler:^(UIImage *result, NSDictionary *info) {
if (result) {
[_images setObject:result forKey:[NSIndexPath indexPathForItem:0 inSection:index-1]];
}
}];
}
}
Which I call in cellForItemAtIndexPath like so...
[self startCachingSurroundingImages:indexPath.section];
and still in cellForItemAtIndexPath, I load the image like so...
if ([_images objectForKey:indexPath]) {
cell.imageView.image = [_images objectForKey:indexPath];
[photoCell.activityIndicator setHidden:YES];
} else {
photoCell.tag = (int)[photoManager requestImageForAsset:asset targetSize:targetSize contentMode:PHImageContentModeAspectFit options:options resultHandler:^(UIImage *result, NSDictionary *info) {
PhotoCell *photoCell = (PhotoCell *)[self.collectionView cellForItemAtIndexPath:indexPath];
if (photoCell && result) {
photoCell.imageView.image = result;
[photoCell.activityIndicator setHidden:YES];
} else {
NSLog(#"BAD IMAGE!!! %#", info);
[photoCell.activityIndicator setHidden:NO];
}
}];
}
Then in didReceiveMemoryWarning I clean up a bit...
- (void)didReceiveMemoryWarning {
NSArray *allKeys = [_images allKeys];
for (int i = 0; i < _images.count; i++) {
NSIndexPath *path = [allKeys objectAtIndex:i];
if (path.section != _lastIndex && path.section != _lastIndex - 1 && path.section != _lastIndex + 1) {
[_images removeObjectForKey:path];
}
}
// Though one could just as easily do this
// [_images removeAllObjects];
}
It's not pretty, but it works.