Vast memory required with UICollectionView images set from NSDictionary - ios

Question

I'm making an app which has lots of entries from a database displayed in a UICollectionView, and each item in the collectionview displays an image. However, there are lots of duplicate images (ie, my UICollectionView has 200 items but there's only about 30 possible images).
Initially I assigned these images by calling cell.imageview.image =[UIImage imageWithContentsOfFile] in my cellForItemAtIndexPath method, but that caused a bit of scrolling lag, presumably due to having to read the image from the internal memory each time.
Now I want to do all the I/O in viewDidLoad by creating an array/dictionary and just referencing that each time cellForItem is called. When I do this and create a dictionary of UIImages in viewDidLoad and have cell.imageview.image = [imageDictionary objectForKey:] it displays images correctly and scrolls really smoothly but the memory used is huge (it just increases till it's scrolled all the way to the bottom so I presume it has all 200 image instances in the RAM).
However, if alternatively I fill the dictionary in viewDidLoad with NSData of the image and set the image using cell.imageView = [UIImage imageWithData:[dataDictionary objectForKey:]], the memory used is far more efficient and reflects only what is currently displayed on the screen. However, the scrolling becomes slightly laggy again.
My question is: why does the memory leak only occur when I fill the dictionary with UIImages rather than NSData? Does iOS automatically cache images unexpectedly or something?
Hope that made sense - it's hard to be clear without going too much into the detail of my code.

Answer 1

Use your first approach to set the image as you scroll. However, load the images in a background thread and then set the image on the main thread so you don't inhibit scrolling. Also, make sure your images are the same size as the image view (ie. not huge) so they don't take up a ton of memory.
Example:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void) {
UIImage *image = [UIImage imageWithContentsOfFile... // do stuff
dispatch_async(dispatch_get_main_queue(), ^(void) {
[[cell imageView] setImage:image];
});
});