UIImage decompression causing scrolling lag

Question

I have this app with a full screen tableView that displays a bunch of tiny images. Those images are pulled from the web, processed on a background thread, and then saved to disk

Answer 1

Your problem is that +imageWithContentsOfFile: is cached and lazy loading. If you want to do something like this, instead use this code on your background queue:

// Assuming ARC
NSData* imageFileData = [[NSData alloc] initWithContentsOfFile:thumbPath];
UIImage* savedImage = [[UIImage alloc] initWithData:imageFileData];

// Dispatch back to main queue and set image...

Now, with this code, the actual decompression of the image data will still be lazy and cost a little bit, but not nearly as much as the file access hit you're getting with the lazy loading in your code example.

Since you're still seeing a performance issue, you can also force UIImage to decompress the image on the background thread:

// Still on background, before dispatching to main
UIGraphicsBeginImageContext(CGSizeMake(100, 100)); // this isn't that important since you just want UIImage to decompress the image data before switching back to main thread
[savedImage drawAtPoint:CGPointZero];
UIGraphicsEndImageContext();

// dispatch back to main thread...