Thread safety of UIImage

Question

I know that Apple officially recommends UIKit to be used in the main thread only. However, I\'ve also heard claims that UIImage is thread-safe since iOS 4.0. I cannot find

Answer 1

Thread-safe is not the issue, in that any thread can attempt to access a context at the same time (concurrently). And, while that is generally okay, in low-memory situations, like with a Photo Editing Extension on iOS devices, two threads accessing one context can crash an app due to low memory.

This happens when mixing Core Image filters with vImage operations. Both are thread-safe, but ARC will not release vImage buffer data before processing a Core Image object, so you have at some point two copies of an image in memory.

Accordingly, you never consider your knowledge of thread safety to be complete without understanding of threading and concurrency—and that goes double for any answer to questions about thread safety. In short: the proper question is, how does thread-safety apply to memory usage, whenever you're talking about image processing.

If you're just kicking the tires here, you need to wait to pose your question until you've encountered a real problem. But, if you're planning your next move, you need to know how to execute image processing commands sequentially, and with manual deallocation. You must design your app so that there is only one copy of the image being processing in memory. Never rely on automatic deallocation—thread-safe or not—to make that call for you. IT WILL NOT WORK.