iOS: dispatch_async and UIImageWriteToSavedPhotosAlbum

Question

Just learning how to allocate tasks among threads, or dispatch asynchronously. I understand that any operation that "touches" a view must be done on the main thread. What about: UIImageWriteToSavedPhotosAlbum? I would assume this could be done on a background thread, but am I mistaken?

Also, if it should be done on a background thread, is there a difference between these two calls below, as one saves a UIImage and the other saves a UIImage from a view?

UIImageWriteToSavedPhotosAlbum(_someUIImage ,nil,nil,nil);

UIImageWriteToSavedPhotosAlbum(_imageView.image ,nil,nil,nil);

By the way I am using this setup to run an HUD in the main thread and to tasks in the background, that is my intention.

[HUD_code showMessage:@"saving image"];
dispatch_queue_t concurrentQueue =
dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(concurrentQueue, ^{
        UIImageWriteToSavedPhotosAlbum(someUIImage ,nil,nil,nil);
            dispatch_async(dispatch_get_main_queue(), ^{
                [HUD_code dismiss];
         });
});

Answer 1

UIKit classes are documented to be usable from the main thread only, except where documented otherwise. (For example, UIFont is documented to be thread-safe.)

There's no explicit blanket statement about the thread safety of UIKit functions (as distinct from classes), so it's not safe to assume they generally thread-safe. The fact that some UIKit functions, like UIGraphicsBeginImageContext, are explicitly documented to be thread-safe, implies that UIKit functions are not generally thread-safe.

Since UIImageWriteToSavedPhotosAlbum can send an asynchronous completion message, you should just call it on the main thread and use its completion support to perform your [HUD_code dismiss].