Preloading assets on iOS

Suppose you have several animations that we want to achieve with a sequence 开发者_高级运维of PNG in a UIImageView. If we have many images in the animation we have a delay from the moment we send the message [myImgView startAnimation] this becausethe loading of all images in memory. I noticed that the loading is lazy: as long as the startAnimation message is not sent images are not loaded into memory. To avoid the problem of delay i load all the animation in the app delegate, attach as subview and than animate once. I want to understand what is the best solution? And if my actual solution have a draback?

You're right about lazy loading. I've never been able to determine if its actually lazy spooling from disk or lazy decompression but in either case, a UIImage (and, underneath, a CGImage) is not necessarily fully processed and ready to draw until it is actually used. I assume that it may conceivably become unready again in the future, depending on exactly how Apple handle memory warnings internally.

If you wanted to be really keen, I guess the best solution would be to use CoreGraphics to load and decompress the images in the background, pausing to finish loading upon startAnimation only if loading hasn't already finished. You can't achieve that directly with UIKit objects since they're not callable anywhere but on the main thread. You'd need to get a CGImage, draw to a bitmap context then create an image from the bitmap context and post that onto the main thread to be wrapped into a UIImage. And it'd probably be smart to use an NSOperationQueue to marshall the complete list of operations.

Supposing you don't mind the additional startup cost of blocking while you load all the PNGs, and are dealing with memory warnings correctly, there shouldn't be any problems with your current approach and I can't think of a better solution while remaining in high level Objective-C stuff.