How Does Apple's On Demand Resource System download files to the App's Main Bundle?

Question

I am currently trying to implement Apple's API for On Demand Resource Management for AWS Cloudfront because Apple's ODR is somehow too unreliable.

I noticed that when I tag images inside of Assets.scnassets/ with an ODR resource tag, I can access that image using

UIImage(name: resourceName)

once it has been downloaded by a NSBundleRequest object. Because I can access the downloaded resource as a UIImage, I know that the resource is located in the app's main bundle but I thought this was impossible because Bundles were read-only. How did apple do this? The most important aspect is being able to create UIImages using this incredibly simple interface.

Answer 1

I know that the resource is located in the app's main bundle but I thought this was impossible because Bundles were read-only. How did apple do this?

It’s an illusion. ODR files are downloaded to a directory outside the app bundle, and calls like UIImage init(named:) are swizzled to look in that directory.