Programming iOS 6, 3rd Edition

The Assets Library Framework

The Assets Library framework does for the photo library roughly what the Media Player framework does for the music library (Chapter 29), letting your code explore the library’s contents. One obvious use of the Assets Library framework might be to implement your own interface for letting the user choose an image, in a way that transcends the limitations of UIImagePickerController. But you can go further with the photo library than you can with the media library: you can save media into the Camera Roll / Saved Photos album, and you can even create a new album and save media into it.

A photo or video in the photo library is an ALAsset. Like a media entity (Chapter 29), an ALAsset can describe itself through key–value pairs called properties. (This use of the word “properties” has nothing to do with the Objective-C properties discussed in Chapter 12.) For example, it can report its type (photo or video), its creation date, its orientation if it is a photo whose metadata contains this information, and its duration if it is a video. You fetch a property value with valueForProperty:. The properties have names like ALAssetPropertyType.

A photo can provide multiple representations (roughly, image file formats). A given photo ALAsset lists these representations as one of its properties, ALAssetPropertyRepresentations, an array of strings giving the UTIs identifying the file formats; a typical UTI might be @"public.jpeg" (kUTTypeJPEG, if you’ve linked to MobileCoreServices.framework ...