I write some photos to the photo library using UIImageWriteToSavedPhotosAlbum() and at the same time I display the contents of this asset group (ALAssetsGroupSavedPhotos) using enumerateAssetsUsingBlock: and friends. Sometimes the assets returned by enumerating the group become sort of “invalid”, meaning that the defaultRepresentation call returns nil, although the asset is still in memory.
I noticed that this seems to happen after the photo library gets modified by the UIImageWriteToSavedPhotosAlbum() call. Is this a documented behaviour? How can I prevent it? Reloading the assets is not a feasible option, as the user might already be somewhere deeper in the UI working with the asset.
this is an unfortunate, but documented behavior. For reference:
"ALAssetsLibraryChangedNotification Sent when the contents of the assets library have changed from under the app that is using the data. When you receive this notification, you should discard any cached information and query the assets library again. You should consider invalid any ALAsset, ALAssetsGroup, or ALAssetRepresentation objects you are referencing after finishing processing the notification."
So what you have to do is to register an observer for ALAssetsLibraryChangedNotification. (And there is a bug in regarding this notification on iOS 5.X, see Open Radar.)
When you receive the notification you have to reenumerate all groups and assets. There is at the moment no other way. This is very unfortunate from a GUI perspective and we can only hope Apple improves this mechanism in the future.
Cheers,
Hendrik