I deleted my last question on this topic in order to research the problem further.
I'm working on a content based image retrieval database that indexes the local photo album on iPhone. I've written filters and methods for feature extraction, but as a preprocessing step, I use CIDetector to detect face rectangles and filter those rectangles through a local binary pattern filter and extract histograms of small rectangles.
The process can obviously take a good amount of time for indexing a large photo album, so I've designed it to be asynchronous, with a background thread that runs the filter and extracts features.
The trouble with this is that the only method to access photo album images seems to be via PHAsset. There are two API's that are commonly used with it, both of which require callback blocks. One of them retrieves a UIImage representation, this is not what I want, primarily because I will need to load many of them and memory constraints are tight and I don't intend to show the image at all. It will also return a dictionary of objects, one of which being the URL for the image file itself, but using +[CIImage imageWithContentsOfURL] will return nil and the iOS console will show that permission is denied to the image when using this API.
The other is -[PHAsset requestContentEditingInputWithOptions]. This particular API will force the callback block to be invoked in the main queue, no matter what queue called the API, the callback block is always dispatched into the main queue. Using this API, a CIImage can be successfully be loaded using +[CIImage imageWithContentsOfURL], meaning that the API has effectively given permission to access the file. I can then put the CIImage on a different queue that will perform all index processing steps.
This approach seems to work, but only for a short time. Eventually the app crashes, and all I see is that during earlier processing a message shows up that says Connection to assetsd was interrupted or assetsd died.
If I don't queue the operation on another queue and just let it run on the main queue which the API forces the callback into, everything is fine and that message doesn't show up in the console. But this leaves my main thread constantly busy doing very expensive image processing.
Edit: It occurs to me that I'm probably misusing this API, but it appeared to be the only thing that would give me the data I wanted in the appropriate format (a CIImage that could be used to load pixel data from a file when necessary, during processing).
The API I'm currently using is probably more suited for an interactive image filtering session where the image edit data is used with UI and the service that is managing it keeps a session open with the app.
