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Editorial Team
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Asked: 2026-05-20T18:52:17+00:00

I’m using a UIImagePickerController to capture a still image. I then need to do

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I’m using a UIImagePickerController to capture a still image. I then need to do some processing work before saving different copies of the image to a Core Data store. The processing and saving work can take up to 4-8 seconds on an iPhone 4 so I’m trying to branch the work off to a background queue so the whole app and UI doesn’t block.

At the root of my question is this. Is it possible to use a UIImage in a background thread so long as the UIImage object is totally confined to that thread? I found the following in apple’s thread safety summary about NSImage. I’m assuming UIImage would work the same way.

NSImage Restrictions:

One thread can create an NSImage object, draw to the image buffer, and pass it off to the main thread for drawing. The underlying image cache is shared among all threads. For more information about images and how caching works, see Cocoa Drawing Guide.

Can anyone confirm this or is it just plain wrong to touch something like a UIImage object outside of the main thread. If using a confined UIImage instance is okay, then I that leads to another issue. The UIImagePickerController returns an NSDictionary which is thread safe, but within that NSDictionary is a UIImage object. Is it safe to pass that dictionary off to another thread then use the contained object to within that thread?

If the UIImage in the imagePicker info dictionary is not safe to use then any suggestions on how best to proceed?

I think I have the actual Core Data threading issues figured out. But for information I currently write and retrieve the image data using an NSValueTransformer to transform a UIImage to and from NSData within a custom NSManagedObject subclass.

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1 Answer

  1. Editorial Team

    I worked on some “Process an image after UIImagePickerController” code in a background thread just as you are and had no problems. Below are the steps I had success with.

    1. Image Picker Delegate returns user image.
    2. Assign image to the background thread class (In my case it was a @property retain’ed IVAR)
    3. close/dismiss Image Picker
    4. Background processing continues and creates a new UIImage after manipulating the original
    5. new UIImage is returned to appropriate VC
    6. Background process ends.
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