Handling Drag & Drop Raw Photos

There is no shortage of tutorials on the topic of Drag & Drop, but I wanted to get into a particular special case which creators of apps that support dropped images should be aware of. If your user is a photographer that shoots with a SLR and imports their images in camera raw format, those images will not be accepted for drop unless you handle them specially.

Specifically, Raw images conform to the kUTTypeRawImage URI, which is not included in UIImage’s readableTypeIdentifiersForItemProvider. This is because UIImage cannot import raw images directly, requiring a detour through Core Image. Luckily, this is pretty easy to accomplish, as Core Image has a simple way to handle Raw images.

Note: My project is using a UICollection view as the drop target, so I’m working with the methods in the UICollectionViewDropDelegate protocol. Working with custom views should be broadly similar, but I haven’t tried it out yet.

Step 1: Getting the Raw Image

When a Raw image is dropped on an app, the image data is not sent as with UIImage-compatible images accessed via UIDropSession’s loadObjects(ofClass:completion:) convenience method. Instead, a URL is provided to your app. However, if you attempt to read or copy the file at the URL, it will always fail as being unavailable. You may be tempted to try to use NSItemProvider’s loadItem(forTypeIdentifier:options:completionHandler:) method, but it has awful ergonomics in Swift (you can’t implicitly coerce a protocol into a conforming type) and what’s more, even if you do force it to give you the URLs for the Raw images, they will all be unavailable and useless, possibly due to sandbox restrictions.

The correct way to do this is to iterate over the list of UIDragItems, filtering by those which have an NSItemProvider that responds affirmatively to hasItemConformingToTypeIdentifier(_:) for kUTTypeRawImage. You can then iterate over the filtered NSItemProviders and call loadFileRepresentation(forTypeIdentifier:completionHandler:) on each one:

This method makes an accessible copy of the Raw image and returns the URL of the copy to the completion block. Why use this method and not, say, loadDataRepresentation(forTypeIdentifier:completionHandler:)? The answer is memory: Raw images tend to be very large (dozens of MB each) and if your user has dropped a bunch of them on your app, attempting to hold all of them in memory could cause the system to kill your app for eating up too much memory. Using the URL instead of the contents of the file consumes basically no memory until a specific image needs to be loaded for processing. In tests, I was able to drop 10+ Raw images onto my app for processing and never see the memory go above about 70MB, dropping back down to 20-30MB when processing completed.

Warning: The URLs provided by loadDataRepresentation only seem to be valid for the scope of the completion closure. You shouldn’t try to hold on to them and load them later, because it will fail. Instead, copy the file in your app’s sandbox (such as the Caches directory) and use the URL of the local copy to access the Raw image later.

Step 2: Converting the Raw Image

Now we have a local URL for the Raw images, but we still can’t do much with them since UIImage is unable to initialize with Raw image data. Thankfully, Apple has a simple and robust conversion mechanism based on Core Image. Instantiating a CIFilter using the init(imageURL:options:) method creates an instance of CIRawFilter, which can process all of the various Raw image formats that Apple officially supports. The filter’s outputImage property (a CIImage) can then be sent to a CIContext for rendering to a CGImage or used directly to apply filter effects and image adjustments:

In practice, the first run of this conversion is pretty slow as the CIContext sets itself up, taking up to several seconds. Subsequent uses are very fast, requiring only a faction of a second. At the end of this process, you have a UIImage that can be used just like a regular dropped image.