Unable to edit screenshots, performChanges block fails

Question

I'm developing an app that allows users to edit photos using PhotoKit. I was previously saving the edited photo to disk as a JPEG. I would like to avoid converting to JPEG and have implemented the modifications in order to do that. It works great for photos taken with the camera, but if you try to edit a screenshot, the PHPhotoLibrary.sharedPhotoLibrary().performChanges block will fail and log The operation couldn’t be completed. (Cocoa error -1.). I am not sure why this is causing the performChanges block to fail, what have I done wrong here?

I've created a sample app available to download that demonstrates the problem, and I've included the relevant code below. The app attempts to edit the newest photo in your photo library. If it succeeds it will prompt for access to edit the photo, otherwise nothing will happen and you'll see the console log. To reproduce the issue, take a screenshot then run the app.

Current code that works with screenshots:

let jpegData: NSData = outputPhoto.jpegRepresentationWithCompressionQuality(0.9)

let contentEditingOutput = PHContentEditingOutput(contentEditingInput: self.input)

var error: NSError?
let success = jpegData.writeToURL(contentEditingOutput.renderedContentURL, options: NSDataWritingOptions.AtomicWrite, error: &error)
if success {
    return contentEditingOutput
} else {
    return nil
}

Replacement code that causes screenshots to fail:

let url = self.input.fullSizeImageURL
let orientation = self.input.fullSizeImageOrientation
var inputImage = CIImage(contentsOfURL: url)
inputImage = inputImage.imageByApplyingOrientation(orientation)

let outputPhoto = createOutputImageFromInputImage(inputImage)!

let originalImageData = NSData(contentsOfURL: self.input.fullSizeImageURL)!
let imageSource = CGImageSourceCreateWithData(originalImageData, nil)

let dataRef = CFDataCreateMutable(nil, 0)
let destination = CGImageDestinationCreateWithData(dataRef, CGImageSourceGetType(imageSource), 1, nil) //getType automatically selects JPG, PNG, etc based on original format

struct ContextStruct {
    static var ciContext: CIContext? = nil
}
if ContextStruct.ciContext == nil {
    let eaglContext = EAGLContext(API: .OpenGLES2)
    ContextStruct.ciContext = CIContext(EAGLContext: eaglContext)
}

let cgImage = ContextStruct.ciContext!.createCGImage(outputPhoto, fromRect: outputPhoto.extent())

CGImageDestinationAddImage(destination, cgImage, nil)

if CGImageDestinationFinalize(destination) {
     let contentEditingOutput = PHContentEditingOutput(contentEditingInput: self.input)

     var error: NSError?
     let imageData: NSData = dataRef
     let success = imageData.writeToURL(contentEditingOutput.renderedContentURL, options: .AtomicWrite, error: &error)
     if success {
          //it does succeed
          return contentEditingOutput
     } else {
          return nil
     }
}

Answer 1

The problem happens due to the fact that adjusted photos are always saved as JPG files, and screenshots are in fact PNG files.

It occurred to me while I was debugging your sample project and saw the in the PhotoEditor, contentEditingOutput.renderedContentURL is a URL to a JPG, while if you examine the result of CGImageSourceGetType(imageSource) it is clear the it's a PNG (returns a PNG UTI: public.png).

So I went and read the documentation for renderedContentURL which states that if editing a photo asset, the altered image is written in JPEG format - which clearly won't work if your image is a PNG. This leads me to think that Apple don't support editing PNG files or don't want you to. Go figure..