Photo Editing Extensions are a powerful way for apps to integrate with the system Photos app. Users can begin editing a photo, jump into a third-party extension, and return seamlessly back to the system editing interface. Let's try making one.

We begin by adding a new Photo Editing Extension target to our project.

Our extension will use CoreImage (covered in Bite #32) to convert a photo to black and white. The basic workflow goes like this:

1. User launches our extension while editing inside Photos.app
2. a PHContentEditingInput object is handed to us via the startContentEditingWithInput... function
3. User performs the edits they would like using our view controller
4. User taps Done button (provided by the system)
5. System asks us for a PHContentEditingOutput object containing our changes to the image via finishContentEditingWithCompletionHandler.

Let's move on to the code.

Along with the input object, we also receive a placeholderImage which we'll use as the initial image in our own image view.

func startContentEditingWithInput(contentEditingInput: PHContentEditingInput?, placeholderImage: UIImage) {
  input = contentEditingInput
  imageView.image = placeholderImage
}

After processing, we create an output using the intial input object we were handed, and write our modified image to it's content URL.

let output = PHContentEditingOutput(contentEditingInput: input)

processedImageData.writeToURL(
  output.renderedContentURL, 
  atomically: true
)

Lastly, we describe our adjustments, so we can be a good citizen in Photos.app's non-destructive editing world.

output.adjustmentData = PHAdjustmentData(
  formatIdentifier: NSBundle.mainBundle().bundleIdentifier!,
  formatVersion: "1.0",
  data: "Grayscale".dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)!
)

Download the complete project here: j.mp/bite058