Darkroom for iPad has been announced and is currently in beta. I don’t use the app too often on iPhone — usually the editing features built into the Photos app are sufficient. But when I need something more, Darkroom is my go-to.
My current workflow for processing photos with my Canon PowerShot G9 X always involves importing those photos into my iPad for editing. This is fine most of the time, because as I said, the Photos app is usually sufficient. But when I want to edit with a bit more power, I have to AirPlay the photo to my iPhone for Darkroom. I look forward to a future when that’s no longer necessary.