You can make a great QRCode scanner using ZXingObjC. It's a barcode image processing library designed to be used on both iOS devices and in Mac applications.
Let the user upload an image from photo library and get the qr code scanned from that uploaded image. I know how to use UIImagePickerController (used it before ...
How to choose a photo from the camera roll using UIImagePickerController ... How to scan a QR code. iOS has built-in support for scanning QR codes using ...