Cannot assign value to stored property from within computed property

Question

I created a struct in which I have a property '_photo' that is lazily assigned when the computed property 'photo' is called. I keep getting the error

Cannot assign to '_photo' in 'self'

Here is the code. I wrote the computed method in both Swift 1.1 (photo1) and swift 1.2 (photo2) syntax. Both give the same compile time error as noted above.

What changes are needed to fix that error?

import UIKit

public struct PhotoStruct {
    var _photo:UIImage?
    var urlString:String?

    init(image:UIImage?, url:String?){
        self._photo = image
        self.urlString = url

    }

    init(url:String?){
        self.urlString = url
    }


    var photo1:UIImage? {

        if let theURL = self._photo {
            return self._photo
        }else{
            if let urlString = self.urlString{
                if let url = NSURL(string: urlString as String){
                    if let imageData :NSData = NSData(contentsOfURL: url){
                        if let image:UIImage = UIImage(data:imageData){

                            self._photo = image //** Cannot assign error is here**
                        }
                    }
                }
            }
            return self._photo
        }
    }

    var photo2:UIImage? {

        if let theURL = self._photo {
            return self._photo
        }else{
            if let  urlString = self.urlString,
                    url = NSURL(string: urlString as String),
                    imageData :NSData = NSData(contentsOfURL: url),
                    image:UIImage = UIImage(data:imageData){

                            self._photo = image //** Cannot assign error is here**
            }
            return self._photo
        }
    }



}

Answer 1

As for struct, If you want to mutate the self property inside computed properties, you have to explicitly declare the getter as mutating get { .. }

public struct PhotoStruct {
    var _photo:UIImage?
    var urlString:String?

    init(image:UIImage?, url:String?){
        self._photo = image
        self.urlString = url
    }

    init(url:String?){
        self.urlString = url
    }

    var photo1:UIImage? {
        mutating get {
    //  ^^^^^^^^^^^^
            // you can set `self._photo = image` here
        }
    }

    var photo2:UIImage? {
        mutating get {
    //  ^^^^^^^^^^^^
            // you can set `self._photo = image` here
        }
    }
}

Of course, the struct itself have to be mutable:

var pVar:PhotoStruct = PhotoStruct(image: nil, url: nil)
pVar.photo1 // no problem
let pLet:PhotoStruct = PhotoStruct(image: nil, url: nil)
pLet.photo1 // < [!] error: immutable value of type 'PhotoStruct' only has mutating members named 'photo1'

One caveat:

As far as I know, mutating get { } feature is undocumented on the language reference.