Cannot assign value to stored property from within computed property - swift

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.

Answer 2

Structs are value types and they are immutable.
This means that you cannot set variable and mutate self.
If you need to mutate struct, you have to make mutating func
public struct PhotoStruct {
var _photo:UIImage?
var urlString:String?
mutating func loadPhoto() -> UIImage {
.. Code here
_photo = UIImage(data:imageData)
// Here you mutate a struct, but it's ok because we make method as mutating
}
}
In you example I would make a mutating method instead of property
mutating func photo () -> UIImage? {
if let photo = self._photo {
return photo
} else {
if let urlString = self.urlString,
url = NSURL(string: urlString),
imageData = NSData(contentsOfURL: url),
image = UIImage(data:imageData) {
self._photo = image
}
return self._photo
}
}

Answer 3

PhotoStruct is a struct and therefore a value type. For value types,
only methods explicitly marked as mutating can modify the properties
of self, so this is not possible within a computed property.
If you change PhotoStruct to be a class then your code compiles
without problems.