Swift - Use of unresolved identifier 'self' - from Closure in a Class

Question

I am trying to reference a property in my class from a closure declared in my class. I cannot access self from inside my closure, and I'm assuming self would refer to the Class API from within my closure.

I want to declare a closure that I use later as a parameter to pass to a URLSession dataTask (It works without the one error line). I get the error listed in the title.

Use of unresolved identifier 'self'

I've been writing swift for an entire day now and am just trying things out as a sandbox, so I fully expect some criticism.

class Api {

    struct Location {
        var name = String()
        var author = String()
        var averageRating: String?
        var id = Int()
        var lat = Double()
        var lon = Double()
        var type = String()
    }

    var locations = [Location]()

    var doSomething = {(data: Data?, response: URLResponse?, error: Error?) -> Void in
        if error != nil {
            print(error!.localizedDescription)
        } else {
            do {
                if let json = try JSONSerialization.jsonObject(with: data!, options: .allowFragments) as? [String: Any] {
                    let myResult = json["results"] as! [[String: Any]]
                    var location : Location! = Location()
                    for jsonLocation in myResult {
                        if let name = jsonLocation["name"]{location.name = name as! String}
                        if let author = jsonLocation["author"]{location.author = author as! String}
                        if let id = jsonLocation["id"]{location.id = id as! Int}
                        if let lat = jsonLocation["lat"]{location.lat = lat as! Double}
                        if let lon = jsonLocation["lon"]{location.lon = lon as! Double}
                        if let type = jsonLocation["type"]{location.type = type as! String}

                        //ERROR IS HERE, Why does self not reference class API?
                        self.locations.append(location)
                    }
                }
            } catch {
                print("error in JSONSerialization")
            }
        }
    }
}

I have found this, but this example is different so I wasn't sure if it was the same bug or me not understanding swift.

Is there a reason to use a computed property instead of a function for doSomething? — Leon, May 09 '17 at 14:26
Nope, simply that I didn't think I needed a function. But I can simply use it. I was mostly just trying to learn to use a closure. I understand a function is a type of closure, but past that, I don't fully know why both exist. — Diesel, May 09 '17 at 21:43
Yah so the fix is making it a function, but I still don't understand why a closure, which a function is a type of, doesn't work. — Diesel, May 10 '17 at 09:58

score 14 · Accepted Answer · answered Jul 06 '17 at 02:21

Rahul's explanation is correct, but the suggested answer is ever so slightly incomplete.

Here is a complete solution:

Declare the doSomething property as lazy as Rahul suggested. A lazy stored property is a property whose initial value is not calculated until the first time it is used. In other words this closure will not be evaluated until the doSomething property is called at run-time, at which point self is guaranteed to exist. See Lazy Stored Properties in the Swift Programming Language for more details.
Add a type annotation to the doSomething property so the compiler doesn't have to infer the type at compile time, which apparently it can't do because the closure includes self. See Type Safety and Type Inference in the Swift Programming Language for more details.

So the complete declaration is:

...
lazy var doSomething: (Data?, URLResponse?, Error?) -> Void = { (data: Data?, response: URLResponse?, error: Error?) -> Void in
...

ps. Welcome to Swift programming! It's a fantastic language and really fun. I hope you enjoy it as much as I do.

The second issue here regarding the required type annotation appears to no longer be required as of Swift 3.1. I just tested this on macOS and IBM's Swift Sandbox which allows you to run different versions of Swift in a Linux environment: https://swift.sandbox.bluemix.net/ — Jeffrey Fulton, Aug 15 '17 at 15:22

score 6 · Answer 2 · answered May 09 '17 at 13:21

6

You are not able to access self because it is not available when you are calling inside the closure as initialization hasn't happened yet and so compiler gives you the error.

The fix would be to user lazy var as this will defer the self call because lazy var get called only after initialisation.

 lazy var doSomething = { your closure goes here }

answered May 09 '17 at 13:21

Rahul

2,056
1
21
37

1

I still get the same issue with adding the word lazy. – Diesel May 09 '17 at 21:44

score 2 · Answer 3 · answered Jul 06 '17 at 00:47

I have the same questions as you and I solve it using lazy var
Here is a quick example

My originally code is:

class MyClass {
    let callback:()->() = {
        self.foo()   // Compile error: Use of unresolved identifier "self"
    }

    func foo() {
        print("bar")
    }
}

It compile error at that line I use self
But I change it to

class MyClass {
    lazy var callback:()->() = {
        [unowned self] in
        self.foo()
    }

    func foo() {
        print("bar")
    }
}

let c = MyClass()
c.callback()

that solved the problem

References: https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Properties.html
http://mikebuss.com/2014/06/22/lazy-initialization-swift/
Shall we always use [unowned self] inside closure in Swift

Excellent answer, thanks. :) Note that with value types you will still run into the *Closure cannot implicitly capture a mutating self parameter* [issue](https://stackoverflow.com/a/41941810) when using `lazy`, so be sure to simply add `[self]` to the closure capture list. EDIT: forgot to link to the aforementioned issue. — Fran Pugl, Apr 24 '19 at 13:22

score 0 · Answer 4 · answered May 09 '17 at 13:40

0

Replace var locations = [Location]() with this var locations : [Location]?

and var location : Location! = Location() with self.locations = [Location]() and self.locations.append(location) with self.locations?.append(location)

You will be good to go!

lazy var is too complex a concept to grasp I guess but you can use it this way:

lazy var locations:[Location] = {
        let locations = Location()
        return locations
    }()

answered May 09 '17 at 13:40

Vamshi Krishna

979
9
19

I still get the same issue with both fixes. If I remove self from the locations after making it lazy it says "Instance member 'locations' cannot be used on type 'Api'" – Diesel May 09 '17 at 21:47

Swift - Use of unresolved identifier 'self' - from Closure in a Class

4 Answers4