Mutating self (struct/enum) inside escaping closure in Swift 3.0

Question

In swift 2.2, We could mutate a struct or enum within a closure, when it was inside a mutating function. But in swift 3.0 its no longer possible. I get the following error

closure cannot implicitly captured a mutating self parameter

Here is a code snippet,

struct Point {
    var x = 0.0, y = 0.0

    mutating func moveBy(x deltaX: Double, y deltaY: Double) {
        x += deltaX
        y += deltaY

        test { (a) -> Void in
            // Get the Error in the below line.
            self.x = Double(a)
        }

    }

    mutating func test(myClosure: @escaping (_ a: Double) -> Void) {
        myClosure(3)
    }
}

I get that value types are not supposed to be mutable. I have cases, where I do have to modify one variable in the struct within one of the functions, when I receive the API response. (In the completion closure)

Is what I was doing in swift 2.2, impossible or is there way to accomplish this?

@MartinR it is escaping, because its Alamofire completion closure. — akshaynhegde, Sep 26 '16 at 12:37
This probably explains it: https://github.com/apple/swift-evolution/blob/master/proposals/0035-limit-inout-capture.md. — Martin R, Sep 26 '16 at 12:39
Related questions: http://stackoverflow.com/questions/37043070/capturing-a-struct-reference-in-a-closure-doesnt-allow-mutations-to-occur, http://stackoverflow.com/questions/38058280/modifying-struct-instance-variables-within-a-dispatch-closure-in-swift. — Martin R, Sep 26 '16 at 12:44
Yeah, I looked at the proposal. Looks like it isn't possible anymore. @MartinR So the only possible way for me to use a class here? — akshaynhegde, Sep 26 '16 at 12:46
I did not investigate that myself so I cannot give an answer. — Martin R, Sep 26 '16 at 12:48

score 15 · Answer 1 · edited Jun 20 '20 at 09:12

The problem is that @escaping closures can be stored for later execution:

Escaping Closures

A closure is said to escape a function when the closure is passed as an argument to the function, but is called after the function returns. ...

One way that a closure can escape is by being stored in a variable that is defined outside the function....

Since the closure can be stored and live outside the scope of the function, the struct/enum inside the closure (self) will be copied (it is a value) as a parameter of the closure. And, if it was allowed to mutate, the closure could have an old copy of it, causing unwanted results.

So, in answer to your question, you cannot; unless you are able to remove "@escaping" (not your case because it's a 3rd party API)

score 0 · Answer 2 · answered Sep 26 '16 at 12:51

0

Yeah, you can do something like this.

struct Point {

    var x = 0.0, y = 0.0

    mutating func moveBy(x deltaX: Double, y deltaY: Double) {
        x += deltaX
        y += deltaY

        test { (a) -> Void in

            self.x = Double(a)
        }
    }

    mutating func test(myClosure: (_ a: Double) -> Void) {
        myClosure(3)
    }
}

answered Sep 26 '16 at 12:51

2

Removing `@escaping` is one way but in case of async calls, you can't just get rid of `@escaping`. So this is not a valid option for this question. – staticVoidMan May 04 '18 at 10:53

score 0 · Answer 3 · answered Jun 21 '17 at 09:27

0

Struct is value type. So when use as Model or ModelView, you can make up a closure with new Value to VC.

struct Point {
    var x = 0.0, y = 0.0

    mutating func moveBy(x deltaX: Double, y deltaY: Double) {
        x += deltaX
        y += deltaY

        test { [x, y](a) -> Point in
            // Get the Error in the below line.
            return Point(x: Double(a), y: y)
        }

    }

    mutating func test(myClosure: @escaping (_ a: Double) -> Point) {
        self = myClosure(3)
    }
}

answered Jun 21 '17 at 09:27

Jenus Dong

304
2
7

2

You have just was moved things around but haven't addressed the core issue that `self` cannot be addressed from **within** an escaped closure. Think about this: What if in `myClosure(3)`, the `3` came from coming from an async call. How would your struct handle that? Bottomline: Your solution will not work with async calls. – staticVoidMan May 04 '18 at 11:09

Mutating self (struct/enum) inside escaping closure in Swift 3.0

3 Answers3

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