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?

Solution

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)

Comments

[staticVoidMan]

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.