Home/ Questions/Is it okay to fix the ''escaping closure captures mutating 'self' parameter'' problem?
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Rowan Evans
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Asked: 2022-05-18T19:08:53+00:00 In:

Is it okay to fix the ”escaping closure captures mutating ‘self’ parameter” problem?

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I am working with programs and getting the error message:

Closure cannot implicitly capture a mutating self parameter

Here is the detail of the code that I ran:

FirebaseRef.observeSingleEvent(of: .value, with: { (snapshot) in
  if let _ = snapshot.value as? NSNull {
  self.img = UIImage(named:"Some-image")!
  } else {
  self.img = UIImage(named: "some-other-image")!
  }
 })

I need an explanation for the problems I’ve encountered. How to fix escaping closure captures mutating ‘self’ parameter?

swift
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2 Answers

  1. Best Answer
    Expert

    The cause:

    FirebaseRef.observeSingleEvent(of:with:) is a struct, it’s also a type of value. In this case a mutating context doesn’t know that you are mutating and binding self in an @escaping closure. Therefore, the error happens.

    Solution:

    For short versions:

    You only need to update your current type to a reference one with class.

    For longer versions:

    You also can change your current type observeSingleEvent(...) into a reference type like a class, it’s better than a struct:

    class Foo {
init() {}
private func bar(with block: @escaping () -> ()) { block() }

func bax() {
bar { print(self) }
}
}

    If you want to capture self weakly or strongly, it’s depends on your case. For example:

    FirebaseRef.observeSingleEvent(of: .value, with: { [weak self] (snapshot) in ...
    • 0

  2. Sync Solution

    If you want to modify a value type (struct), it may not work for async calls. However, it will work synchronously if you write it this way: 

    struct Banana {
 var isPeeled = false
 mutating func peel() {
 var result = self

 SomeService.synchronousClosure { foo in
 result.isPeeled = foo.peelingSuccess
 }

 self = result
 }
}

    A “mutating self”, with different value types, cannot be captured by any other means than providing a copy (hence var).

    Why not Async instead?

    This is because you can’t mutate result with no compiler error. However, you cannot return the mutated result to self. Despite the fact that there will be no error, self won’t change since the closure of peel() is not dispatched.

    You can try to circumvent this by changing your code to make the async calls synchronous. Wait for the call to complete. Although technically possible, it is unlikely that this will serve the purpose of the async API with which you are interacting. You should change your approach.

    Although technically sound, changing struct to class doesn’t solve the problem. As an example, the class Banana property can be changed synchronously by anyone-knows-when. This is difficult to understand and will lead to problems. It’s better to write an API handler outside of the model and then fetch and modify the model object upon completion. This is because it doesn’t have enough context. I assume that this is model code since self.img has been mutated in OP’s source code.

    It may be possible to add “async corruption” objects.

    This is what I am thinking of:

    • A BananaNetworkRequestHandler executes requests simultaneously and reports the result to a BananaStore
    • The BananaStore will then look for peelingResult.bananaID on its inside and take the Banana appropriate Banana.
    • After finding an object with banana.bananaID == peelingResult.bananaID it sets banana.isPeeled = peelingResult.isPeeled.
    • Finally, replace the original object by the mutated instance.

    It is easy to see that the search for a simple solution can quickly become complicated, especially if you need to change the architecture of an app.

    • 9

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