Swift 5: What's 'Escaping Closure Captures Mutating 'Self' Parameter' and How to Fix It

Swift 5 : What's 'Escaping closure captures mutating 'self' parameter' and how to fix it

The problem is that ContentView is a struct, which means it's a value type. You can't pass that to a closure and mutate it. If you did, nothing would change, because the closure would have its own independent copy of the struct.

Your problem is that you've mixed your View and your Model. There can be many, many copies of a given View (every time it's passed to a function, a copy is made). You wouldn't want every one of those copies to initiate a request. Instead move this request logic into a Model object and just let the View observe it.

Escaping closure captures mutating 'self' parameter: struct

As you have found, the quick solution is to use a reference type, a class. But why is this the case?

Swift structs are value types, so they are immutable. You can mark a function as mutating to indicate to the compiler that a function mutates the struct, but what does that actually mean?

Consider a simple struct:

struct Counter {
   var count

   init(_ count: Int = 0)
   {
       self.count = count
   }

   mutating func increment() {
       self.count+=1
   }
}

Now, try and assign an instance of this to a let constant:

let someCounter = Counter()
someCounter.increment()
print(someCounter.count)

You will get an error; you need to use a var.

var someCounter = Counter()
someCounter.increment()
print(someCounter.count)

What actually happens when you call a mutating func is that a new Counter is created, with the new count and it is assigned to someCounter. It is effectively saying someCounter = Counter(someCounter.count+1)

Now, think what would happen if you could mutate self in an escaping closure - That new Counter is going to be created at some unspecified time in the future, but execution has already moved on. It is too late to update someCounter.

The other advantage of using a class, as you have found, is that you can use ObservableObject, which makes updating your SwiftUI views much easier.

SwiftUI Escaping closure captures mutating 'self' parameter

An object's initializer cannot do anything asynchronous. Its job is to produce the object immediately, with all its properties initialized.

SwiftUI: What's 'Escaping closure captures mutating 'self' parameter' and how to fix it

EDIT:
Seems like you cannot mutate structs anymore in escaping closure without removing @escaping which not be possible in your case. You might want to consider changing your implementation to a class.

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Structs are immutable. Which mean they cannot be mutated. In your case you are modifying the value of self.bool1 = true which is changing the value of self.

~~A better way (IMO) would be to create a mutating func to do your firebase call and update the values inside mutating function.~~

Also, you shouldn’t use State property wrappers in models. They should only be used in views.

Escaping closure captures mutating 'self' parameter

All instances methods receive a reference to self as an implicit first parameter, which is why you have to capture self to call the instance method. Is you don't actually need any instance variables then make doCoolStuff() a static function and you will no longer need to call it with self. (you can use Self. as of Swift 5, or just the type name ViewModel.). You can similarly avoid this issue if doCoolStuff is a closure defined inside of init.

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