Swift mutable structs in closure of class and struct behave differently

Question

I have a class(A) that has a struct variable (S). In one function of this class I call a mutating function on struct variable,this function takes a closure. Body of this closure

Answer 1

This is not a solution but with this code we can see that the ViewController's, viewModel.data is properly set for both class and struct cases. What is different is that the viewModel.changeFromClass closure captures a stale self.viewModel.data. Notice in particular that only the '3 self' print for class is wrong. Not the '2 self' and '4 self' prints wrapping it.

class NetworkingClass {
  func fetchDataOverNetwork(completion:()->()) {
    // Fetch Data from netwrok and finally call the closure
    print("\nclass: \(self)")
    completion()
  }
}

struct NetworkingStruct {
  func fetchDataOverNetwork(completion:()->()) {
    // Fetch Data from netwrok and finally call the closure
    print("\nstruct: \(self)")
    completion()
  }
}

struct ViewModelStruct {

  /// Initial value
  var data: String = "A"

  /// Mutate itself in a closure called from a struct
  mutating func changeFromStruct(completion:()->()) {
    let networkingStruct = NetworkingStruct()
    networkingStruct.fetchDataOverNetwork {
      print("1 \(self)")
      self.data = "B"
      print("2 \(self)")
      completion()
      print("4 \(self)")
    }
  }

  /// Mutate itself in a closure called from a class
  mutating func changeFromClass(completion:()->()) {
    let networkingClass = NetworkingClass()
    networkingClass.fetchDataOverNetwork {
      print("1 \(self)")
      self.data = "C"
      print("2 \(self)")
      completion()
      print("4 \(self)")
    }
  }
}

class ViewController {
  var viewModel: ViewModelStruct = ViewModelStruct()

  func changeViewModelStruct() {
    print(viewModel.data)

    /// This never changes self.viewModel, Why Not?
    viewModel.changeFromClass {
      print("3 \(self.viewModel)")
      print(self.viewModel.data)
    }

    /// This changes self.viewModel, Why?
    viewModel.changeFromStruct {
      print("3 \(self.viewModel)")
      print(self.viewModel.data)
    }
  }
}

var c = ViewController()
c.changeViewModelStruct()