Swift: Should ViewModel be a struct or class?

Question

I am trying to use MVVM pattern in my new project. First time, I created all my view model to struct. But when I implemented async business logic such as fetchDataFromNetwork with closures, closures capture old view model value then updated to that. Not a new view model value.

Here is a test code in playground.

import Foundation
import XCPlayground

struct ViewModel {
  var data: Int = 0

  mutating func fetchData(completion:()->()) {
    XCPlaygroundPage.currentPage.needsIndefiniteExecution = true
    NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: "http://stackoverflow.com")!) {
      result in
      self.data = 10
      print("viewModel.data in fetchResponse : \(self.data)")
      completion()
      XCPlaygroundPage.currentPage.finishExecution()
      }.resume()
  }
}

class ViewController {
  var viewModel: ViewModel = ViewModel() {
    didSet {
      print("viewModel.data in didSet : \(viewModel.data)")
    }
  }

  func changeViewModelStruct() {
    print("viewModel.data before fetch : \(viewModel.data)")

    viewModel.fetchData {
      print("viewModel.data after fetch : \(self.viewModel.data)")
    }
  }
}

var c = ViewController()
c.changeViewModelStruct()

Console prints

viewModel.data before fetch : 0
viewModel.data in didSet : 0
viewModel.data in fetchResponse : 10
viewModel.data after fetch : 0

The problem is View Model in ViewController does not have new Value 10.

If I changed ViewModel to class, didSet not called but View Model in ViewController has new Value 10.

Answer 1

You should use a class.

If you use a struct with a mutating function, the function should not perform the mutation within a closure; you should not do the following:

struct ViewModel {
  var data: Int = 0

  mutating func myFunc() {
      funcWithClosure() {
          self.data = 1
      }
  }
}

If I changed ViewModel to class, didSet not called

Nothing wrong here - that's the expected behavior.

---

If you prefer to use struct, you can do

  func fetchData(completion: ViewModel ->()) {
    XCPlaygroundPage.currentPage.needsIndefiniteExecution = true
    NSURLSession.sharedSession().dataTaskWithURL(NSURL(string: "http://stackoverflow.com")!) {
      result in
      var newViewModel = self
      newViewModel.data = 10
      print("viewModel.data in fetchResponse : \(self.data)")
      completion(newViewModel)
      XCPlaygroundPage.currentPage.finishExecution()
      }.resume()
  }


  viewModel.fetchData { newViewModel in
     self.viewModal = newViewModel
      print("viewModel.data after fetch : \(self.viewModel.data)")
    }

---

Also note that the closure provided to dataTaskWithURL does not run on the main thread. You might want to call dispatch_async(dispatch_get_main_queue()) {...} in it.