Swift: Should ViewModel be a struct or class? - swift

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.

Answer 2

You could get the self.data in two options: either use a return parameter in your closure for fetchResponse( using viewModel as struct) OR you can create your own set-method/closure and use it in your init method(using viewModel as class).
class ViewModel {
var data: Int = 0
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! { didSet { print("viewModel.data in didSet : \(viewModel.data)") } }
init( viewModel: ViewModel ) {
// closure invokes didSet
({ self.viewModel = viewModel })()
}
func changeViewModelStruct() {
print("viewModel.data before fetch : \(viewModel.data)")
viewModel.fetchData {
print("viewModel.data after fetch : \(self.viewModel.data)")
}
}
}
let viewModel = ViewModel()
var c = ViewController(viewModel: viewModel)
c.changeViewModelStruct()
Console prints:
viewModel.data in didSet : 0
viewModel.data before fetch : 0
viewModel.data in fetchResponse : 10
viewModel.data after fetch : 10
Apple Document
says like this:
willSet and didSet observers are not called when a property is first initialized. They are only called when the property’s value is set outside of an initialization context.