SwiftUI not releasing model instances - swiftui

Question

Having a SwiftUI project generated by Xcode, and adding a custom MyView with a MyViewModel.
The ContentView just renders MyView.
The problem:
When the ContentView gets reloaded (the reload button changes its state), MyViewModel gets somehow disconnected from MyView (the MyView counter stops incrementing in the UI when the button is clicked), but the console logs show the incrementation works.
If the model subscribes to a publisher, it does not get unsubscribed because the instance is not released. Therefore, the instances still process incoming messages and alter the app's data and state.
Looking at the instance counters and memory addresses in the console:
Every time the ContentView gets refreshed, a new MyView and MyViewModel instances get created. However, the counter incrementation uses the original first-created model instance.
Some model instances did not get released.
EDIT: The model needs to be recreated every time MyView is recreated.
import SwiftUI
struct ContentView: View {
#State
private var reloadCounter = 0
var body: some View {
VStack {
Button(action: { self.reloadCounter += 1 },
label: { Text("Reload view") })
Text("Reload counter: \(reloadCounter)")
MyView().environmentObject(MyViewModel())
}
}
}
import SwiftUI
struct MyView: View {
#EnvironmentObject
private var model: MyViewModel
var body: some View {
VStack {
Button(action: { self.model.counter += 1 },
label: { Text("Increment counter") })
Text("Counter value: \(model.counter)")
}
.frame(width: 480, height: 300)
}
init() { withUnsafePointer(to: self) { print("Initialising MyView struct instance \(String(format: "%p", $0))") }}
}
import Combine
class MyViewModel: ObservableObject {
private static var instanceCount: Int = 0 { didSet {
print("SettingsViewModel: \(instanceCount) instances")
}}
#Published
var counter: Int = 0 { didSet {
print("Model counter: \(counter), self: \(Unmanaged.passUnretained(self).toOpaque())")
}}
init() { print("Initialising MyViewModel class instance \(Unmanaged.passUnretained(self).toOpaque())"); Self.instanceCount += 1 }
deinit { print("Deinitialising MyViewModel class instance \(Unmanaged.passUnretained(self).toOpaque())"); Self.instanceCount -= 1 }
}
Any clue what did I do wrong?
The image below depicts the app's UI after all the actions in the logs were performed.

Answer 1

You create new view model on every refresh, so just move it outside body, like
struct ContentView: View {
#State
private var reloadCounter = 0
private let vm = MyViewModel() // << here !!
var body: some View {
VStack {
Button(action: { self.reloadCounter += 1 },
label: { Text("Reload view") })
Text("Reload counter: \(reloadCounter)")
MyView().environmentObject(vm) // << reference only !!
}
}
}
Note: if you don't need it deeply in sub-view hierarchy then consider to use #ObservedObject instead of #EnvironmentObject and pass reference via constructor (because environment object is stored somewhere outside and you have less control over its life-cycle)