Referencing instance method requires equivalency (SWIFT) - swift

Question

I am trying to leverage SwiftUI and Combine to store user defaults for my application. Looking at suggestions in a few other posts, I have updated my code as you see below. However, I am now getting the error of "Referencing instance method 'send()' on 'Subject' requires the types 'Setup' and 'Void' be equivalent". It has been suggested that I change "Setup" to void in the PassthroughSubject, however this then gives a hard crash in the app at startup - " Fatal error: No observable object of type Setup.Type found."
I am at a bit of loss... any pointers would be welcomed.
============== DataStoreClass ============
import SwiftUI
import Foundation
import Combine
class Setup: ObservableObject {
private var notificationSubscription: AnyCancellable?
let objectWillChange = PassthroughSubject<Setup,Never>()
#UserDefault(key: "keyValueBool", defaultValue: false)
var somevalueBool: Bool {
didSet{
objectWillChange.send() // <====== Referencing instance method 'send()' on 'Subject' requires the types 'Setup' and 'Void' be equivalent
}
}
init() {
notificationSubscription = NotificationCenter.default.publisher(for: UserDefaults.didChangeNotification).sink { _ in
self.objectWillChange.send()
}
}
}
============= property wrapper ===========
import Foundation
#propertyWrapper
struct UserDefault<T> {
let key: String
let defaultValue: T
var wrappedValue: T {
get {
UserDefaults(suiteName: "group.com.my.app")!.value(forKey: key) as? T ?? defaultValue
} set {
UserDefaults(suiteName: "group.com.my.app")!.set(newValue, forKey: key)
}
}
}

Answer 1

The error comes from the fact that you have declared your Output type as Setup, but you are calling objectWillChange with Void.
So you have to pass self to objectWillChange:
self.objectWillChange.send(self)
Important thing to notice is that you should call objectWillChange not in didSet but in willSet:
var somevalueBool: Bool {
willSet{
objectWillChange.send(self
}
}
You never set somevalueBool, so this bit of code will not get called anyway.
Your setup should look roughly like this:
class Setup: ObservableObject {
private var notificationSubscription: AnyCancellable?
public let objectWillChange = PassthroughSubject<Setup,Never>()
#UserDefault(key: "keyValueBool", defaultValue: false)
var somevalueBool: Bool
init() {
notificationSubscription = NotificationCenter.default.publisher(for: UserDefaults.didChangeNotification).sink { _ in
self.objectWillChange.send(self)
}
}
}

Answer 2

The send method requires you to pass the input type of the subject, or a failure completion. So your send lines should pass the Setup;
objectWillChange.send(self)
That said, in most SwiftUI code, PassthroughSubject is <Void, Never> (such that send() does not require a parameter). It's not clear what the source of the crash you're describing is; we would need to see the code that's involved in the crash to debug that. I haven't reproduced it so far.

Answer 3

SwiftUI doesn't use a PassthroughSubject, it uses an ObservableObjectPublisher. I am pretty sure that that is an alias for PassthroughSubject<Void, Never> but I'm not sure. The ObservableObject protocol defines a correct objectWillChange for you so the best thing you can do is to remove your definition.
The publisher is objectWillChange and as its name suggests it should be sent in willSet and not didSet, I don't suppose that it matters much but Apple changed from didSet to willSet and my guess is that they had a good reason.