How to create a generic persistent array of ObservableObjects in Swift - swift

Question

I have a class that implements an array of ObservableObjects that will save itself to persistent storage whenever one of the objects changes. Here is a very simplified version of the working code:
class MyObject : ObservableObject {
#Published var name: String = ""
// Lots of other #Published variables
}
class MyArray {
#Published var objects: [MyObject] = []
#Published private var objectWillChange: Void = ()
private var objectChanged: AnyCancellable?
init() {
loadFromStorage()
for object in objects {
object.objectWillChange.assign(to: &$objectWillChange)
}
objectChanged = $objectWillChange.dropFirst().sink() {
self.saveToStorage()
}
}
func loadFromStorage() {
// Loads objects from persistent storage
}
func saveToStorage() {
// Saves objects to persistent storage
}
}
This all works fine. Now I would like to create a generic version of this code, but I can't get it to compile.
Here is what I have so far
class MyPersistentArray: PersistentArray<MyObject> {
}
class PersistentArray<Object: ObservableObject> {
#Published var objects: [Object] = []
#Published private var objectWillChange: Void = ()
private var objectChanged: AnyCancellable?
init() {
loadFromStorage()
for object in objects {
object.objectWillChange.assign(to: &$objectWillChange) // DOES NOT COMPILE
}
objectChanged = $objectWillChange.dropFirst().sink() {
self.saveToStorage()
}
}
func loadFromStorage() {
// Loads objects from persistent storage
}
func saveToStorage() {
// Saves objects to persistent storage
}
}
In Xcode 14.2, the call to object.objectWillChange gives the following compiler error:
Cannot convert parent type 'Published<Void>' to expected type 'Published<Object.ObjectWillChangePublisher.Output>'
How can I fix this compiler error?

Answer 1

I will answer my own question. I figured out that I had to constrain the ObservableObject using a where clause to limit the type of ObjetWillChangePublisher.Output to Void, like this:
protocol PersistentObject: ObservableObject where ObjectWillChangePublisher.Output == Void {
}
class MyObject : PersistentObject {
#Published var name: String = ""
// Lots of other #Published variables
}
class MyPersistentArray: PersistentArray<MyObject> {
}
class PersistentArray<Object: PersistentObject> {
#Published var objects: [Object] = []
#Published private var objectWillChange: Void = ()
private var objectChanged: AnyCancellable?
init() {
loadFromStorage()
for object in objects {
object.objectWillChange.assign(to: &$objectWillChange)
}
objectChanged = $objectWillChange.dropFirst().sink() { _ in
self.saveToStorage()
}
}
func loadFromStorage() {
// Loads objects from persistent storage
}
func saveToStorage() {
// Saves objects to persistent storage
}
}