Observing a #Published var from another Object - swift

Question

I am trying to get one object to listen to changes in the property of another object. I have it working as shown below, but I would prefer the observing object knew nothing of the Model, just the property.
class Model : ObservableObject{
#Published var items: [Int] = []
}
class ObjectUsingItems{
var itemObserver: AnyCancellable?
var items: [Int] = []
func observeItems(model: Model){
itemObserver = model.$items
.sink{ newItems in
self.items = newItems
print("New Items")
}
}
}
At the moment I begin observing the model.items as follows - which works:
let model = Model()
let itemUser = ObjectUsingItems()
itemUser.observeItems(model: model)
model.items.append(1) // itemUser sees changes
Unfortunately I can’t seem to figure out just what is required as the parameter to the observeItems method so that it works without knowing anything about the Model - like this:
class ObjectUsingItems{
var itemObserver: AnyCancellable?
var items: [Int] = []
func observeItems(propertyToObserve: WhatGoesHere?){
itemObserver = propertyToObserve
.sink{ newItems in
// etc.
}
}
}
And then call it like so:
itemUser.observeItems(XXX: model.$items)
Can anyone explain what I need to do? Thanks!

Answer 1

You can just accept a publisher as a parameter, if you don't care where the value comes from.
In your very specific case, it could be:
func observeItems(propertyToObserve: Published<[Int]>.Publisher) {
itemObserver = propertyToObserve
.sink { self.items = $0 }
}
But this might be too restrictive - why only this specific publisher? In principle, you shouldn't care what the publisher is - all you care about is the output value and error type. You can make it generic to any publisher, so long as its Output is [Int] and Failure is Never (like that of the #Published one):
func observeItems<P: Publisher>(propertyToObserve: P)
where P.Output == [Int], P.Failure == Never {
itemObserver = propertyToObserve
.sink { self.items = $0 }
}
The usage would be something like this:
let model = Model()
let itemUser = ObjectUsingItems()
itemUser.observeItems(propertyToObserve: model.$items)