Why objectWillChange has no effect - swift

Question

I got this sample code about ObservableObject from Apple official website
import Foundation
class Contact: ObservableObject {
#Published var name: String
#Published var age: Int
init(name: String, age: Int) {
self.name = name
self.age = age
}
func changeAge() -> Int {
self.age += 1
return self.age
}
}
class Test {
init() {
let john = Contact(name: "John Appleseed", age: 24)
_ = john.objectWillChange
.sink { _ in
print("\(john.age) will change")
}
print(john.changeAge())
}
}
let test = Test()
When running on terminal by swift Contact.swift, the result is only 25, but official website shows the result should be
// Prints "24 will change"
// Prints "25"
Why the first line Prints "24 will change" is not shown?
Thank you.

Answer 1

You have to store the AnyCancellable that's returned by sink, otherwise it cancels the subscription as soon as it's deinitialized when you assign to _.
In your simple example just assigning to a local variable is enough to get the printout that you want, because the variable still lives when you change the age:
let c = john.objectWillChange.sink {...}
But in a real app, you'd want to assign it to a property, which would keep the subscription for the duration that you need. Typically, it's done like this:
class Test {
private var c: Set<AnyCancellable> = []
init() {
let john = Contact(name: "John Appleseed", age: 24)
john.objectWillChange
.sink { _ in
print("\(john.age) will change")
}
.store(in: &c) // store here
print(john.changeAge())
}
}