#Published requires willSet to fire - swift

Question

I have a class:
final class LocationManager: NSObject, ObservableObject, CLLocationManagerDelegate {
let objectWillChange = PassthroughSubject<Void, Never>()
private let locationManager = CLLocationManager()
#Published var status: String? {
willSet { objectWillChange.send() }
}
#Published var location: CLLocation? {
willSet { objectWillChange.send() }
}
// ...other code
}
And then I have a view that observes this class:
struct MapView: UIViewRepresentable {
#ObservedObject var lm = LocationManager()
// ...other view code
}
Everything works fine and the view updates when the published property changes. However, if I remove the willSet { objectWillChange.send() } then the view that observes an instance of LocationManager does not update when the published location changes. Which brings me to my question: I thought that by putting #Published next to a var that any #ObservedObject will invalidate the current view when the published property changes, essentially a default objectWillChange.send() implementation but this doesn't seem to be happening. Instead I have to manually call the update. Why is that?

Answer 1

You're doing too much work. It looks like you're trying to write ObservableObject. You don't need to; it already exists. The whole point is that ObservableObject is already observable, automatically. Here's a non-SwiftUI example:
final class Thing: NSObject, ObservableObject {
#Published var status: String?
}
class ViewController: UIViewController {
var storage = Set<AnyCancellable>()
let thing = Thing()
override func viewDidLoad() {
self.thing.objectWillChange
.sink {_ in print("will change")}.store(in: &self.storage)
self.thing.$status
.sink { print($0) }.store(in: &self.storage)
}
#IBAction func doButton (_ sender:Any) {
self.thing.status = (self.thing.status ?? "") + "x"
}
}
The thing to notice is that, although the observable object contains no code at all, it is emitting a signal every time its status property is set, before the property changes. Then the status property itself emits a signal, namely its new value.
The same thing happens in SwiftUI.

Answer 2

Apple documentation states:
By default an ObservableObject synthesizes an objectWillChange
publisher that emits the changed value before any of its #Published
properties changes.
This means you don't need to declare your own objectWillChange. You can just remove objectWillChange from your code including the following line:
let objectWillChange = PassthroughSubject<Void, Never>()