Observable is both a macro and a protocol

ObservableProtocol.swift

import Observation

protocol P: AnyObject, Observable {
    var value: Int { get set }
}

@Observable
class MyObject: P {
    var value: Int = 0
}

// Usage: somewhere else in your code:

let obj: any P = MyObject()
withObservationTracking {
    // Observe
    print("willSet, value is", obj.value)
} onChange: {
    print("onChange")
}
obj.value += 1
// Prints:
// willSet, value is 0
// onChange