enum State {
    case ready(TState)
    case inProgress(TState)

    var isPaused: Bool {
        switch self {
        case .ready: return false
        case .inProgress: return false
        }
    }
}

enum Drain {
    case foo(String)
    case bar(Int, String)
}

struct TState {
    let drain: Drain
}

class Task {
    var state = State.ready(TState(drain: .foo("wow"))) { 
        willSet {
            if newValue.isPaused { }
        }
    }

    func resume() {
        if case .ready(let tState) = state {
            print("before => \(state)")
            state = .inProgress(tState) //this assignment does not happen
            print("after => \(state)")
        }
    }
}    

//main
Task().resume()

before => ready(Test.TState(drain: Test.Drain.foo("wow")))
after => inProgress(Test.TState(drain: Test.Drain.foo("wow")))

before => ready(Test.TState(drain: Test.Drain.foo("wow")))
after => ready(Test.TState(drain: Test.Drain.foo("wow")))