Implementing all states through structs, and moving the logic

Each state will be responsible for its own logic and transitions, so we can start moving the logic into each state:

struct UnknownState: CardReaderState {
    static let shared = UnknownState()
    func perform(context: CardReader) {
        // Perform local initialization, When everything is ready
        // Toggle the state to Waiting
        context.state = WaitingState.shared
     }
}

As you can see, this is cleaner, as our initialization states are encapsulated. In the example code following, the rest of the logic is even more encapsulated:

struct WaitingState: CardReaderState {
    static let shared = WaitingState()
    func perform(context: CardReader) {
        guard seenCard() else { return }
        // we have seen a card!
        context.state = ReadingState.shared
    }
}

struct ReadingState: CardReaderState {
    func perform(context: CardReader) {
        // Read the contents of the card over the radio
        // This is quite complext and can take a while
        if readCard() {
            // Card was read
            context.state = ReadState(card: CardInfo())
        } else {
            context.state = ErrorState(error: ReadError())
        }
    }
}

struct ReadState: CardReaderState {
    let card: CardInfo
    func perform(context: CardReader) {
        // Open the gates to the metro doors
        // And reset back to waiting
        context.state = WaitingState.shared
    }
}

struct ErrorState: CardReaderState {
    let error: Error
    func perform(context: CardReader) {
        // display an error, and go back to waiting
        context.state = WaitingState.shared
    }
}