RUNNING_THE_GAME.md

Running the game

All the code examples in this document are taken from the SimpleHandler and SimpleGame modules of the companion package.

The essential tool for running a game built with Narratore is the Runner actor. In turn. Runner uses an instance of some type that conforms to the Handler protocol in order to communicate with the game engine (thus, with the player) and send it the various game events, like new narration steps, or choices to make. Before diving into Runner, let's see what's required from a Handler type.

Handler is a protocol that has the following definition:

public protocol Handler {
  associatedtype Game: Setting

  func acknowledge(narration: Player<Game>.Narration) async -> Next<Game, Void>
  func make(choice: Player<Game>.Choice) async -> Next<Game, Player<Game>.Option>
  func handle(event: Player<Game>.Event)
}

Each callback function declared by Handler has a specific meaning, and a signature that's specific to the requirements of the particular functionality that the function expresses. The value that are expected as input or output are typically included in the Player<Game> namespace, that defines types that represent internal concepts of Narratore (like Narration and Choice) but in a "player-facing" fashion, that hides private details and restricts the options. Let's see each function in detail.

Also, while exploring in detail the Handler protocol and the Runner APIs, let's define a SimpleHandler class that can be used to run a game on the command line:

public final class SimpleHandler<Game: Story> {
  ...
}

We want Game to conform to Story because SimpleHandler will also take care of game data serialization.

acknowledge(narration:)

This will be called each time a new narration step must be presented to the user. A Player.Narration value is passed in the function, with the following structure:

public enum Player<Game: Setting> {
  public struct Narration {
    public let messages: [Game.Message]
    public let tags: [Game.Tag]
  }
  ...
}

The Narration will contain a list of Messages and a list of Tags (both possibly empty), of the types declared in the game Setting definition: Runner will actually skip the call to acknowledge(narration:) if both messages and tags are empty, because there would actually be nothing to acknowledge. This function is async, so it can be suspended, in order, for example, to wait for the user to acknowledge the message. The function must return an instance of type Next with the following structure:

public struct Next<Game: Setting, A> {
  public var action: Action
  public var update: Update<Game>?
  
  public enum Action {
    case advance(A)
    case replay
    case stop
  }
  ...
}

Next describes the request that the game Runner will need to satisfy right after the narration step. Next defines several convenient constructors, and defines 2 properties:

• action: what the Runner should do next;
• update: an optional update to the game World that should be handled by the Runner before passing the next step to the player.

The action could be one of the following:

• advance: advance to the next step, passing a generic A value to the Runner (in the case of Narration, A is simply Void);
• replay: replay the last step (the update to the game World will be executed before replaying it);
• stop: stop the game.

Here's a possible implementation of acknowledge(narration:) in our SimpleHandler class:

extension SimpleHandler: Handler {
  public func acknowledge(narration: Player<Game>.Narration) async -> Next<Game, Void> {
    if !narration.tags.isEmpty {
      print("[\(narration.tags.map { "\($0)" }.joined(separator: "|"))]")
    }
    
    for message in narration.messages {
      print(message)

      _ = readLine()
    }

    return .advance
  }
  ...
}

SimpleHandler will simply print the Tags in a single line, then the messages one by one, using readLine after each message in order for the player to acknowledge the latter.

make(choice:)

This will be called each time the player is expected to make a choice. The Player.Choice type that's passed in will contain a (possibly empty) list of Tags, and a list of Player.Options that's guaranteed to not be empty.

Player.Option has the following structure:

public enum Player<Game: Setting> {
  ...
  public struct Option {
    let id: String
    public let message: Game.Message
    public let tags: [Game.Tag]
  }
  ...
}

The message is the one to be shown to present the option to the player, and there is a (possibly empty) list of Tags associated with the Option. This function is also async, so it can be suspended while waiting for the player's selection. The return type is, again, Next, but the A type is now Player.Option: if .advance is retuned, it must associated with one of the Options from the Choice itself. The structure of the Player.Option type will guarantee that only an Option from the list defined in Player.Choice can be passed, and this will be done via a internal id that will be generated via the uniqueString function from the Generate type defined in Setting. Nevertheless, in case an invalid Player.Option is returned, the Handler will receive an error event, and the same choice step will be sent again, without affecting the story and the game World.

Here's a possible implementation of make(choice:) in our SimpleHandler class:

public final class SimpleHandler<Game: Story> {
  ...
  private func input(accepted: [String]) -> String {
    while true {
      guard let captured = readLine(), accepted.contains(captured) else {
        print("[Invalid input. Valid inputs: \(accepted.joined(separator: ", "))]")
        continue
      }
      return captured
    }
  }
}

extension SimpleHandler: Handler {
  ...
  public func make(choice: Player<Game>.Choice) async -> Next<Game, Player<Game>.Option> {
    for (index, option) in choice.options.enumerated() {
      print(index, "-", option.message)
    }

    let received = input(accepted: choice.options.indices.map { "\($0)" })

    guard
      let selected = Int(received),
      choice.options.indices.contains(selected)
    else {
      print("[Invalid option. Valid options: \(choice.options.indices)]")
      return .replay
    }

    return .advance(with: choice.options[selected])
  }
  ...
}

The function will print the possible options, together with their index (starting from 0). Then, it will continuously readLine until the player enters an input that matches one of the possible ones, that is, the option indices. The logic is achieved through the input(accepted:) function, but in case something goes wrong, the make(choice:) function will return .replay.

handle(event:)

Event is a flexible type that expresses various events that the Runner will communicate to the Handler in a "fire-and-forget" fashion: Runner will simply relay these events to the Handler, without expecting anything particular in return. The possible events are the following:

public enum Player<Game: Setting> {
  public enum Event {
    case gameStarted(Status<Game>)
    case gameEnded
    case errorProduced(Failure<Game>)
    case statusUpdated(Status<Game>)
  }
}

In order:

• gameStarted will be sent when the game starts, and will pass over the initial Status (if the game is restored from persistence, this will be the restored Status);
• gameEnded will be sent when the game ends because the story is completed: it will not be sent if the the game ends "forcibly", either because the game process is terminated, or if the handler returns .stop;
• errorProduced will be sent in case of an error in the game engine;
• statusUpdated will be sent each time the Status is updated, so the game engine has the opportunity to log or persist it, if needed.

Here's a possible implementation of handle(event:) in our SimpleHandler class:

private let directoryPath = "SimpleGameSupportingFiles"
private let filePath = "\(directoryPath)/Status.json"

public final class SimpleHandler<Game: Story> {
  ...
  private let encoder = JSONEncoder()
  private let fm = FileManager.default
  ...
}

extension SimpleHandler: Handler {
  ...

  public func handle(event: Player<Game>.Event) {
    switch event {
    case .statusUpdated(let status):
      guard let data = try? encoder.encode(status) else {
        break
      }
      let currentDirectory = fm.currentDirectoryPath
      try? fm.createDirectory(atPath: "\(currentDirectory)/\(directoryPath)", withIntermediateDirectories: true)
      try? fm.removeItem(atPath: "\(currentDirectory)/\(filePath)")
      try? data.write(to: URL(fileURLWithPath: "\(currentDirectory)/\(filePath)"))

    case .gameStarted(let status):
      print("""
      Welcome to Narratore!
      ...
      """)

      _ = readLine()

      for message in status.info.script.words.suffix(4) {
        print(message)
      }

    case .gameEnded:
      print("The story ended. Until next time.")
      try? fm.removeItem(atPath: filePath)

    case .errorProduced(let failure):
      print("ERROR: \(failure)")
    }
  }
}

By using FileManager and JSONEncoder, SimpleHandler is able to persist the state of the story (expressed by the Status type) each time it changes via the statusUpdated event, into a .json file at a certain path. Also, in gameStarted, it will show a simple welcome message, and print the last 4 messages from status.info.script.words, if available, for example when the game is restored from a previous Status. When the game ends, and gameEnded is received, SimpleHandler will delete the .json file with the game state. Finally, in case of errorProduced, the error will be simply printed.

The Runner

In order to actually execute the game, the Runner actor must be used. Runner will simply require an Handler and a Status to be initialized, and the only public function is start(). Runner also exposes a public info: Info property, that can be accessed at any time and will contain the latest game Info value, but given that Runner is an actor, the property access will be async.

As mentioned, Runner requires an Handler and a Status to be initialized: the Status could be the one retrieved from a previously run and interrupted game, but can also be constructed with a starting value depending on a state of the the game World, and a Scene:

public struct Status<Game: Setting> {
  public init<Scene>(world: Game.World, scene: S) where S: Scene, S.Game == Game {
    ...
  }
  
  ...
}

Generally, when starting a fresh game, this init will be used. Status doesn't have any other public init, but it's Encodable in general, and Decodable if Game is a Story. So it can be serialized, and retrieved from a serialized state.

We want our SimpleHandler to also handle the serialization of Status so it makes sense to also equip it a function to retrieve and decode a serialized Status, if available (and if the player wants to continue a previously interrupted game):

public final class SimpleHandler<Game: Story> {
  private let decoder = JSONDecoder()
  ...
  public func askToRestoreStatusIfPossible() -> Status<Game>? {
    let useFile: Bool
    if fm.fileExists(atPath: filePath) {
      print("""
      A Narratore status file was found at '\(filePath)': do you want to continue where you left?
      "[Valid inputs: y, n. Hit RETURN after entering a valid input.]"
      """)
      useFile = input(accepted: ["y", "n"]) == "y"
    } else {
      useFile = false
    }

    guard useFile else {
      return nil
    }

    guard let data = try? Data(contentsOf: URL(fileURLWithPath: filePath)) else {
      print("""
      Cannot read file, will start story from scratch.
      """)
      return nil
    }

    do {
      return try decoder.decode(Status<Game>.self, from: data)
    } catch {
      print("""
      Cannot read file, will start story from scratch.
      ERROR: \(error)
      """)
      return nil
    }
  }
  ...
}

The function askToRestoreStatusIfPossible will check for the existence of a Status file at a certain path and, if found, it will ask the player if they want to restore the story. If the file is not corrupted, a Status is returned, that can be passed to Runner in order to start the game where the player left it.

Executing the game

Finally, to actually run the game we can define an executableTarget in which we call Runner.start() in the @main function:

import Narratore
import SimpleHandler
import SimpleSetting
import SimpleStory

@main
enum Main {
  static func main() async {
    let handler = SimpleHandler<SimpleStory>.init()
    
    let runner = Runner<SimpleStory>.init(
      handler: handler,
      status: handler.askToRestoreStatusIfPossible() ?? .init(
        world: .init(),
        scene: Start.init()
      )
    )
    
    await runner.start()
  }
}

This code can be found in the SimpleGame target of the SimpleGame companion Swift package. Notice that this module imports all other modules (it's not strictly necessary, due to transitive dependencies, but it's useful for documentation purposes), because:

• to construct a Runner we need a Handler and a Status;
• to construct a Status we need a World, defined in the game Setting, and an initial Scene, defined in the game Story.