Building a custom peer-to-peer protocol

At a glance

Project structure

Source bundle/
└── TicTacToe/
    ├── TicTacToe-iOS/
    │   ├── AppDelegate.swift
    │   └── Views/
    │       ├── GameViewController.swift
    │       ├── PeerListViewController.swift
    │       ├── GameScene.swift
    │       └── PasscodeViewController.swift
    ├── TicTacToe-tvOS/
    │   ├── AppDelegate.swift
    │   └── Views/
    │       ├── GameViewController.swift
    │       ├── PeerListViewController.swift
    │       └── GameScene.swift
    └── TicTacToe-watchOS Extension/
        ├── GameViewController.swift
        ├── ExtensionDelegate.swift
        └── GameScene.swift

Structure observations

• Architecturally prominent files are ranked from entry points and role-named declarations; resource-only paths are omitted.
• Primary languages: Swift.
• The verified tree contains 11 project/configuration file(s) and 30 source declaration(s).

Overall architecture

flowchart LR
    N1["AppDelegate"]
    N2["GameViewController"]
    N3["Network APIs"]
    N1 --> N2
    N2 --> N3

Reference code

TicTacToe/TicTacToe-iOS/AppDelegate.swift:10 — architecture anchor

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
}

Interpretation

The arrows summarize the source-visible entry, role-named types or folders, and framework direction; when nodes come from structural folders, the sequence is a high-level interpretation rather than proof that every adjacent node calls the next. Ownership is claimed only where the next section cites a stored property or assignment. The diagram is intentionally limited to the dominant path into Network.

Ownership and state

classDiagram
    AppDelegate o-- UIWindow : window
    GameViewController o-- SKView : sceneView
    GameViewController o-- UILabel : instructionLabel
    GameViewController o-- UIButton : leftButton

Ownership evidence

TicTacToe/TicTacToe-iOS/AppDelegate.swift:12 — stored dependency or nearest verified ownership anchor

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
    var window: UIWindow?
}

Composition arrows indicate a source-visible construction expression or locally owned value state; aggregation means the owner stores or receives a dependency without proving exclusive lifetime ownership.

Class and protocol design

TicTacToe/Networking/PeerBrowser.swift:13 — representative type boundary

protocol PeerBrowserDelegate: AnyObject {
    func refreshResults(results: Set<NWBrowser.Result>)
    func displayBrowseError(_ error: NWError)
}

The source explicitly defines local protocol relationships: GameViewController → PeerConnectionDelegate, PeerListViewController → PeerBrowserDelegate, PeerListViewController → PeerConnectionDelegate, GameViewController → PeerConnectionDelegate, PeerListViewController → PeerConnectionDelegate, GameViewController → PeerConnectionDelegate.

Access control

Reference code

TicTacToe/Networking/NWParameters+InitWithPasscode.swift:32 — representative boundary

    private static func tlsOptions(passcode: String) -> NWProtocolTLS.Options {
        // ...
    }

Swift declarations without a modifier are internal; explicit private, fileprivate, private(set), public, or open entries above are interpreted by language semantics. Objective-C/C samples instead rely on header and implementation boundaries, which are not equivalent to Swift lexical privacy.

Logic ownership and placement

Design patterns

Main application flow

Receiving one peer move passes framed Network content through the connection delegate into game state, while the receive callback immediately arms the next message.

sequenceDiagram
    participant Peer as Remote peer
    participant Network as NWConnection
    participant Connection as PeerConnection
    participant Controller as GameViewController
    participant Scene as GameScene
    Peer->>Network: Send framed move
    Network-->>Connection: receiveMessage callback
    Connection-->>Controller: receivedMessage(content, metadata)
    Controller->>Controller: Decode move and validate row/column
    Controller->>Scene: placeCharacter(...)
    Connection->>Network: receiveNextMessage()
    loop until an error occurs
        Network-->>Connection: Next framed message callback
    end

Reference code

TicTacToe/Networking/PeerConnection.swift:155 — the callback forwards decoded metadata to the delegate and recursively schedules the next receive.

func receiveNextMessage() {
    guard let connection = connection else { return }
    connection.receiveMessage { (content, context, isComplete, error) in
        if let gameMessage = context?.protocolMetadata(definition: GameProtocol.definition) as? NWProtocolFramer.Message {
            self.delegate?.receivedMessage(content: content, message: gameMessage)
        }
        if error == nil {
            self.receiveNextMessage()
        }
    }
}

TicTacToe/TicTacToe-iOS/Views/GameViewController.swift:252 — the delegate dispatches a move and applies validated coordinates to the scene.

func receivedMessage(content: Data?, message: NWProtocolFramer.Message) {
    guard let content = content else { return }
    switch message.gameMessageType {
    case .invalid:
        print("Received invalid message")
    case .selectedCharacter:
        handleSelectCharacter(content, message)
    case .move:
        handleMove(content, message)
    }
}

Naming conventions

• Types: Controller: GameViewController, PasscodeViewController, PeerListViewController; Delegate: AppDelegate, ExtensionDelegate, PeerBrowserDelegate, PeerConnectionDelegate; Scene: GameScene.
• Protocols: PeerBrowserDelegate, PeerConnectionDelegate.
• Methods: emojiArray, hideButtons, disableButtons, enableButtons, declareWinner, handleMyTurnSelectFamily, handleWaitingToSelectFamily, handleMyTurn.
• Files: TicTacToe/TicTacToe-iOS/AppDelegate.swift, TicTacToe/TicTacToe-tvOS/AppDelegate.swift, TicTacToe/TicTacToe-iOS/Views/GameViewController.swift, TicTacToe/TicTacToe-tvOS/Views/GameViewController.swift, TicTacToe/TicTacToe-watchOS Extension/GameViewController.swift, TicTacToe/TicTacToe-iOS/Views/PeerListViewController.swift.

Architecture takeaways

• AppDelegate is the main source-visible entry or composition anchor for this sample.
• Framework work reaches Network, UIKit, SpriteKit, GameplayKit through a deliberately small high-level chain; the detailed API graph remains inside the cited implementation files.
• Stored-property evidence identifies lifecycle collaboration; it does not by itself prove exclusive object ownership.
• Access-control conclusions separate verified language visibility from the likely design rationale.
• Local protocol relationships provide an explicit substitution boundary.

Source map

• Official Apple documentation