Multiplayer Counter Demo with GameKit

https://github.com/user-attachments/assets/85a9807b-00bf-47da-bfe1-4e1269c08588

A simple SwiftUI multiplayer demo using GameKit, where two players can increment a counter, and the updated value is synchronized between devices in real-time.

Introduction
Features
Requirements
Installation
Usage
ProjectStructure
Author

Introduction

This project demonstrates how to implement real-time multiplayer functionality in a SwiftUI app using GameKit. It provides a simple counter that two (or more) players can increment, and the counter value is synchronized between both players' devices. The project serves as a starting point for building more complex multiplayer games or apps.

Features

Real-time multiplayer using GameKit.
Asynchronous programming with Swift's async/await.
State management with @Published properties and ObservableObject.
Clean and maintainable code with MVVM architecture.
Automatic matchmaking with Game Center.
Error handling and user feedback.
Supports iOS 15.0 and above.

Requirements

Xcode 13.0 or later.
Swift 5.9 or later.
iOS 15.6 or later.
Two devices or simulators signed into Game Center with different accounts.

Installation

Update the bundle identifier to a unique value associated with your Apple Developer account.
Go to your project's Signing & Capabilities and set up the necessary capabilities:

Make sure to create the Project in AppstoreConnect and enable GameKit else it will not work in real devices:

Usage

Starting a Match

Open the app on both devices or simulators.
Initiate Matchmaking
On each device, tap the Find Match button.
The Game Center matchmaking UI will appear.
Connect with Another Player

https://github.com/user-attachments/assets/85a9807b-00bf-47da-bfe1-4e1269c08588

The matchmaking service will automatically connect the two devices.
Once connected, the game will start automatically.
On either device, tap the Increment Counter button.
The updated counter value will appear on both devices.
Repeat tapping the button on either device to continue incrementing.

Tap the End Game button to disconnect and return to the main menu.

ProjectStructure

Models

MultiplayerState: Enum representing the different states of the multiplayer session.
AuthenticationError: Enum for handling authentication-related errors.

ViewModels

MultiplayerViewModel: Manages the game logic, state transitions, and communication with GameKitManager.

@MainActor
final class MultiplayerViewModel: ObservableObject {

    @Published var state: MultiplayerState = .idle
    @Published var connectedPlayers: [GKPlayer] = []
    @Published var counter: Int = 0
    private var isHost: Bool = false

    // Initialization and subscriptions...

    func incrementCounter() {
        counter += 1
        sendCounterUpdate()
    }

    private func sendCounterUpdate() {
        let messageData = [
            "action": "updateCounter",
            "counterValue": counter
        ] as [String : Any]
        
        do {
            let data = try JSONSerialization.data(withJSONObject: messageData, options: [])
            sendData(data)
        } catch {
            state = .error("Failed to send counter update: \(error.localizedDescription)")
        }
    }

    // Other methods...
}

Views

MultiplayerView: The main SwiftUI view that updates based on the MultiplayerViewModel's state.

Managers

GameKitManager: Handles all Game Center-related functionality, including authentication, matchmaking, and data transmission.

final class GameKitManager: NSObject, ObservableObject {

    static let shared = GameKitManager()
    
    // Authentication Properties
    @Published private(set) var isAuthenticated: Bool = GKLocalPlayer.local.isAuthenticated
    @Published var authenticationError: AuthenticationError?
    @Published var authenticationViewController: UIViewController?

    // Multiplayer Properties
    @Published var currentMatch: GKMatch?
    @Published var matchError: Error?
    @Published var receivedData: (data: Data, player: GKPlayer)?
    @Published var playerConnectionState: (player: GKPlayer, state: GKPlayerConnectionState)?

    func findMatch(minPlayers: Int, maxPlayers: Int, viewController: UIViewController) throws {
        guard isAuthenticated else {
            throw AuthenticationError.custom(message: "Local player is not authenticated. Please sign in to Game Center.")
        }
        
        let request = GKMatchRequest()
        request.minPlayers = minPlayers
        request.maxPlayers = maxPlayers
        
        let mmvc = GKMatchmakerViewController(matchRequest: request)
        mmvc?.matchmakerDelegate = self
        
        if let mmvc {
            viewController.present(mmvc, animated: true)
        }
    }
    
    func sendData(_ data: Data, mode: GKMatch.SendDataMode = .reliable) throws {
        guard let match = currentMatch else {
            throw NSError(domain: "No active match", code: 0, userInfo: nil)
        }
        
        try match.sendData(toAllPlayers: data, with: mode)
    }
    
    func disconnectMatch() {
        currentMatch?.disconnect()
        currentMatch = nil
    }
    // Other methods...
}

Author

James Thang, find me on LinkedIn

Learn more about SwiftUI, check out my book :books: on Amazon

GameKitDemo

Install / Use

README