MPCAndSwift6

A practical MultipeerConnectivity reference project built with Swift 6 and modern concurrency

This repository demonstrates a working, real-device MultipeerConnectivity (MPC) setup using Swift 6, with a clear separation between UI, session management, and Apple’s delegate-based APIs.

It exists because converting legacy MPC code to Swift 6 is not straightforward, and many examples online either:

• rely on outdated concurrency rules
• ignore Swift 6 actor isolation
• break when run on real devices
• or silently fail without explanation

This project was built from the ground up after repeated failures trying to retrofit Swift 6 concurrency onto an older MPC architecture.

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What This Project Does

• Discovers nearby peers
• Displays peers in a UITableView
• Sends and receives invitations
• Accepts / rejects connections via UI
• Tracks per-peer connection state
• Sends and receives messages
• Sends and receives small files
• Shows per-peer file transfer progress
• Runs on real iOS devices (not simulators)

Important: MultipeerConnectivity does work on a Simulator. This project is designed to be tested on two physical iOS devices.

Architecture Overview

Apple’s MultipeerConnectivity API is:

• delegate-driven
• Objective-C based
• not concurrency-aware

Swift 6 is:

• strict about actor isolation
• strict about Sendable
• unforgiving of cross-actor UI access

The solution used here

This project uses three layers:

MPCActor

An actor that owns all MPC objects:

• MCSession
• MCNearbyServiceBrowser
• MCNearbyServiceAdvertiser

It is responsible for:

• starting/stopping MPC
• sending messages
• sending files
• exposing async-safe access to session state

No UIKit code should live here.

Delegate Bridges (critical)

Because MPC delegates cannot live inside actors, each delegate is bridged:

• MCSessionDelegateBridge
• MPCBrowserDelegateBridge
• MPCAdvertiserDelegateBridge

These are:

• NSObject subclasses
• isolated to @MainActor
• forward events safely into MPCActor using Task {}

This avoids:

• Swift 6 isolation errors
• crashes
• undefined delegate behavior

PeerSessionManager

A UI-facing coordinator that:

• tracks peers and connection state
• exposes callbacks like: onPeersUpdated onPeerStateChanged onInvitationReceived

• feeds data into the UITableView
  

• sures all UI updates occur on the main actor

Why This Repo Exists

During development, these were encountered:

• Swift 6 compile-time isolation failures
• Delegates that compiled but never fired
• Invitations sent but never received
• Sessions connecting but immediately dropping
• Xcode debugger stalling when devices disconnected
• Hardware flakiness (USB / Wi-Fi) affecting MPC testing
• Actor-isolated properties not accessible where expected
• Perfectly “correct” Xcode error messages that were completely unhelpful

The final conclusion:

Trying to “convert” an older MPC project to Swift 6 is a trap. A clean-slate rebuild is faster, clearer, and safer.

This repo is the result of that rebuild.

Swift & Xcode Versions

• Swift 6 (strict concurrency enabled)
• Xcode 16.2+ (tested)
• iOS 15+

Earlier Swift versions may compile but are not supported.

How to Run

1.    Clone the repo
2.    Open the .xcodeproj
3.    Set a valid development team
4.    Run on two physical iOS devices
5.    Ensure:
    i.        Wi-Fi is enabled
    ii.        Bluetooth is enabled
    iii.    Devices are unlocked

Known Limitations

• Large file streaming is not implemented (for this app, reliability was preferred over complexity)
• Debugging MPC with two devices is fragile

•  Simulator support is intentionally not provided
  

Lessons Learned (Key Takeaways)

• MPC delegates must not live inside actors
• Delegate bridges are unavoidable in Swift 6
• UI must never touch actor-isolated properties directly
• browser.invitePeer(...) is correct — but only if:
• advertiser is running
• delegates are wired correctly
• service types match exactly
• Swift 6 improves correctness, not developer ergonomics
• Xcode’s debugger can severely distort perceived app behavior

Who This Is For

This repo is for developers who:

• are using Swift 6
• need MultipeerConnectivity
• want a working reference
• are tired of half-broken tutorials

It is intentionally minimal, explicit, and boring — because MPC reliability matters more than elegance.

License