shareplay-activities

What this skill does

# GroupActivities / SharePlay

Build shared real-time experiences using the GroupActivities framework. SharePlay
connects people over FaceTime, Messages, AirDrop, and nearby visionOS sharing,
synchronizing media playback, app state, or custom data. Targets Swift 6.3 / iOS 26+.

## Contents

- [Setup](#setup)
- [Defining a GroupActivity](#defining-a-groupactivity)
- [Session Lifecycle](#session-lifecycle)
- [Sending and Receiving Messages](#sending-and-receiving-messages)
- [Coordinated Media Playback](#coordinated-media-playback)
- [Starting SharePlay from Your App](#starting-shareplay-from-your-app)
- [GroupSessionJournal: File Transfer](#groupsessionjournal-file-transfer)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#references)

## Setup

### Capability

Add the **Group Activities** capability to the app target in Xcode. Xcode adds
the required entitlement and updates the provisioning profile:

```xml
<key>com.apple.developer.group-session</key>
<true/>
```

Configure this only for app targets. Group Activities are not available in
widgets, extensions, or App Clips.

### Checking Eligibility

```swift
import GroupActivities

let observer = GroupStateObserver()

// Check if a FaceTime call or Messages conversation is active
if observer.isEligibleForGroupSession {
    showSharePlayButton()
}
```

Observe changes reactively:

```swift
for await isEligible in observer.$isEligibleForGroupSession.values {
    showSharePlayButton(isEligible)
}
```

## Defining a GroupActivity

Conform to `GroupActivity` and provide metadata:

```swift
import GroupActivities

struct WatchTogetherActivity: GroupActivity {
    let movieID: String
    let movieTitle: String

    var metadata: GroupActivityMetadata {
        var meta = GroupActivityMetadata()
        meta.title = movieTitle
        meta.type = .watchTogether
        meta.fallbackURL = URL(string: "https://example.com/movie/\(movieID)")
        return meta
    }
}
```

### Activity Types

| Type | Use Case |
|---|---|
| `.generic` | Default for custom activities |
| `.watchTogether` | Video playback |
| `.listenTogether` | Audio playback |
| `.createTogether` | Collaborative creation (drawing, editing) |
| `.exploreTogether` | Shared browsing, planning, or exploration |
| `.learnTogether` | Shared learning or studying |
| `.readTogether` | Shared reading |
| `.shopTogether` | Shared shopping |
| `.workoutTogether` | Shared fitness sessions |

`GroupActivity` is `Codable`; stored activity data must be codable. Add
`Transferable` only for SwiftUI `ShareLink`, SharePlay over AirDrop, or
AppKit/UIKit share sheets. Keep payloads minimal: use identifiers or URLs
instead of large data.

## Session Lifecycle

### Listening for Sessions

Set up a long-lived task to receive sessions when another participant starts
the activity:

```swift
@Observable
@MainActor
final class SharePlayManager {
    private var session: GroupSession<WatchTogetherActivity>?
    private var messenger: GroupSessionMessenger?
    private var sessionTasks: [Task<Void, Never>] = []

    func observeSessions() {
        Task {
            for await session in WatchTogetherActivity.sessions() {
                self.configureSession(session)
            }
        }
    }

    private func configureSession(
        _ session: GroupSession<WatchTogetherActivity>
    ) {
        self.session = session
        self.messenger = GroupSessionMessenger(session: session)

        // Observe session state changes
        let stateTask = Task {
            for await state in session.$state.values {
                handleState(state)
            }
        }
        sessionTasks.append(stateTask)

        // Observe participant changes
        let participantTask = Task {
            for await participants in session.$activeParticipants.values {
                handleParticipants(participants)
            }
        }
        sessionTasks.append(participantTask)

        // Join the session
        session.join()
    }

    private func cleanUp() {
        sessionTasks.forEach { $0.cancel() }
        sessionTasks.removeAll()
        session = nil
        messenger = nil
    }
}
```

### Session States

| State | Description |
|---|---|
| `.waiting` | Session exists but local participant has not joined |
| `.joined` | Local participant is actively in the session |
| `.invalidated(reason:)` | Session ended (check reason for details) |

### Handling State Changes

```swift
private func handleState(_ state: GroupSession<WatchTogetherActivity>.State) {
    switch state {
    case .waiting:
        print("Waiting to join")
    case .joined:
        print("Joined session")
        loadActivity(session?.activity)
    case .invalidated(let reason):
        print("Session ended: \(reason)")
        cleanUp()
    @unknown default:
        break
    }
}

private func handleParticipants(_ participants: Set<Participant>) {
    print("Active participants: \(participants.count)")
}
```

### Leaving and Ending

```swift
// Leave the session (other participants continue)
session?.leave()

// End the session for all participants
session?.end()
```

## Sending and Receiving Messages

Use `GroupSessionMessenger` to sync small, time-sensitive app state between
participants.

### Defining Messages

Messages must be `Codable`; keep each message under 256 KB.

```swift
struct SyncMessage: Codable {
    let action: String
    let timestamp: Date
    let data: [String: String]
}
```

### Sending

```swift
func sendSync(_ message: SyncMessage) async throws {
    guard let messenger else { return }

    try await messenger.send(message, to: .all)
}

// Send to specific participants
try await messenger.send(message, to: .only(participant))
```

### Receiving

```swift
func observeMessages() {
    guard let messenger else { return }

    Task {
        for await (message, context) in messenger.messages(of: SyncMessage.self) {
            let sender = context.source
            handleReceivedMessage(message, from: sender)
        }
    }
}
```

### Delivery Modes

```swift
// Reliable (default) -- checked and retried for crucial state
let reliableMessenger = GroupSessionMessenger(
    session: session,
    deliveryMode: .reliable
)

// Unreliable -- lower latency, no delivery guarantee
let unreliableMessenger = GroupSessionMessenger(
    session: session,
    deliveryMode: .unreliable
)
```

Use `.reliable` for state-changing actions such as selections or turns. Use
`.unreliable` for high-frequency ephemeral data such as cursor positions,
drawing strokes, and reactions.

## Coordinated Media Playback

For video/audio, use `AVPlaybackCoordinator` with `AVPlayer`:

```swift
import AVFoundation
import GroupActivities

func configurePlayback(
    session: GroupSession<WatchTogetherActivity>,
    player: AVPlayer
) {
    // Connect the player's coordinator to the session
    let coordinator = player.playbackCoordinator
    coordinator.coordinateWithSession(session)
}
```

Once connected, AVFoundation synchronizes play/pause, seeking, rate, playback speed,
and time. Do not put AVPlayer transport fields in messenger messages or snapshots,
including late-joiner snapshots; use custom messages only for state outside playback.

## Starting SharePlay from Your App

### Using GroupActivitySharingController (UIKit)

```swift
import GroupActivities
import UIKit

func startSharePlay() async throws {
    let activity = WatchTogetherActivity(
        movieID: "123",
        movieTitle: "Great Movie"
    )

    switch await activity.prepareForActivation() {
    case .activationPreferred:
        // A conversation is active and the user chose to share.
        _ = try await activity.activate()

    case .activationDisabled:
        // The user chose local playback, or sharing is unavailable.
        startLocalExperience()

    case .cancelled:
        break

    @unknown default:
        break
    }
}
```

When no conversation is active (i.e., `isEligibleForGroupSe