Amphlify/Packages/AmphlifyKit/Sources/Playback/AVFoundationEngine.swift

#if canImport(AVFoundation)
    import AVFoundation
    import Domain
    import Foundation

    public actor AVFoundationEngine: OutputEngine {
        public typealias Resolve = @Sendable (Track) async throws -> StreamRequest

        private struct Slot {
            let item: QueueItem
            let playerItem: AVPlayerItem
            let statusObservation: NSKeyValueObservation
        }

        public nonisolated let events: AsyncStream<OutputEvent>
        private let continuation: AsyncStream<OutputEvent>.Continuation
        private let resolve: Resolve
        private let player = AVQueuePlayer()
        private let bufferAhead: TimeInterval = 60
        private let anchorInterval: TimeInterval = 5
        private var current: Slot?
        private var prepared: Slot?
        private var adoptable = false
        private var wantsPlaying = false
        private var sessionActive = false
        private var loadGeneration = 0
        private var prepareGeneration = 0
        private var observations: [NSKeyValueObservation] = []
        private var tokens: [any NSObjectProtocol] = []
        private var timeObserver: Any?

        public init(resolve: @escaping Resolve) {
            self.resolve = resolve
            (events, continuation) = AsyncStream<OutputEvent>.makeStream()
            player.automaticallyWaitsToMinimizeStalling = true
            player.audiovisualBackgroundPlaybackPolicy = .continuesIfPossible
        }

        isolated deinit {
            tokens.forEach { NotificationCenter.default.removeObserver($0) }
            if let timeObserver {
                player.removeTimeObserver(timeObserver)
            }
        }

        public var outputLatency: TimeInterval {
            AudioSession.outputLatency
        }

        public func load(_ item: QueueItem, at position: TimeInterval) async {
            startObserving()
            loadGeneration += 1
            let generation = loadGeneration
            if adoptable, current?.item.id == item.id {
                adoptable = false
            } else if let prepared, prepared.item.id == item.id {
                player.advanceToNextItem()
                current = prepared
                self.prepared = nil
            } else {
                guard let slot = await makeSlot(item), generation == loadGeneration else { return }
                player.removeAllItems()
                prepared = nil
                player.insert(slot.playerItem, after: nil)
                current = slot
            }
            adoptable = false
            if position > 0 {
                await seek(to: position)
            }
        }

        public func prepare(_ item: QueueItem?) async {
            startObserving()
            guard prepared?.item.id != item?.id else { return }
            prepareGeneration += 1
            let generation = prepareGeneration
            if let prepared {
                player.remove(prepared.playerItem)
                self.prepared = nil
            }
            guard let item, current != nil else { return }
            guard let slot = await makeSlot(item), generation == prepareGeneration, current != nil else { return }
            player.insert(slot.playerItem, after: nil)
            prepared = slot
        }

        public func play() async {
            wantsPlaying = true
            if !sessionActive {
                sessionActive = await (try? AudioSession.activate()) != nil
            }
            player.play()
        }

        public func pause() {
            wantsPlaying = false
            player.pause()
        }

        public func seek(to position: TimeInterval) async {
            let time = CMTime(seconds: position, preferredTimescale: 600)
            _ = await player.seek(to: time, toleranceBefore: .zero, toleranceAfter: .zero)
            reportAnchor()
        }

        public func stop() {
            wantsPlaying = false
            player.pause()
            player.removeAllItems()
            current = nil
            prepared = nil
            adoptable = false
        }

        private func makeSlot(_ item: QueueItem) async -> Slot? {
            do {
                let request = try await resolve(item.track)
                let playerItem = makePlayerItem(request)
                let observation = playerItem.observe(\.status, options: [.new]) { @Sendable [weak self] playerItem, _ in
                    guard playerItem.status == .failed else { return }
                    let id = ObjectIdentifier(playerItem)
                    Task { await self?.itemFailed(id) }
                }
                return Slot(item: item, playerItem: playerItem, statusObservation: observation)
            } catch {
                continuation.yield(.failed(item.id))
                return nil
            }
        }

        private func makePlayerItem(_ request: StreamRequest) -> AVPlayerItem {
            var options: [String: any Sendable] = [AVURLAssetPreferPreciseDurationAndTimingKey: true]
            if !request.resource.headers.isEmpty {
                options["AVURLAssetHTTPHeaderFieldsKey"] = request.resource.headers
            }
            let asset = AVURLAsset(url: request.resource.url, options: options)
            let playerItem = AVPlayerItem(asset: asset)
            playerItem.preferredForwardBufferDuration = bufferAhead
            return playerItem
        }

        private func startObserving() {
            guard timeObserver == nil else { return }
            let interval = CMTime(seconds: anchorInterval, preferredTimescale: 600)
            timeObserver = player.addPeriodicTimeObserver(forInterval: interval, queue: .main) { [weak self] _ in
                Task { await self?.reportAnchor() }
            }
            observations.append(player.observe(\.timeControlStatus, options: [.new]) { @Sendable [weak self] _, _ in
                Task { await self?.reportAnchor() }
            })
            tokens.append(observeItems(.AVPlayerItemDidPlayToEndTime) { [weak self] id in await self?.itemEnded(id) })
            tokens.append(observeItems(.AVPlayerItemFailedToPlayToEndTime) { [weak self] id in await self?.itemFailed(id) })
            tokens.append(contentsOf: AudioSession.observe(
                interrupted: { [weak self] began in Task { await self?.interruption(began: began) } },
                routeLost: { [weak self] in Task { await self?.routeLost() } }
            ))
        }

        private func observeItems(
            _ name: Notification.Name,
            handler: @escaping @Sendable (ObjectIdentifier) async -> Void
        ) -> any NSObjectProtocol {
            NotificationCenter.default.addObserver(forName: name, object: nil, queue: nil) { notification in
                guard let playerItem = notification.object as? AVPlayerItem else { return }
                let id = ObjectIdentifier(playerItem)
                Task { await handler(id) }
            }
        }

        private func itemEnded(_ id: ObjectIdentifier) {
            guard let current, ObjectIdentifier(current.playerItem) == id else { return }
            continuation.yield(.finished(current.item.id))
            self.current = prepared
            prepared = nil
            adoptable = self.current != nil
        }

        private func itemFailed(_ id: ObjectIdentifier) {
            if let current, ObjectIdentifier(current.playerItem) == id {
                continuation.yield(.failed(current.item.id))
            } else if let prepared, ObjectIdentifier(prepared.playerItem) == id {
                player.remove(prepared.playerItem)
                self.prepared = nil
            }
        }

        private func reportAnchor() {
            guard let playing = player.currentItem,
                  let slot = [current, prepared].compactMap(\.self).first(where: { $0.playerItem === playing })
            else { return }
            let seconds = player.currentTime().seconds
            guard seconds.isFinite else { return }
            let rate = player.timeControlStatus == .playing ? Double(player.rate) : 0
            continuation.yield(.anchor(slot.item.id, position: seconds, rate: rate))
        }

        private func interruption(began: Bool) {
            if began {
                player.pause()
                reportAnchor()
            } else if wantsPlaying {
                player.play()
            }
        }

        private func routeLost() {
            wantsPlaying = false
            player.pause()
            reportAnchor()
        }
    }
#endif