Flaro/Packages/FlixyKit/Sources/FlixyPlayback/Engine/AVPlayerEngine.swift

import AVFoundation
import Foundation

/// Wiedergabe über den Systemplayer.
///
/// Der bevorzugte Weg, wann immer das Format ihn zulässt: Bild-in-Bild,
/// AirPlay, die Steuerung im Sperrbildschirm und der Umgang mit
/// Unterbrechungen durch Anrufe kommen alle mit, ohne dass etwas davon
/// nachgebaut werden müsste.
@MainActor
public final class AVPlayerEngine: VideoPlayerEngine {
    /// Wird von der Ansicht gebraucht, um die Bildebene zu bauen. Der einzige
    /// Grund, warum AVFoundation überhaupt nach außen sichtbar ist.
    public let player = AVPlayer()

    public private(set) var state: PlaybackState = .idle {
        didSet {
            guard state != oldValue else { return }
            continuation?.yield(.stateChanged(state))
        }
    }

    public private(set) var duration: Duration?
    public private(set) var audioTracks: [MediaTrack] = []
    public private(set) var subtitleTracks: [MediaTrack] = []

    public var currentTime: Duration {
        let seconds = player.currentTime().seconds
        guard seconds.isFinite, seconds >= 0 else { return .zero }
        return .seconds(seconds)
    }

    public let supportsPictureInPicture = true

    public private(set) lazy var events: AsyncStream<PlaybackEvent> = {
        AsyncStream { continuation in
            self.continuation = continuation
        }
    }()

    private var continuation: AsyncStream<PlaybackEvent>.Continuation?
    private var timeObserver: Any?
    private var observations: [NSKeyValueObservation] = []
    private var endObserver: (any NSObjectProtocol)?

    public init() {
        AudioSession.activate()
    }

    public func load(_ source: PlaybackSource) async throws(PlaybackFailure) {
        teardownObservers()
        state = .loading

        let asset = AVURLAsset(url: source.url)
        let item = AVPlayerItem(asset: asset)
        player.replaceCurrentItem(with: item)

        observe(item: item)
        observePlayer()

        do {
            let assetDuration = try await asset.load(.duration)
            if assetDuration.isNumeric {
                let value = Duration.seconds(assetDuration.seconds)
                duration = value
                continuation?.yield(.durationResolved(value))
            }
            try await loadTracks(from: asset)
        } catch {
            let failure = PlaybackFailure(
                kind: .sourceUnavailable,
                underlyingDescription: String(describing: error)
            )
            state = .failed(failure)
            throw failure
        }

        if source.startTime > .zero {
            await seek(to: source.startTime)
        }
    }

    public func play() {
        player.play()
    }

    public func pause() {
        player.pause()
    }

    public func seek(to time: Duration) async {
        let target = CMTime(seconds: time.seconds, preferredTimescale: 600)
        // Genau springen statt zum nächsten Schlüsselbild: beim Zurückspringen
        // um zehn Sekunden landet man sonst je nach Datei bis zu mehreren
        // Sekunden daneben.
        await player.seek(to: target, toleranceBefore: .zero, toleranceAfter: .zero)
        continuation?.yield(.timeChanged(time))
    }

    public func setRate(_ rate: Double) {
        player.rate = Float(rate)
    }

    public func selectAudioTrack(_ track: MediaTrack?) {
        select(track, in: .audible)
    }

    public func selectSubtitleTrack(_ track: MediaTrack?) {
        select(track, in: .legible)
    }

    public func teardown() {
        teardownObservers()
        player.replaceCurrentItem(with: nil)
        continuation?.finish()
        continuation = nil
        state = .idle
        AudioSession.deactivate()
    }

    // MARK: - Spurwahl

    private func select(_ track: MediaTrack?, in characteristic: AVMediaCharacteristic) {
        guard let item = player.currentItem,
              let group = item.asset.mediaSelectionGroup(forMediaCharacteristic: characteristic)
        else { return }

        guard let track, let index = track.engineIdentifier, group.options.indices.contains(index) else {
            // `nil` schaltet ab — bei Untertiteln der Normalfall, bei Ton
            // sollte es nicht vorkommen.
            item.select(nil, in: group)
            return
        }
        item.select(group.options[index], in: group)
    }

    private func loadTracks(from asset: AVURLAsset) async throws {
        func tracks(
            for characteristic: AVMediaCharacteristic,
            kind: MediaTrack.Kind
        ) async throws -> [MediaTrack] {
            guard let group = try await asset.loadMediaSelectionGroup(for: characteristic) else { return [] }
            return group.options.enumerated().map { index, option in
                MediaTrack(
                    id: "av-\(kind)-\(index)",
                    kind: kind,
                    displayName: option.displayName,
                    languageCode: option.extendedLanguageTag,
                    engineIdentifier: index,
                    isForced: option.hasMediaCharacteristic(.containsOnlyForcedSubtitles)
                )
            }
        }

        audioTracks = try await tracks(for: .audible, kind: .audio)
        subtitleTracks = try await tracks(for: .legible, kind: .subtitle)
        continuation?.yield(.tracksResolved(audio: audioTracks, subtitle: subtitleTracks))
    }

    // MARK: - Beobachtung

    private func observePlayer() {
        observations.append(
            player.observe(\.timeControlStatus, options: [.new]) { [weak self] player, _ in
                MainActor.assumeIsolated {
                    guard let self else { return }
                    switch player.timeControlStatus {
                    case .playing: self.state = .playing
                    case .paused: if self.state != .ended { self.state = .paused }
                    case .waitingToPlayAtSpecifiedRate: self.state = .buffering
                    @unknown default: break
                    }
                }
            }
        )

        // Viermal je Sekunde: fein genug, dass der Schieberegler flüssig
        // wirkt, grob genug, dass er nicht jede Bildwiederholung antreibt.
        timeObserver = player.addPeriodicTimeObserver(
            forInterval: CMTime(seconds: 0.25, preferredTimescale: 600),
            queue: .main
        ) { [weak self] time in
            MainActor.assumeIsolated {
                guard let self, time.isNumeric else { return }
                self.continuation?.yield(.timeChanged(.seconds(time.seconds)))
            }
        }
    }

    private func observe(item: AVPlayerItem) {
        observations.append(
            item.observe(\.status, options: [.new]) { [weak self] item, _ in
                MainActor.assumeIsolated {
                    guard let self, item.status == .failed else { return }
                    self.state = .failed(PlaybackFailure(
                        kind: .sourceUnavailable,
                        underlyingDescription: item.error.map { String(describing: $0) }
                    ))
                }
            }
        )

        endObserver = NotificationCenter.default.addObserver(
            forName: AVPlayerItem.didPlayToEndTimeNotification,
            object: item,
            queue: .main
        ) { [weak self] _ in
            MainActor.assumeIsolated {
                guard let self else { return }
                self.state = .ended
                self.continuation?.yield(.reachedEnd)
            }
        }
    }

    private func teardownObservers() {
        observations.forEach { $0.invalidate() }
        observations.removeAll()

        if let timeObserver {
            player.removeTimeObserver(timeObserver)
            self.timeObserver = nil
        }
        if let endObserver {
            NotificationCenter.default.removeObserver(endObserver)
            self.endObserver = nil
        }
    }
}

extension Duration {
    /// Die Dauer als Gleitkommazahl in Sekunden — AVFoundation rechnet darin.
    public var seconds: Double {
        Double(components.seconds) + Double(components.attoseconds) / 1e18
    }
}

/// Die Tonsitzung des Systems.
///
/// Nur auf iOS vorhanden. Gekapselt statt an drei Stellen mit `#if` versehen —
/// macOS steht in diesem Paket ohnehin nur, damit die Logik-Tests ohne
/// Simulator laufen.
enum AudioSession {
    static func activate() {
        #if os(iOS)
        // Ohne diese Kategorie schweigt die Wiedergabe, sobald der
        // Stummschalter am Gerät umgelegt ist — und Bild-in-Bild gäbe es gar
        // nicht.
        try? AVAudioSession.sharedInstance().setCategory(.playback, mode: .moviePlayback)
        try? AVAudioSession.sharedInstance().setActive(true)
        #endif
    }

    static func deactivate() {
        #if os(iOS)
        // `notifyOthersOnDeactivation` lässt die Musik-App wieder anlaufen,
        // die vor dem Film gespielt hat.
        try? AVAudioSession.sharedInstance().setActive(false, options: .notifyOthersOnDeactivation)
        #endif
    }
}