Dreamio turn document

Fix VLC Playback Sync

VLC native playback now uses state-aware play and pause commands, reports readiness once per media item, and separates fast progress refreshes from slower track-menu work.

2026-05-26 Beads issue dreamio-e3u MobileVLCKit

Summary

This change hardens native VLC playback against pause lag and UI churn. It adds debug snapshots around playback commands and state changes, gates rapid toggles, uses VLC state rather than only isPlaying, and applies conservative caching options for streamed media.

Changes Made

Added a pure toggle policy that maps playback states to play, pause, or wait actions.
Instrumented VLC play, pause, toggle, stop, state changes, buffering, current time, position, and audio delay in DEBUG builds.
Updated pause handling to require canPause plus an active VLC state before sending pause().
Added a ready-once flag so onReady is only emitted once for each play(request:).
Reduced repeated main-thread work by splitting progress updates from audio and subtitle menu refreshes.
Added conservative stream caching and clock jitter options, while leaving :clock-synchro=0 out until device testing proves it helps.

Context

The reported failure looked like either VLC draining audio after the video paused, or the UI flipping state from stale isPlaying. The new logs are designed to distinguish those cases by showing command timing, VLC state, play status, media time, position, buffering transitions, and currentAudioPlaybackDelay.

Important Implementation Details

.playing and .buffering are pauseable; .paused, .stopped, .ended, and .error are playable; .opening and unknown states wait for VLC to finish transitioning.
Rapid toggles are ignored for 350 ms to avoid sending contradictory play and pause commands while MobileVLCKit is transitioning.
Audio delay is reset on new streams and after audio track selection, with debug snapshots available before considering a user-facing delay control.
Readiness still hides startup UI and starts progress polling, but repeated buffering and playing transitions now only update state.

Relevant Diff Snippets

Dreamio/NativePlaybackBackend.swift · state-aware toggle policy

Dreamio/NativePlaybackBackend.swift

+30

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    func stop()
}

enum NativePlaybackError: LocalizedError {
    case backendUnavailable
    case startupTimedOut33 unmodified lines
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    func stop()
}

enum NativePlaybackToggleState {
    case opening
    case buffering
    case playing
    case paused
    case stopped
    case ended
    case error
    case unknown
}

enum NativePlaybackToggleAction {
    case play
    case pause
    case waitForTransition
}

enum NativePlaybackTogglePolicy {
    static func action(for state: NativePlaybackToggleState) -> NativePlaybackToggleAction {
        switch state {
        case .playing, .buffering:
            return .pause
        case .paused, .stopped, .ended, .error:
            return .play
        case .opening, .unknown:
            return .waitForTransition
        }
    }
}

enum NativePlaybackError: LocalizedError {
    case backendUnavailable
    case startupTimedOut

Dreamio/VLCNativePlaybackBackend.swift · VLC command hardening and instrumentation

Dreamio/VLCNativePlaybackBackend.swift

-4+134

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    private var hasPendingExternalSubtitleSelection = false
    private var pendingExternalSubtitleDisplayNames: [String] = []
    private var externalSubtitleDisplayNamesByTrackID: [Int32: String] = [:]

    override init() {
        super.init()
19 unmodified lines
        hasPendingExternalSubtitleSelection = false
        pendingExternalSubtitleDisplayNames.removeAll()
        externalSubtitleDisplayNamesByTrackID.removeAll()
        let media = VLCMedia(url: request.playbackURL)
        let headerValue = request.headers
            .map { "\($0.key): \($0.value)" }
5 unmodified lines
        if !headerValue.isEmpty {
            media.addOption(":http-header=\(headerValue)")
        }

        mediaPlayer.media = media
#if DEBUG
        print("[DreamioVLC] opening url=\(URLRedactor.redactedURLString(request.playbackURL.absoluteString))")
#endif
        mediaPlayer.play()
#else
        onFailure?(NativePlaybackError.backendUnavailable)
#endif
1 unmodified line

    func play() {
#if canImport(MobileVLCKit)
        mediaPlayer.play()
#endif
    }

    func pause() {
#if canImport(MobileVLCKit)
        mediaPlayer.pause()
#endif
    }

    func togglePlayPause() {
        isPlaying ? pause() : play()
    }

    func seek(to position: Float) {
21 unmodified lines
        logAudioTracks(reason: "before-select-\(id)")
#endif
        mediaPlayer.currentAudioTrackIndex = id
#if DEBUG
        logAudioTracks(reason: "after-select-\(id)")
#endif
        onAudioTracksChange?()
#endif
38 unmodified lines

    func stop() {
#if canImport(MobileVLCKit)
        mediaPlayer.stop()
        mediaPlayer.drawable = nil
        mediaPlayer.media = nil
98 unmodified lines
    }

#if canImport(MobileVLCKit)
    private func attachSubtitles(_ candidates: [SubtitleCandidate]) -> Int {
        var attachedCount = 0
        var duplicateCount = 0
155 unmodified lines
extension VLCNativePlaybackBackend: VLCMediaPlayerDelegate {
    func mediaPlayerStateChanged(_ aNotification: Notification) {
#if DEBUG
        print("[DreamioVLC] state=\(stateName(mediaPlayer.state))")
#endif
        switch mediaPlayer.state {
        case .buffering, .playing:
            reapplyAutoSelectedSubtitleTrackIfNeeded(reason: stateName(mediaPlayer.state))
            onReady?()
            onStateChange?()
            onAudioTracksChange?()
        case .error:
            onFailure?(NativePlaybackError.playbackFailed)
        case .paused, .stopped, .ended:
            onStateChange?()
        case .esAdded:
            selectPreferredSubtitleTrackIfNeeded(reason: "esAdded")
8 unmodified lines
        }
    }

    private func stateName(_ state: VLCMediaPlayerState) -> String {
        switch state {
        case .opening:30 unmodified lines
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    private var hasPendingExternalSubtitleSelection = false
    private var pendingExternalSubtitleDisplayNames: [String] = []
    private var externalSubtitleDisplayNamesByTrackID: [Int32: String] = [:]
    private var didReportReadyForCurrentMedia = false
    private var lastToggleDate = Date.distantPast
    private let minimumToggleInterval: TimeInterval = 0.35

    override init() {
        super.init()
19 unmodified lines
        hasPendingExternalSubtitleSelection = false
        pendingExternalSubtitleDisplayNames.removeAll()
        externalSubtitleDisplayNamesByTrackID.removeAll()
        didReportReadyForCurrentMedia = false
        lastToggleDate = .distantPast
        let media = VLCMedia(url: request.playbackURL)
        let headerValue = request.headers
            .map { "\($0.key): \($0.value)" }
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        if !headerValue.isEmpty {
            media.addOption(":http-header=\(headerValue)")
        }
        addConservativePlaybackOptions(to: media)
        mediaPlayer.currentAudioPlaybackDelay = 0

        mediaPlayer.media = media
#if DEBUG
        print("[DreamioVLC] opening url=\(URLRedactor.redactedURLString(request.playbackURL.absoluteString)) options=conservative-low-latency")
        logPlaybackSnapshot(reason: "before-initial-play")
#endif
        mediaPlayer.play()
#if DEBUG
        logPlaybackSnapshot(reason: "after-initial-play-command")
#endif
#else
        onFailure?(NativePlaybackError.backendUnavailable)
#endif
1 unmodified line

    func play() {
#if canImport(MobileVLCKit)
#if DEBUG
        logPlaybackSnapshot(reason: "before-play")
#endif
        mediaPlayer.play()
#if DEBUG
        logPlaybackSnapshot(reason: "after-play-command")
#endif
#endif
    }

    func pause() {
#if canImport(MobileVLCKit)
        let state = mediaPlayer.state
#if DEBUG
        logPlaybackSnapshot(reason: "before-pause canPause=\(mediaPlayer.canPause) pauseable=\(isPauseableState(state))")
#endif
        guard mediaPlayer.canPause, isPauseableState(state) else {
#if DEBUG
            print("[DreamioVLC] pause skipped state=\(stateName(state)) canPause=\(mediaPlayer.canPause)")
#endif
            return
        }
        mediaPlayer.pause()
#if DEBUG
        logPlaybackSnapshot(reason: "after-pause-command")
        DispatchQueue.main.asyncAfter(deadline: .now() + 0.25) { [weak self] in
            self?.logPlaybackSnapshot(reason: "pause-follow-up-250ms")
        }
#endif
#endif
    }

    func togglePlayPause() {
#if canImport(MobileVLCKit)
        let now = Date()
        guard now.timeIntervalSince(lastToggleDate) >= minimumToggleInterval else {
#if DEBUG
            print("[DreamioVLC] toggle skipped rapid-repeat elapsed=\(String(format: "%.3f", now.timeIntervalSince(lastToggleDate)))")
#endif
            return
        }
        lastToggleDate = now

        let state = playbackToggleState(for: mediaPlayer.state)
        let action = NativePlaybackTogglePolicy.action(for: state)
#if DEBUG
        logPlaybackSnapshot(reason: "toggle action=\(action) mappedState=\(state)")
#endif
        switch action {
        case .play:
            play()
        case .pause:
            pause()
        case .waitForTransition:
            break
        }
#else
        isPlaying ? pause() : play()
#endif
    }

    func seek(to position: Float) {
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        logAudioTracks(reason: "before-select-\(id)")
#endif
        mediaPlayer.currentAudioTrackIndex = id
        mediaPlayer.currentAudioPlaybackDelay = 0
#if DEBUG
        logAudioTracks(reason: "after-select-\(id)")
        logPlaybackSnapshot(reason: "after-audio-select-\(id)")
#endif
        onAudioTracksChange?()
#endif
38 unmodified lines

    func stop() {
#if canImport(MobileVLCKit)
#if DEBUG
        logPlaybackSnapshot(reason: "before-stop")
#endif
        didReportReadyForCurrentMedia = false
        mediaPlayer.stop()
        mediaPlayer.drawable = nil
        mediaPlayer.media = nil
98 unmodified lines
    }

#if canImport(MobileVLCKit)
    private func addConservativePlaybackOptions(to media: VLCMedia) {
        [
            ":network-caching=1000",
            ":file-caching=1000",
            ":live-caching=1000",
            ":clock-jitter=0"
        ].forEach { media.addOption($0) }
    }

    private func isPauseableState(_ state: VLCMediaPlayerState) -> Bool {
        switch state {
        case .playing, .buffering:
            return true
        default:
            return false
        }
    }

    private func playbackToggleState(for state: VLCMediaPlayerState) -> NativePlaybackToggleState {
        switch state {
        case .opening:
            return .opening
        case .buffering:
            return .buffering
        case .playing:
            return .playing
        case .paused:
            return .paused
        case .stopped:
            return .stopped
        case .ended:
            return .ended
        case .error:
            return .error
        default:
            return .unknown
        }
    }

#if DEBUG
    private func logPlaybackSnapshot(reason: String) {
        let mediaLength = mediaPlayer.media?.length.intValue ?? 0
        print("[DreamioVLC] snapshot reason=\(reason) state=\(stateName(mediaPlayer.state)) isPlaying=\(mediaPlayer.isPlaying) canPause=\(mediaPlayer.canPause) seekable=\(mediaPlayer.isSeekable) currentTime=\(String(format: "%.3f", currentTime)) duration=\(String(format: "%.3f", TimeInterval(max(0, mediaLength)) / 1000)) position=\(String(format: "%.4f", mediaPlayer.position)) audioDelay=\(mediaPlayer.currentAudioPlaybackDelay) readyReported=\(didReportReadyForCurrentMedia)")
    }
#endif

    private func attachSubtitles(_ candidates: [SubtitleCandidate]) -> Int {
        var attachedCount = 0
        var duplicateCount = 0
155 unmodified lines
extension VLCNativePlaybackBackend: VLCMediaPlayerDelegate {
    func mediaPlayerStateChanged(_ aNotification: Notification) {
#if DEBUG
        logPlaybackSnapshot(reason: "state-change")
#endif
        switch mediaPlayer.state {
        case .buffering, .playing:
            reapplyAutoSelectedSubtitleTrackIfNeeded(reason: stateName(mediaPlayer.state))
            reportReadyIfNeeded()
            onStateChange?()
        case .error:
            onFailure?(NativePlaybackError.playbackFailed)
        case .paused, .stopped, .ended:
#if DEBUG
            DispatchQueue.main.asyncAfter(deadline: .now() + 0.25) { [weak self] in
                self?.logPlaybackSnapshot(reason: "inactive-state-follow-up-250ms")
            }
#endif
            onStateChange?()
        case .esAdded:
            selectPreferredSubtitleTrackIfNeeded(reason: "esAdded")
8 unmodified lines
        }
    }

    private func reportReadyIfNeeded() {
        guard !didReportReadyForCurrentMedia else {
#if DEBUG
            print("[DreamioVLC] ready skipped already-reported state=\(stateName(mediaPlayer.state))")
#endif
            return
        }
        didReportReadyForCurrentMedia = true
#if DEBUG
        print("[DreamioVLC] ready reported state=\(stateName(mediaPlayer.state))")
#endif
        onReady?()
        onAudioTracksChange?()
        onSubtitleTracksChange?()
    }

    private func stateName(_ state: VLCMediaPlayerState) -> String {
        switch state {
        case .opening:

Dreamio/NativePlayerViewController.swift · progress/menu refresh split

Dreamio/NativePlayerViewController.swift

-12+19

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        }
        backend.onStateChange = { [weak self] in
            DispatchQueue.main.async {
                self?.refreshControls()
            }
        }
        backend.onSubtitleTracksChange = { [weak self] in
            DispatchQueue.main.async {
                self?.refreshControls()
            }
        }
        backend.onAudioTracksChange = { [weak self] in
            DispatchQueue.main.async {
                self?.refreshControls()
            }
        }
    }
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    private func startProgressUpdates() {
        progressTimer?.invalidate()
        progressTimer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats: true) { [weak self] _ in
            self?.refreshControls()
        }
    }

    private func refreshControls() {
        let audioTracks = backend.audioTracks
        let subtitleTracks = backend.subtitleTracks
        playPauseButton.setImage(UIImage(systemName: backend.isPlaying ? "pause.fill" : "play.fill"), for: .normal)
        scrubber.isEnabled = backend.isSeekable
        jumpBackButton.isEnabled = backend.isSeekable
        jumpForwardButton.isEnabled = backend.isSeekable
        updateAudioMenuIfNeeded(audioTracks: audioTracks)
        updateCaptionsMenuIfNeeded(subtitleTracks: subtitleTracks)
        elapsedLabel.text = PlaybackTimeFormatter.label(for: backend.currentTime)
        remainingLabel.text = "-\(PlaybackTimeFormatter.label(for: backend.remainingTime))"
        scrubber.accessibilityValue = "\(elapsedLabel.text ?? "0:00") elapsed, \(remainingLabel.text ?? "-0:00") remaining"
        if !isScrubbing {
            scrubber.value = backend.position
        }
        [scrubber, jumpBackButton, jumpForwardButton].forEach { $0.alpha = backend.isSeekable ? 1 : 0.45 }
    }

    private func updateAudioMenuIfNeeded(audioTracks: [AudioTrack]) {287 unmodified lines
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        }
        backend.onStateChange = { [weak self] in
            DispatchQueue.main.async {
                self?.refreshProgressControls()
            }
        }
        backend.onSubtitleTracksChange = { [weak self] in
            DispatchQueue.main.async {
                self?.refreshTrackMenus()
            }
        }
        backend.onAudioTracksChange = { [weak self] in
            DispatchQueue.main.async {
                self?.refreshTrackMenus()
            }
        }
    }
341 unmodified lines
    private func startProgressUpdates() {
        progressTimer?.invalidate()
        progressTimer = Timer.scheduledTimer(withTimeInterval: 0.5, repeats: true) { [weak self] _ in
            self?.refreshProgressControls()
        }
    }

    private func refreshControls() {
        refreshProgressControls()
        refreshTrackMenus()
    }

    private func refreshProgressControls() {
        let isSeekable = backend.isSeekable
        playPauseButton.setImage(UIImage(systemName: backend.isPlaying ? "pause.fill" : "play.fill"), for: .normal)
        scrubber.isEnabled = isSeekable
        jumpBackButton.isEnabled = isSeekable
        jumpForwardButton.isEnabled = isSeekable
        elapsedLabel.text = PlaybackTimeFormatter.label(for: backend.currentTime)
        remainingLabel.text = "-\(PlaybackTimeFormatter.label(for: backend.remainingTime))"
        scrubber.accessibilityValue = "\(elapsedLabel.text ?? "0:00") elapsed, \(remainingLabel.text ?? "-0:00") remaining"
        if !isScrubbing {
            scrubber.value = backend.position
        }
        [scrubber, jumpBackButton, jumpForwardButton].forEach { $0.alpha = isSeekable ? 1 : 0.45 }
    }

    private func refreshTrackMenus() {
        updateAudioMenuIfNeeded(audioTracks: backend.audioTracks)
        updateCaptionsMenuIfNeeded(subtitleTracks: backend.subtitleTracks)
    }

    private func updateAudioMenuIfNeeded(audioTracks: [AudioTrack]) {

Tests/StreamResolverTests.swift · toggle policy coverage

Tests/StreamResolverTests.swift

+11

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        testSubtitleDisplayNameNormalization()
        testSubtitleDisplayNameUsesPreservedNamesForGenericVLCTracks()
        testSubtitleOptionMappingIncludesNone()
        print("StreamResolverTests passed")
    }

475 unmodified lines
        assertEqual(options.map(\.name), ["None", "English", "Commentary"])
    }

    private static func assertEqual<T: Equatable>(_ actual: T?, _ expected: T, file: StaticString = #file, line: UInt = #line) {
        assert(actual == expected, "Expected \(String(describing: expected)), got \(String(describing: actual))", file: file, line: line)
    }23 unmodified lines
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        testSubtitleDisplayNameNormalization()
        testSubtitleDisplayNameUsesPreservedNamesForGenericVLCTracks()
        testSubtitleOptionMappingIncludesNone()
        testNativePlaybackTogglePolicy()
        print("StreamResolverTests passed")
    }

475 unmodified lines
        assertEqual(options.map(\.name), ["None", "English", "Commentary"])
    }

    private static func testNativePlaybackTogglePolicy() {
        assertEqual(NativePlaybackTogglePolicy.action(for: .playing), .pause)
        assertEqual(NativePlaybackTogglePolicy.action(for: .buffering), .pause)
        assertEqual(NativePlaybackTogglePolicy.action(for: .paused), .play)
        assertEqual(NativePlaybackTogglePolicy.action(for: .stopped), .play)
        assertEqual(NativePlaybackTogglePolicy.action(for: .ended), .play)
        assertEqual(NativePlaybackTogglePolicy.action(for: .opening), .waitForTransition)
        assertEqual(NativePlaybackTogglePolicy.action(for: .unknown), .waitForTransition)
    }

    private static func assertEqual<T: Equatable>(_ actual: T?, _ expected: T, file: StaticString = #file, line: UInt = #line) {
        assert(actual == expected, "Expected \(String(describing: expected)), got \(String(describing: actual))", file: file, line: line)
    }

Diffs were generated with @pierre/diffs/ssr at documentation time. Each file diff stays inside its own shell so the page remains readable as a static HTML artifact.

Expected Impact for End-Users

Pause and resume should feel more deterministic, rapid taps should be less likely to desynchronize audio and video, and normal playback should spend less time rebuilding audio and subtitle menus during ordinary progress ticks.

Validation

xcodebuild -workspace Dreamio.xcworkspace -scheme Dreamio -configuration Debug -sdk iphonesimulator build succeeded.
The focused Swift compile for the existing test sources plus NativePlaybackBackend.swift succeeded for the iOS simulator target. It cannot execute as a standalone iOS binary from the shell.
Manual device validation is still required for MobileVLCKit timing and real audio-buffer behavior.

Issues, Limitations, and Mitigations

No device was available in this session, so the logs are in place to confirm whether remaining lag is buffer drain or stale UI state.
The workaround options of setting playback rate to 0 or briefly muting remain intentionally unused until the new instrumentation proves they are necessary.
:clock-synchro=0 was not added because it can affect normal sync on unstable streams and needs device evidence first.

Follow-up Work

Run the manual validation matrix on device for pause, resume, rapid toggles, seeking, audio switching, subtitles, and 10 to 15 minutes of smooth playback.
If DEBUG logs show persistent audio after pause, test MobileVLCKit-specific mitigation options in isolation.
If glitches correlate with elementary stream events, consider more granular menu refresh debouncing.