YouTubeKit returns stream URLs only; its own README punts on the
progressive-vs-adaptive problem. Most modern YouTube videos above 360p
serve video and audio as separate DASH tracks, so the previous "any MP4"
fallback silently grabbed a video-only track and the downloaded file had
no audio.
VideoDownloadService now resolves a DownloadPlan up front. Progressive
MP4 (when one exists) still streams straight to the final destination.
Otherwise the service downloads the best MP4 video track + best M4A
audio track to temp files, then combines them into a single MP4 via
AVMutableComposition + AVAssetExportSession. Passthrough preset first
(lossless container rewrite) with a fallback to highestQuality if the
codec combination requires re-encoding. Temp files are cleaned up on
both success and failure.
DownloadStatus replaces the bare Double progress so the UI can show
per-phase labels (Video %, Audio %, Finalizing…). Muxing progress is
rendered as an indeterminate spinner since AVAssetExportSession's
progress property doesn't cross actor boundaries cleanly.
Also retires the deprecated Stream.subtype comparison in favor of
Stream.fileExtension, matching YouTubeKit's current API.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Trey t
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