When do you use AVAssetReader instead of a player, and what do the output settings change?

Predict first

What do the reads return?

try await makeVideo(at: clip, frames: 30, fps: 30)
defer { try? FileManager.default.removeItem(at: clip) }

let asset = AVURLAsset(url: clip)
let track = try await asset.loadTracks(withMediaType: .video)[0]
let bgra = [kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_32BGRA]

func readAll(range: CMTimeRange? = nil, settings: [String: Any]?) throws -> (frames: Int, first: CMSampleBuffer?) {
    let reader = try AVAssetReader(asset: asset)
    if let range { reader.timeRange = range }
    let output = AVAssetReaderTrackOutput(track: track, outputSettings: settings)
    reader.add(output)
    reader.startReading()
    var count = 0, first: CMSampleBuffer?
    while let sample = output.copyNextSampleBuffer() { if first == nil { first = sample }; count += 1 }
    return (count, first)
}
let all = try readAll(settings: bgra)
let pixels = CMSampleBufferGetImageBuffer(all.first!)!
print("decoded frames:", all.frames, "| BGRA pixels:", CVPixelBufferGetPixelFormatType(pixels) == kCVPixelFormatType_32BGRA,
      "| size:", CVPixelBufferGetWidth(pixels), "x", CVPixelBufferGetHeight(pixels))

let window = CMTimeRange(start: CMTime(seconds: 0.5, preferredTimescale: 600), duration: CMTime(seconds: 0.2, preferredTimescale: 600))
print("frames with timeRange 0.5s..0.7s:", try readAll(range: window, settings: bgra).frames)

let compressed = try readAll(settings: nil)
print("outputSettings nil: samples are still compressed, so no image buffer:", CMSampleBufferGetImageBuffer(compressed.first!) == nil)

• 30 BGRA 64x64 frames, 6 frames for a 0.2 s range, and nil image buffers when settings are nil
• 0 frames because the reader needs a player
• compressed samples have an image buffer