#title: AVAsset Reader / Writer / Export description: Low-level companions to AVAsset: AVAssetExportSession (preset-based transcode), AVAssetReader + Video/AudioOutput (pull decoded frames as UIImage / interleaved Float32 PCM), and AVAssetWriter + AVAssetWriterInput (push frames / samples into mp4 / m4a). tag: PRO

Three lower-level companions to AVAsset, mirroring the iOS AVAssetExportSession / AVAssetReader / AVAssetWriter APIs:

• AVAssetExportSession — high-level transcoding / re-encoding with a preset name.
• AVAssetReader + outputs — pull decoded video frames as UIImage or PCM audio as Float32Array.
• AVAssetWriter + AVAssetWriterInput — push frames / samples produced in JS into a new mp4 / m4a file.

Local files only. Both AVAssetReader and AVAssetExportSession are unreliable when fed an HTTP-streamed asset. Download remote sources to a local path first (fetch() + FileManager.writeAsBytes()), then construct the asset from that path.

#AVAssetExportSession

High-level transcode / re-encode using a preset.

const asset = new AVAsset("/path/to/input.mov")
const session = new AVAssetExportSession(asset, "HEVCHighestQuality")
session.outputFileType = "mp4"
session.onProgress = p => console.log(`${(p * 100).toFixed(0)}%`)
await session.exportTo("/path/to/output.mp4")

#Constructor

#new AVAssetExportSession(asset: AVAsset, presetName: AVAssetExportPreset)

Throws if asset is not a non-disposed AVAsset. The preset's compatibility with the asset is validated lazily inside exportTo().

type AVAssetExportPreset =
  | '640x480' | '960x540' | '1280x720' | '1920x1080' | '3840x2160'
  | 'AppleM4A'
  | 'LowQuality' | 'MediumQuality' | 'HighestQuality'
  | 'HEVC1920x1080' | 'HEVC3840x2160' | 'HEVCHighestQuality'
  | 'AppleProRes422LPCM' | 'AppleProRes4444LPCM'

#Properties

#Methods

#exportTo(outputPath: string): Promise<void>

Resolves on success, rejects with the underlying session.error.localizedDescription on failure (including the cancellation case). Calling while another export is running rejects immediately.

If outputFileType is incompatible with the preset, the promise rejects with a list of supported types — it does not throw an uncatchable ObjC NSInvalidArgumentException.

#cancel(): void

Cancels the in-flight export. The pending exportTo promise rejects with "Export cancelled.".

#dispose(): void

Releases the underlying session.

#AVAssetReader

Pull decoded samples from an asset, frame-by-frame or chunk-by-chunk.

const asset = new AVAsset("/path/to/input.mp4")
const tracks = await asset.loadTracks("video")

const reader = new AVAssetReader(asset)
const out = new AVAssetReaderVideoOutput(tracks[0])
reader.add(out)
reader.startReading()

for (;;) {
  const frame = await out.copyNextFrame()
  if (frame == null) break  // true end-of-stream
  console.log(frame.presentationTime.seconds, frame.image.size)
}

#AVAssetReader

#AVAssetReaderVideoOutput

class AVAssetReaderVideoOutput {
  constructor(track: AVAssetTrack, options?: { pixelFormat?: 'bgra' | 'rgba' })
  copyNextFrame(): Promise<{ image: UIImage; presentationTime: MediaTime } | null>
}

copyNextFrame() returns null only on a clean end-of-stream. If the reader transitions to 'failed' or 'cancelled', the promise rejects so you don't silently see "0 frames decoded".

#AVAssetReaderAudioOutput

class AVAssetReaderAudioOutput {
  constructor(tracks: AVAssetTrack[], options?: { sampleRate?: number; channels?: number })
  copyNextSamples(): Promise<{
    samples: Float32Array       // length === frameCount * channels (interleaved)
    frameCount: number
    sampleRate: number
    channels: number
    presentationTime: MediaTime
  } | null>
}

Always interleaved 32-bit float PCM. Pass multiple tracks to mix; options.sampleRate and options.channels resample / re-channel.

#AVAssetWriter

Manually push UIImage frames or PCM chunks to produce a new mp4 / m4a.

const sampleRate = 44100
const writer = new AVAssetWriter("/tmp/out.m4a", "m4a")
const input = AVAssetWriterInput.audio({ sampleRate, channels: 1, codec: "pcm" })
writer.add(input)
writer.startWriting()
writer.startSession(MediaTime.zero())

const samples = new Float32Array(sampleRate)  // 1 second of silence
input.appendPCMSamples(samples, sampleRate, 1, MediaTime.make({ value: 0, timescale: sampleRate }))
input.markAsFinished()
await writer.finishWriting()

#AVAssetWriter

#AVAssetWriterInput (static factories)

AVAssetWriterInput.video({
  width: number, height: number,
  codec?: 'h264' | 'hevc',          // default 'h264'
  bitRate?: number,                 // bits per second
  frameRate?: number,
})

AVAssetWriterInput.audio({
  sampleRate?: number,              // default 44100
  channels?: number,                // default 1
  codec?: 'aac' | 'pcm',            // default 'aac'
  bitRate?: number,                 // only meaningful for AAC
})