AVAssetExportSession safety when using structured concurrency in Swift 5 and 6

MovieExporter.swift

import AVFoundation

/**
  * AVAssetExportSession is initialized with an AVComposition, and that composition and all of its constituent
  * components are not sendable. Because `composition` is isolated to the main actor here, the call to the
  * non-isolated method `export(to:as:)` sends it across an isolation region and that is unsafe.
  */
@MainActor
class UnsafeMovieExporter {
    let composition = AVMutableComposition()

    func export() async throws {
        // the compiler is noticing here that `exportSession` depends on `composition`
        // but composition is isolated to MainActor...
        let exportSession = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetPassthrough)
        let url = URL(filePath: "/tmp/whatever.mp4")

        // ... and this makes the non-isolated async call here unsafe
        try await exportSession?.export(to: url, as: .mp4)
    }
}


/**
 * AVAssetExportSession is initialized with an AVComposition, and that composition and all of its constituent
 * components are not sendable. Because `composition` is *not* isolated to the main actor here, the call to the
 * non-isolated method `export(to:as:)` doesn't send it across an isolation region.
 */
class SafeMovieExporter {
    let composition = AVMutableComposition()

    func export() async throws {
        // the compiler is noticing here that `exportSession` depends on `composition`
        // and composition is no longer isolated to any actor...
        let exportSession = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetPassthrough)
        let url = URL(filePath: "/tmp/whatever.mp4")

        // ... and this makes the non-isolated async call here kosher
        try await exportSession?.export(to: url, as: .mp4)
    }
}

@DePasqualeOrg Haven’t tried it but if the caller holds onto the export session and then the exportAsynchronously callback calls into the caller’s context to finish up and read out the status that might work. Observing progress and reading the status is a pain. I’d probably just deal with warnings since it is safe to use the export session in the callback.