useBackgroundDownloadSession in WhisperKitConfig is ignored during initialization

[bww-ios]

When initializing WhisperKit with a WhisperKitConfig, the useBackgroundDownloadSession argument is not correctly passed to the HubAPI downloader. This prevents the use of a background URLSession for model downloads during the initialization process. The issue appears to stem from the WhisperKit convenience initializer, which doesn't seem to propagate the useBackgroundDownloadSession setting to the underlying downloader.

Steps to Reproduce

1. Create a WhisperKitConfig with useBackgroundDownloadSession set to true:

let config = WhisperKitConfig(
    verbose: true,
    logLevel: .debug,
    prewarm: true,
    load: true,
    download: true,
    useBackgroundDownloadSession: true
)

2. Initialize WhisperKit using this configuration:

self.whisperPipe = try await WhisperKit(config)

3. Set a breakpoint in Downloader.swift for the swift-transformers here: https://github.com/huggingface/swift-transformers/blob/main/Sources/Hub/Downloader.swift#L64

Expected Behavior: When useBackgroundDownloadSession is true, the code execution should enter this conditional block to configure a background URLSession:

if inBackground {
    config = URLSessionConfiguration.background(withIdentifier: sessionIdentifier)
    config.isDiscretionary = false
    config.sessionSendsLaunchEvents = true
}

Actual Behavior: The if inBackground conditional evaluates to false, and the background URLSession configuration is skipped. The download proceeds using a standard URLSession.

Current Workaround: AFAIK, to enable background downloading, you must invoke the download() method directly on an already-initialized WhisperKit instance.

[ZachNagengast]

Having trouble replicating this. Are you using the static download() method? That one doesn't have access to the config setting so it needs to be passed in. If you're using the class method setupModels() or just the initializer, it should be set on the WhisperKit object from the config. If you put a breakpoint here can you check the value of useBackgroundDownloadSession?

WhisperKit/Sources/WhisperKit/Core/WhisperKit.swift

Line 317 in cf0d29b

useBackgroundSession: useBackgroundDownloadSession,

                self.modelFolder = try await Self.download(
                    variant: modelVariant,
                    downloadBase: downloadBase,
                    useBackgroundSession: useBackgroundDownloadSession,
                    from: repo,
                    token: modelToken
                )