可以通过实现AVAssetResourceLoaderDelegate的方法来限制流的选择，从而提高质量。具体来说，可以使用preferredPeakBitRate属性来设定最大比特率。以下是一段示例代码：

class ResourceLoaderDelegate: NSObject, AVAssetResourceLoaderDelegate {
    private let preferredPeakBitrate: Double
    
    init(preferredPeakBitrate: Double) {
        self.preferredPeakBitrate = preferredPeakBitrate
    }
    
    func resourceLoader(_ resourceLoader: AVAssetResourceLoader, shouldWaitForLoadingOfRequestedResource loadingRequest: AVAssetResourceLoadingRequest) -> Bool {
        if let url = loadingRequest.request.url,
            url.absoluteString.contains("master.m3u8") {
            
            // Load the master playlist
            let session = URLSession(configuration: .default, delegate: self, delegateQueue: nil)
            let task = session.dataTask(with: url) { [weak self] (data, response, error) in
                guard let self = self else { return }
                if let data = data {
                    // Modify the playlist to limit the stream choices by setting the maximum peak bitrate
                    var playlistString = String(data: data, encoding: .utf8) ?? ""
                    playlistString = playlistString.replacingOccurrences(of: "\n", with: "")
                    var lines = playlistString.components(separatedBy: "#EXT-X-STREAM-INF")
                    lines.removeFirst()
                    lines = lines.filter { line in
                        if let range = line.range(of: "BANDWIDTH=") {
                            let bitrateString = String(line[range.upperBound...].split(separator: ",")[0])
                            if let bitrate = Double(bitrateString) {
                                return bitrate <= self.preferredPeakBitrate
                            }
                        }
                        return true
                    }
                    playlistString = "#EXTM3U\n" + lines.joined(separator: "#EXT-X-STREAM-INF")
                    
                    // Provide the modified playlist to AVAssetResourceLoader
                    let dataRequest = loadingRequest.dataRequest ?? AVAssetResourceLoadingDataRequest()
                    dataRequest.respond(with: playlistString.data(using: .utf8)!)
                    
                    loadingRequest.finishLoading()