要在AVPlayerViewController中显示字幕控制，您需要先确保字幕文件与视频文件关联，并正确设置字幕轨道。这是一种解决方法的示例代码：

import AVKit

class ViewController: UIViewController {
    var playerViewController: AVPlayerViewController!

    override func viewDidLoad() {
        super.viewDidLoad()

        // 创建视频URL
        let videoURL = Bundle.main.url(forResource: "video", withExtension: "mp4")

        // 创建字幕URL
        let subtitleURL = Bundle.main.url(forResource: "subtitle", withExtension: "srt")

        // 创建AVPlayer对象
        let player = AVPlayer(url: videoURL!)

        // 创建AVPlayerItem对象
        let playerItem = AVPlayerItem(url: videoURL!)

        // 创建字幕轨道
        let subtitleTrack = AVAssetTrack(url: subtitleURL!)
        let subtitleMetadata = AVMutableMetadataItem()
        subtitleMetadata.identifier = AVMetadataIdentifier.iTunesMetadataKeyLyrics as NSString
        subtitleMetadata.value = "subtitle" as NSString
        subtitleMetadata.extendedLanguageTag = "srt"
        subtitleTrack.addMetadataItem(subtitleMetadata)

        // 将字幕轨道添加到AVPlayerItem中
        playerItem.add(subtitleTrack)

        // 将AVPlayer对象与AVPlayerItem关联
        player.replaceCurrentItem(with: playerItem)

        // 创建AVPlayerViewController对象
        playerViewController = AVPlayerViewController()
        playerViewController.player = player

        // 显示字幕控制
        playerViewController.showsPlaybackControls = true
        playerViewController.allowsPictureInPicturePlayback = true

        // 将AVPlayerViewController添加到视图中
        addChild(playerViewController)
        view.addSubview(playerViewController.view)
        playerViewController.view.frame = view.bounds
        playerViewController.didMove(toParent: self)
    }
}

请注意，此示例假设您已将视频文件和字幕文件添加到项目中，并将其命名为"video.mp4"和"subtitle.srt"。您可以根据您自己的文件名称进行相应的更改。

另外，请确保您已在项目的Info.plist文件中添加了适当的权限，以允许访问媒体和字幕文件。您可以在"Privacy - Media Library Usage Description"和"Privacy - Files and Folders Usage Description"键中添加适当的描述。