Vegaでクローズドキャプションと字幕を実装する方法

[[wants.privilege]]
id = "com.amazon.devconf.privilege.accessibility"

import { VTTCue } from '@amazon-devices/react-native-w3cmedia';

// ポリフィルクラス内：
static install() {
    console.log('W3CMediaポリフィルをインストールします');
    global.window.VTTCue = global.VTTCue = VTTCue;
    if(!window.VTTCue) {
      console.log("VTTCueのポリフィルは適用されませんでした");
    }
}

import { KeplerCaptionsView } from '@amazon-devices/react-native-w3cmedia';

const onCaptionViewCreated = (captionsHandle: string): void => {
    videoPlayer.setCaptionViewHandle(captionsHandle);
}

// コンポーネントのreturnステートメント内：
<View style={{ backgroundColor: "white", alignItems: "stretch",
    width: deviceWidth, height: deviceHeight}}>
        <KeplerCaptionsView
            onCaptionViewCreated={onCaptionViewCreated}
            show={true}
            style={{ width: '100%',
            height: '100%',
            top: 0,
            left: 0,
            position: 'absolute',
            backgroundColor: 'transparent',
            flexDirection: 'column',
            alignItems: 'center',
            zIndex: 2}}
        />
</View>

const textTrack = videoPlayer.addTextTrack('subtitles', 'SampleTextTrack', 'en');

const vttCue = new VTTCue(startTime, endTime, payload);
vttCue.lineAlign = lineAlign;
vttCue.positionAlign = positionAlign;
if (size) {
    vttCue.size = size;
}
textTrack.addCue(vttCue);

videoPlayer.textTracks.addEventListener("addtrack", onTextTrackAdded);

const onTextTrackAdded = (event: Event): void => {
    const textTrackList: TextTrackList | undefined | null = media.current?.textTracks;
    if(textTrackList === undefined || textTrackList === null) {
        return;
    }
    for (let i = 0; i < textTrackList.length; i++) {
        const textTrack = textTrackList[i];
        console.log(`テキストトラックID = ${textTrack.id} \
            kind = ${textTrack.kind} \
            language = ${textTrack.language} \
            label = ${textTrack.label} \
            mode = ${textTrack.mode}`);
   }
}

const textTrack = videoPlayer.addTextTrack("subtitles", 'SampleTextTrack', 'en',
    contentUri, contentMimeType);

textTrack.mode = 'showing';

shakaPlayer.configure({
    autoShowText: shaka.config.AutoShowText.ALWAYS,
});

async load(content: any) {
    await shakaPlayer.load(content.uri);
    shakaPlayer.setTextTrackVisibility(true);
}