SubtitlePlayable.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Playables;
using UnityEngine.Timeline;

// The instance of the clip for a playing timeline
[System.Serializable]
public class SubtitlePlayable : ScriptPlayable
{
	public UnityEngine.UI.Text ui;
	public string message;

	//public override void OnGraphStart() { }
	//public override void OnGraphStop()  { }

	//public override void PrepareFrame(FrameData info) { }

	public override void OnPlayStateChanged(FrameData info, PlayState newState) {

		if (ui == null)
			return;

		ui.enabled = handle.playState == PlayState.Playing;
		if (newState == PlayState.Playing) {
			ui.text = message;
		}
	}
}

// The representation of the Timeline Clip
[System.Serializable]
public class SubtitlePlayableAsset : PlayableAsset
{
	public ExposedReference<UnityEngine.UI.Text> ui;
	public string message;

	// Create the runtime version of the clip, by creating a copy of the template
	public override PlayableHandle CreatePlayable(PlayableGraph graph, GameObject go)
    {
		var handle = graph.CreateScriptPlayable<SubtitlePlayable> ();
		handle.GetObject<SubtitlePlayable>().ui = ui.Resolve(graph.resolver);
		handle.GetObject<SubtitlePlayable> ().message = message;
		return handle;
    }
}