Playing a Coroutine in Timeline

I would recommend against trying to use a Coroutine inside a PlayableBehaviour. A PlayableBehaviour method like ProcessFrame is called every frame that the clip is active.

Coroutines start a process that last for a certain amount of time. It appears that you need a method that given a certain time, set the text appropriately.

Here's a simple example how to do the text effect completely in timeline.

Code (CSharp):

1. using UnityEngine;
2. using UnityEngine.Playables;
3. using UnityEngine.Timeline;
4. using UnityEngine.UI;
5.  
6. class TypewriterEffectBehaviour : PlayableBehaviour
7. {
8.     public string DefaultText;
9.  
10.     // called every frame the clip is active
11.     public override void ProcessFrame(Playable playable, FrameData info, object playerData)
12.     {
13.         var textObject = playerData as Text;
14.         if (textObject == null)
15.             return;
16.  
17.         // given the current time, determine how much of the string will be displayed
18.         var progress = (float) (playable.GetTime() / playable.GetDuration());
19.         var subStringLength = Mathf.RoundToInt(Mathf.Clamp01(progress) * DefaultText.Length);
20.         textObject.text = DefaultText.Substring(0, subStringLength);
21.     }
22. }
23.  
24. public class TypewriterEffectPlayableAsset : PlayableAsset, IPropertyPreview
25. {
26.     public string Text = "This is the default text to display";
27.     public override Playable CreatePlayable(PlayableGraph graph, GameObject owner)
28.     {
29.         var playable = ScriptPlayable<TypewriterEffectBehaviour>.Create(graph);
30.         playable.GetBehaviour().DefaultText = Text;
31.         return playable;
32.     }
33.  
34.     // this will put the text field into preview mode when editing, avoids constant dirtying the scene
35.     public void GatherProperties(PlayableDirector director, IPropertyCollector driver)
36.     {
37.         driver.AddFromName<Text>("m_Text");
38.     }
39. }
40.  

And the associated track

Code (CSharp):

1. [TrackClipType(typeof(TypewriterEffectPlayableAsset))]
2. [TrackBindingType(typeof(Text))]
3. public class TypewriterEffectTrack : TrackAsset
4. {
5. }

This will use the clip duration and text length to determine the speed of the effect.

Alternately you can use a control track, and on your Monobehaviour use the ITimeControl interface to have Timeline pass the current clip time. In that case you should not use a Coroutine either. This might be better if you only need the time, and the duration or speed is determined by your monobehaviour. It very much depends on how you like it work.

Code (CSharp):

1. public class TypewriterEffect : MonoBehaviour, ITimeControl
2. {
3.     ...
4.     private float m_Time = 0;
5.    
6.     static string GetSubStringForTime(string text, float t, float duration)
7.     {
8.         int subStringLength = Mathf.RoundToInt(Mathf.Clamp01(t / duration) * text.Length);
9.         return text.Substring(0, subStringLength);
10.     }
11.  
12.     // called by timeline control track to pass the current clip time
13.     public void SetTime(double time)
14.     {
15.         m_Time = (float) time;
16.         m_Text = GetSubStringForTime(m_DefaultText, t, m_Duration);
17.     }
18.  
19.     // called by timeline control track when the clip starts
20.     public void OnControlTimeStart()
21.     {
22.     }
23.  
24.     // called by timeline control track when the clip stops
25.     // not necessarily at the end of the clip.
26.     public void OnControlTimeStop()
27.     {
28.     }
29. }
30.  
31.  

Timeline works well when you can express your algorithm as a function of the time. Coroutines work well when you only need forward progress at runtime and can accumulate time yourself.