Strange Timeline bug with custom track

Hey, I've encountered a strange bug with timeline, and I'm mostly sure it's not something I'm doing. I managed to boil down an example to this:

I have a custom track, that extends TrackAsset and ILayerable:

Code (CSharp):

1. [Serializable]
2. [TrackClipType(typeof(AnimationPlayableAsset), false)]
3. [TrackBindingType(typeof(Animator))]
4. public class CustomAnimationTrack : TrackAsset, ILayerable {
5.  
6.     public override Playable CreateTrackMixer(PlayableGraph graph, GameObject go, int inputCount) {
7.         return AnimationMixerPlayable.Create(graph, inputCount);
8.     }
9.  
10.     public Playable CreateLayerMixer(PlayableGraph graph, GameObject go, int inputCount) {
11.         AnimationPlayableOutput playableOutput = AnimationPlayableOutput.Create(graph, "LayerMixerOutput123", null);
12.         return Playable.Null;
13.     }
14.  
15. }

The track just creates an AnimationMixerPlayable for the TrackMixer, and returns Playable.Null for the LayerMixer, but it also creates an instance of AnimationPlayableOutput there. This example does nothing with it but in my "actual" track I do use it.

So this particular track doesn't do anything exciting (I removed all the irrelevant code), except for one thing. If I have two tracks, each with one clip (or more), the ControlTrack that comes after them will not work. So for example, in this setup:


The ControlTrack will not work. The clip that is on it is driving a GameObject in the scene with an ITimeControl script on it, that logs all the methods. I get no log in the console. I also don't see anything weird in the playable graph. If I remove one of the custom tracks (or all the clips on one), the ControlTrack starts working normally. Other types of track (e.g. ActivationTrack) work as expected.

This is happening on Unity 2020.3.11f1. I attached an example project showing the issue.

Does anyone have an idea what's going on?

 

Hey @DavidGeoffroy, thanks for the feedback, I didn't know about "outputs". Doing it that way fixed the issue, thanks! Just for reference, this is my actual track:

Code (CSharp):

1. [Serializable]
2. [TrackClipType(typeof(AnimationPlayableAsset), false)]
3. [TrackBindingType(typeof(Animator))]
4. public class AdditiveAnimationTrack : TrackAsset, ILayerable {
5.  
6.     public override Playable CreateTrackMixer(PlayableGraph graph, GameObject go, int inputCount) {
7.         return AnimationMixerPlayable.Create(graph, inputCount);
8.     }
9.  
10.     public Playable CreateLayerMixer(PlayableGraph graph, GameObject go, int inputCount) {
11.         // This mixes the the first layer ("base" track) with all the other layers as additive
12.         AnimationLayerMixerPlayable layerMixer = AnimationLayerMixerPlayable.Create(graph, inputCount);
13.         layerMixer.SetLayerAdditive(0, false);
14.         layerMixer.SetInputWeight(0, 1f);
15.         for (uint i = 1; i < inputCount; i++) {
16.             layerMixer.SetLayerAdditive(i, true);
17.             layerMixer.SetInputWeight((int) i, 1f);
18.         }
19.  
20.         // Create an output to the Animator
21.         PlayableDirector director = go.GetComponent<PlayableDirector>();
22.         Animator controller = director.GetGenericBinding(this) as Animator;
23.         AnimationPlayableOutput playableOutput = AnimationPlayableOutput.Create(graph, "LayerMixer", controller);
24.         playableOutput.SetSourcePlayable(layerMixer);
25.  
26.         return layerMixer;
27.     }
28.  
29. }

It's basically an "animation" track with support for additive animations. If I remove the "Create an output to the Animator" section and add an override for outputs, it works and the issue is gone:

Code (CSharp):

1. [Serializable]
2. [TrackClipType(typeof(AnimationPlayableAsset), false)]
3. [TrackBindingType(typeof(Animator))]
4. public class AdditiveAnimationTrack : TrackAsset, ILayerable {
5.  
6.     public override Playable CreateTrackMixer(PlayableGraph graph, GameObject go, int inputCount) {
7.         return AnimationMixerPlayable.Create(graph, inputCount);
8.     }
9.  
10.     public Playable CreateLayerMixer(PlayableGraph graph, GameObject go, int inputCount) {
11.         // This mixes the the first layer ("base" track) with all the other layers
12.         AnimationLayerMixerPlayable layerMixer = AnimationLayerMixerPlayable.Create(graph, inputCount);
13.         layerMixer.SetLayerAdditive(0, false);
14.         layerMixer.SetInputWeight(0, 1f);
15.         for (uint i = 1; i < inputCount; i++) {
16.             layerMixer.SetLayerAdditive(i, true);
17.             layerMixer.SetInputWeight((int) i, 1f);
18.         }
19.  
20.         return layerMixer;
21.     }
22.  
23.     public override IEnumerable<PlayableBinding> outputs {
24.         get {
25.             yield return AnimationPlayableBinding.Create(name, this);
26.         }
27.     }
28.  
29. }

However, the graph looks the same in both cases, which is still weird.

 

Hey David, I was a bit wrong here, the graphs in the two examples do look different:

Using "outputs":


Creating output in CreateLayerMixer:


But the strange part is, if I move the Control Track one space up, so it's between the two animation tracks, it starts working again, but the only thing that changes in the graph is the order:


That part still confuses me. Why would just changing the ordering of the track "fix" the issue?