ObserveTransition.cs

using Arbor;
using UniRx;

[BuiltInBehaviour]
public class ObserveTransition : StateBehaviour
{
    public StateBehaviour target;
    public string methodName = "";
    public StateLink next;

    public override void OnStateBegin()
    {
        IObservable<Unit> obs = target.GetType().GetMethod(methodName).Invoke(target, null) as IObservable<Unit>;
        obs.Take(1).Subscribe(__ => Transition(next));
    }
}