ObserveTransitionInspector.cs

using UnityEditor;
using System.Linq;
using UniRx;
[CustomEditor(typeof(ObserveTransition))]
public class ObserveTransitionInspector : Editor {
    public override void OnInspectorGUI()
    {
        serializedObject.Update();

        var transition = serializedObject.targetObject as ObserveTransition;
        var behaviours = transition.state.behaviours
                            .Where(b => !b.Equals(transition)).ToList();
        if (behaviours.Count > 0)
        {
            if(transition.target == null) { transition.target = behaviours.First(); }
            int index = EditorGUILayout.Popup("BehaviourName", behaviours.FindIndex(b => transition.target.Equals(b)) , behaviours.Select(o => o.GetType().Name).ToArray());
            transition.target = behaviours[index];
            var methods =  transition.target.GetType().GetMethods();
            var obs = methods.Where(m => m.ReturnType == typeof(IObservable<Unit>)).ToList();
            if (obs.Count > 0)
            {
                int methodIndex = EditorGUILayout.Popup("ObservableMethod", obs.FindIndex(o => o.Name == transition.methodName) , obs.Select(o => o.Name).ToArray());
                if(methodIndex < 0) { methodIndex = 0; }
                transition.methodName = obs[methodIndex].Name;
            }
        }
        serializedObject.ApplyModifiedProperties();
    }
}