gistfile1.cs

using UnityEngine;
using UniRx;

namespace UniRxSamples
{
    public class OnGroundedScript : ObservableMonoBehaviour
    {
        public override void Start()
        {
            var characterController = GetComponent<CharacterController>();
            var particleSystem = GetComponentInChildren<ParticleSystem>();

            UpdateAsObservable()
                .Select(_ => characterController.isGrounded)
                .DistinctUntilChanged()
                .Where(x => x)
                .Subscribe(_ => particleSystem.Play());
        }
    }
}