EditorParticles.cs

//Steffen Vetne made this
//Creative Commons 0

#if UNITY_EDITOR
using UnityEngine;
using UnityEditor;

[ExecuteInEditMode]
[RequireComponent(typeof(ParticleSystem))]
public class EditorParticles : MonoBehaviour
{
    private float previousTime;
    private bool previousSelected;
    private int playAfterFrames;

    private void OnEnable()
    {
        Play();

        previousTime = Time.realtimeSinceStartup;

        EditorApplication.update -= EditorUpdate;
        if (!Application.isPlaying)
            EditorApplication.update += EditorUpdate;
        else
            Destroy(this);
    }

    private void OnDestroy()
    {
        OnDisable();
    }

    private void OnDisable()
    {
        EditorApplication.update -= EditorUpdate;
    }

    private void Play()
    {
        var ps = GetComponent<ParticleSystem>();
        ps.Stop(true, ParticleSystemStopBehavior.StopEmitting);
        ps.Play(true);
    }

    private void EditorUpdate()
    {
        if (this == null)
        {
            EditorApplication.update -= EditorUpdate;
            return;
        }

        if (Application.isPlaying)
        {
            enabled = false;
            return;
        }

        float time = Time.realtimeSinceStartup;

        bool selected = Selection.Contains(gameObject.GetInstanceID());
        if (!selected)
        {
            var ps = GetComponent<ParticleSystem>();

            float deltaTime = Mathf.Min(0.1f, time - previousTime); //I do this because UnityEditor.EditorApplication.update can happen multiple times in a single frame
            if (deltaTime != 0)
                ps.Simulate(deltaTime, true, false);
        }

        playAfterFrames--;
        if (playAfterFrames == 0)
        {
            Play();
        }

        if (selected && !previousSelected)
            playAfterFrames = 2; //These frames are used because it seems that the ParticleSystem's prewarm is not used on when auto-started in editor when selected. Only when pressing Restart

        previousSelected = selected;


        previousTime = time;
    }
}
#endif