Specifies how to sort particles within a system.
                        using UnityEngine;
using UnityEditor;
using System.Collections;
[RequireComponent(typeof(ParticleSystem))]
public class ExampleClass : MonoBehaviour {
    private ParticleSystem ps;
    private ParticleSystemRenderer psr;
    public ParticleSystemSortMode sortMode;
    void Start() {
        ps = GetComponent<ParticleSystem>();
        psr = GetComponent<ParticleSystemRenderer>();
        // set a slow start speed and high emission rate, to cause more overlap, to show sorting
        var main = ps.main;
        main.startSpeedMultiplier = 0.2f;
        var emission = ps.emission;
        emission.rateOverTimeMultiplier = 100.0f;
        // set color over life, so we can see the sorting more easily
        var colorOverLifetime = ps.colorOverLifetime;
        colorOverLifetime.enabled = true;
        Gradient gradient = new Gradient();
        gradient.SetKeys(
            new GradientColorKey[] { new GradientColorKey(Color.green, 0.0f), new GradientColorKey(Color.red, 1.0f) },
            new GradientAlphaKey[] { new GradientAlphaKey(1.0f, 0.0f), new GradientAlphaKey(1.0f, 1.0f) }
            );
        colorOverLifetime.color = new ParticleSystem.MinMaxGradient(gradient);
    }
    void Update() {
        psr.sortMode = sortMode;
    }
    void OnGUI() {
        sortMode = (ParticleSystemSortMode)GUI.SelectionGrid(new Rect(25, 25, 900, 30), (int)sortMode, new GUIContent[] { new GUIContent("None"), new GUIContent("Distance"), new GUIContent("OldestInFront"), new GUIContent("YoungestInFront"), new GUIContent("Depth"), new GUIContent("DistanceReverse"), new GUIContent("DepthReverse") }, 7);
    }
}