ParticleSystem.ExternalForcesModule.influenceMask

Description

Particle System Force Field Components with a matching Layer affect this Particle System.

Additional resources: LayerMask.

using UnityEngine;

[RequireComponent(typeof(ParticleSystem))]
public class ExampleClass : MonoBehaviour
{
    private ParticleSystem ps;
    public bool layerToggle;
    private readonly int layer0 = 0;
    private readonly int layer1 = 1;

    void Start()
    {
        ps = GetComponent<ParticleSystem>();

        var main = ps.main;
        main.startColor = Color.red;

        var shape = ps.shape;
        shape.enabled = false;

        var externalForces = ps.externalForces;
        externalForces.enabled = true;

        var forceField1 = new GameObject("Force Field 1", typeof(ParticleSystemForceField)).GetComponent<ParticleSystemForceField>();
        forceField1.transform.parent = ps.transform;
        forceField1.transform.localPosition = new Vector3(-3.0f, 0.0f, 3.0f);
        forceField1.transform.localRotation = Quaternion.Euler(0.0f, 0.0f, 180.0f);
        forceField1.transform.localScale = new Vector3(5.0f, 5.0f, 5.0f);
        forceField1.gameObject.layer = layer0;

        forceField1.gravity = 0.04f;
        forceField1.rotationSpeed = 2.0f;
        forceField1.rotationAttraction = 0.02f;

        var forceField2 = new GameObject("Force Field 2", typeof(ParticleSystemForceField)).GetComponent<ParticleSystemForceField>();
        forceField2.transform.parent = ps.transform;
        forceField2.transform.localPosition = new Vector3(3.0f, 0.0f, 3.0f);
        forceField2.transform.localRotation = Quaternion.identity;
        forceField2.transform.localScale = new Vector3(5.0f, 5.0f, 5.0f);
        forceField2.gameObject.layer = layer1;

        forceField2.gravity = 0.04f;
        forceField2.rotationSpeed = 2.0f;
        forceField2.rotationAttraction = 0.02f;
    }

    void Update()
    {
        var externalForces = ps.externalForces;
        externalForces.influenceMask = layerToggle ? (1 << layer0) : (1 << layer1);
    }

    void OnGUI()
    {
        layerToggle = GUI.Toggle(new Rect(25, 40, 100, 30), layerToggle, "Toggle Layer");
    }
}