Triggers module

The Built-in Particle System’s Triggers module allows you to access and modify particles based on their interaction with one or more CollidersAn invisible shape that is used to handle physical collisions for an object. A collider doesn’t need to be exactly the same shape as the object’s mesh - a rough approximation is often more efficient and indistinguishable in gameplay. More info
See in Glossary in the SceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary. When you enable this module, the Particle System calls the OnParticleTrigger() callback on attached scriptsA piece of code that allows you to create your own Components, trigger game events, modify Component properties over time and respond to user input in any way you like. More info
See in Glossary, which you can use to access lists of particles depending on where they are in respect to the Colliders in the Scene.

Using the Triggers module

This module is part of the Particle SystemA component that simulates fluid entities such as liquids, clouds and flames by generating and animating large numbers of small 2D images in the scene. More info
See in Glossary component. When you create a new Particle System GameObjectThe fundamental object in Unity scenes, which can represent characters, props, scenery, cameras, waypoints, and more. A GameObject’s functionality is defined by the Components attached to it. More info
See in Glossary, or add a Particle System component to an exiting GameObject, Unity adds the Triggers module to the Particle System. By default, Unity disables this module. To create a new Particle System and enable this module:

1. Click GameObject > Effects > Particle System.
2. In the InspectorA Unity window that displays information about the currently selected GameObject, asset or project settings, allowing you to inspect and edit the values. More info
   See in Glossary, find the Particle System component.
3. In the Particle System component, find the Triggers module fold-out.
4. To the left of the fold-out header, enable the checkbox.

To begin, specify which Colliders in the Scene the particles can interact with. To do this, assign one or more Colliders to the Colliders list property. To increase the number of Colliders in the list, click the Add (+) button beneath the list of Colliders. To remove a Collider from the list, select the Collider and click the Remove (-) button. If you have not yet assigned a Collider to an index of the list, you can use the smaller Add (+) button, to the right of the empty entry, to create and assign a new Collider. This creates a new GameObject as a child of the Particle System and attaches a Sphere ColliderA sphere-shaped collider component that handles collisions for GameObjects like balls or other things that can be roughly approximated as a sphere for the purposes of physics. More info
See in Glossary to it, then assigns the Collider to the empty entry.

After you add the Colliders, you can then specify what a particle does when it meets the criteria for passing a particular trigger event type. There are four event types that describe how a particle can interact with a Collider. They are:

• Inside: A particle is inside a Collider’s bounds.
• Outside: A particle is outside a Collider’s bounds.
• Enter: A particle enters a Collider’s bounds.
• Exit: A particle exits a Collider’s bounds.

In the Inspector, there is a drop-down for each of these event types that lets you select what should happen to a particle if it passes the trigger event’s conditions. The options are:

• Callback: Allows you to access the particle in the OnParticleTrigger() callback function.
• Kill: Destroys the particle. You can not access the particle in the OnParticleTrigger() callback function.
• Ignore: Ignores the particle. You can not access the particle in the OnParticleTrigger() callback function.

API

Since this module is part of the Particle System component, you access it through the ParticleSystem class. For information on how to access it and change values at runtime, see the Triggers module API documentation.

Accessing particles within OnParticleTrigger()

If you select Callback as the reaction to one of the trigger events, you can access the particles that fulfill the event condition from an attached script. To do this, you first need to add the OnParticleTrigger() function to an attached script. Inside this function, call the ParticlePhysicsExtensions.GetTriggerParticles() function to get the list of particles that fulfill the trigger event’s criteria. This function takes a ParticleSystemTriggerEventType, which specifies the trigger event you want to get the particles for (Inside, Outside, Enter, or Exit), and a list of ParticlesA small, simple image or mesh that is emitted by a particle system. A particle system can display and move particles in great numbers to represent a fluid or amorphous entity. The effect of all the particles together creates the impression of the complete entity, such as smoke. More info
See in Glossary which the function populates with the result. From the list, you can access, modify, or destroy any particle. The function can also take an optional parameter which outputs collisionA collision occurs when the physics engine detects that the colliders of two GameObjects make contact or overlap, when at least one has a Rigidbody component and is in motion. More info
See in Glossary information, such as which Collider each particle triggered. The Collider Query Mode property controls what information is available through this parameter.

For more information on the API and how to use it, see the below example.

Properties

For some properties in this section, you can use different modes to set their value. For information on the modes you can use, see Varying properties over time.

Examples

Interaction with a collider

The example below causes particles to turn red as they enter the Collider’s bounds, then turn green as they leave the Collider’s bounds.

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

[ExecuteInEditMode]
public class TriggerScript : MonoBehaviour
{
    ParticleSystem ps;

    // these lists are used to contain the particles which match
    // the trigger conditions each frame.
    List<ParticleSystem.Particle> enter = new List<ParticleSystem.Particle>();
    List<ParticleSystem.Particle> exit = new List<ParticleSystem.Particle>();

    void OnEnable()
    {
        ps = GetComponent<ParticleSystem>();
    }

    void OnParticleTrigger()
    {
        // get the particles which matched the trigger conditions this frame
        int numEnter = ps.GetTriggerParticles(ParticleSystemTriggerEventType.Enter, enter);
        int numExit = ps.GetTriggerParticles(ParticleSystemTriggerEventType.Exit, exit);

        // iterate through the particles which entered the trigger and make them red
        for (int i = 0; i < numEnter; i++)
        {
            ParticleSystem.Particle p = enter[i];
            p.startColor = new Color32(255, 0, 0, 255);
            enter[i] = p;
        }

        // iterate through the particles which exited the trigger and make them green
        for (int i = 0; i < numExit; i++)
        {
            ParticleSystem.Particle p = exit[i];
            p.startColor = new Color32(0, 255, 0, 255);
            exit[i] = p;
        }

        // re-assign the modified particles back into the particle system
        ps.SetTriggerParticles(ParticleSystemTriggerEventType.Enter, enter);
        ps.SetTriggerParticles(ParticleSystemTriggerEventType.Exit, exit);
    }
}

See the images below for the result of this example:

Interaction with multiple colliders

The following example takes advantage of the additional collision data you can extract from the GetTriggerParticles() function. It causes particles inside the first Collider’s bounds to turn red, inside the second Collider’s bounds to turn blue, or inside both Colliders to turn green. It also turns particles white if they are not inside a Collider. For this example, Collider Query Mode is set to All.

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

[ExecuteInEditMode]
public class TriggerScript : MonoBehaviour
{
    void OnParticleTrigger()
    {
        ParticleSystem ps = GetComponent();
 
        // particles
        List inside = new List();
        List exit = new List();
 
        // get
        int numInside = ps.GetTriggerParticles(ParticleSystemTriggerEventType.Inside, inside, out var insideData);
        int numExit = ps.GetTriggerParticles(ParticleSystemTriggerEventType.Exit, exit);
 
        // iterate
        for (int i = 0; i < numInside; i++)
        {
            ParticleSystem.Particle p = inside[i];
            if (insideData.GetColliderCount(i) == 1)
            {
                if (insideData.GetCollider(i, 0) == ps.trigger.GetCollider(0))
                    p.startColor = new Color32(255, 0, 0, 255);
                else
                    p.startColor = new Color32(0, 0, 255, 255);
            }
            else if (insideData.GetColliderCount(i) == 2)
            {
                p.startColor = new Color32(0, 255, 0, 255);
            }
            inside[i] = p;
        }
        for (int i = 0; i < numExit; i++)
        {
            ParticleSystem.Particle p = exit[i];
            p.startColor = new Color32(1, 1, 1, 255);
            exit[i] = p;
        }
 
        // set
        ps.SetTriggerParticles(ParticleSystemTriggerEventType.Inside, inside);
        ps.SetTriggerParticles(ParticleSystemTriggerEventType.Exit, exit);
    }
}

See the following image for the result of this example: