# PhysicsCallbacks.IContactCallback

> An interface that when implemented, can be called as a target by PhysicsWorld.SendContactCallbacks.

## Definition

* **Type:** Interface
* **Namespace:** [Unity.U2D.Physics](/engine/6000.7/script-reference/unity/u2d/physics.md)
* **Assembly:** UnityEngine.PhysicsCore2DModule

```csharp
public interface PhysicsCallbacks.IContactCallback
```

## Remarks

Unity calls `OnContactBegin2D` and `OnContactEnd2D` on the main thread after the simulation finishes.

Additional Resources: [PhysicsWorld.SendContactCallbacks](/engine/6000.7/script-reference/unity/u2d/physics/physicsworld/sendcontactcallbacks.md), [PhysicsCallbacks.ITriggerCallback](/engine/6000.7/script-reference/unity/u2d/physics/physicscallbacks/itriggercallback.md)

## Examples

```csharp
using UnityEngine;
using Unity.U2D.Physics;

// Add the PhysicsCallbacks.IContactCallback interface.
public class DetectCollisions : MonoBehaviour, PhysicsCallbacks.IContactCallback
{
    void Start()
    {
        PhysicsWorld world = PhysicsWorld.defaultWorld;

        // Create a small falling circle.
        PhysicsBody object1 = world.CreateBody(new PhysicsBodyDefinition
        {
            position = new Vector2(0.5f, 8f),
            type = PhysicsBody.BodyType.Dynamic
        });
        PhysicsShape objectShape1 = object1.CreateShape(CircleGeometry.defaultGeometry);

        // Create a larger static circle below.
        PhysicsBody object2 = world.CreateBody(new PhysicsBodyDefinition
        {
            position = new Vector2(0f, 0f),
            type = PhysicsBody.BodyType.Static
        });
        object2.CreateShape(new CircleGeometry { radius = 3f });

        // Set object 1 to activate collisions.
        objectShape1.contactEvents = true;
        objectShape1.callbackTarget = this;
    }

    // Log when the small circle collides.
    public void OnContactBegin2D(PhysicsEvents.ContactBeginEvent eventData)
    {
        var contact = eventData.contactId.contact;
        Debug.Log("Collision started between shapes: " + contact.shapeA + " and " + contact.shapeB);
    }

    // Log when the small circle stops colliding.
    public void OnContactEnd2D(PhysicsEvents.ContactEndEvent eventData)
    {
        var contact = eventData.contactId.contact;
        Debug.Log("Collision ended between shapes: " + contact.shapeA + " and " + contact.shapeB);
    }
}
```

## Methods

| Method                                                                                                                      | Description                                                                                                                                                                                                                                   |
| --------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| [OnContactBegin2D](/engine/6000.7/script-reference/unity/u2d/physics/physicscallbacks/icontactcallback/oncontactbegin2d.md) | Called when a [PhysicsEvents.ContactBeginEvent](/engine/6000.7/script-reference/unity/u2d/physics/physicsevents/contactbeginevent.md) for the object occurs. This will always be called on the main-thread after the simulation has finished. |
| [OnContactEnd2D](/engine/6000.7/script-reference/unity/u2d/physics/physicscallbacks/icontactcallback/oncontactend2d.md)     | Called when a [PhysicsEvents.ContactEndEvent](/engine/6000.7/script-reference/unity/u2d/physics/physicsevents/contactendevent.md) for the object occurs. This will always be called on the main-thread after the simulation has finished.     |