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Connect Physics Core 2D API objects with joints

Create a connection or constraint between two physics objects.
Read time 2 minutesLast updated 12 days ago
Note
This documentation is about writing C# scripts using the 
Unity.U2D.Physics
API. To use 2D physics in the Unity Editor using components like the Rigidbody 2D component, refer to 2D physics instead.
To create a connection or constraint between two Physics Core 2D API objects, create a joint that connects the two physics bodies.
For example, create a fixed joint that welds two bodies together, or a distance joint that keeps them a set distance apart. Use joints to simulate real-world mechanical behaviours like a spring or a hinge.
All joints allow you to set the anchor points on the bodies they connect, and limits on the force and torque they apply. For more information, refer to the 
PhysicsJoint
 API.
Follow these steps:
  1. Create two 
    PhysicsBody
    objects. For more information, refer to Create a physics object.
  2. Create a definition object for the type of joint you want. For example, a 
    PhysicsDistanceJointDefinition
    object. Make it a 
    public
    field so it displays its properties in the
    Inspector

    ?

    window.
    For example:
    public PhysicsDistanceJointDefinition jointDefinition = new PhysicsDistanceJointDefinition{    // Specify the two bodies to connect    bodyA = body1,    bodyB = body2,    // Set the distance    distance = 2f};
    A new definition has a set of default values. For more information about definitions and changing the default values, refer to Configure 2D physics properties using a definition.
    For the full list of joint types, refer to 
    PhysicsJoint.JointType
    .
  3. To create the joint, use the 
    CreateJoint
    method of your world object, and pass in the definition. For example:
    PhysicsJoint distanceJoint = world.CreateJoint(jointDefinition);
  4. If your script is in a 
    MonoBehaviour
    class attached to a GameObject, adjust the properties in the Inspector window.
    To configure the properties in your script instead, set the properties of the definition object before you create the joint. For more information, refer to Configure 2D physics properties using a definition.
If you destroy a body, any joints connected to that body are also destroyed. For more information, refer to Destroy physics objects and manage memory.

Example

The following example uses a fixed joint to create a fixed point that a large circle swings from.
using UnityEngine;using Unity.U2D.Physics; public class CreateJoint : MonoBehaviour{    public PhysicsDistanceJointDefinition jointDefinition = new PhysicsDistanceJointDefinition();    void Awake()    {        PhysicsWorld world = PhysicsWorld.defaultWorld;        // Create a static fixed body        PhysicsBodyDefinition bodyDefinition1 = new PhysicsBodyDefinition{            type = PhysicsBody.BodyType.Static,            position = new Vector2(-5f, 0f),        };        PhysicsBody object1 = world.CreateBody(bodyDefinition1);        // Create a large circle below the fixed body        PhysicsBodyDefinition bodyDefinition2 = new PhysicsBodyDefinition{            type = PhysicsBody.BodyType.Dynamic,            position = new Vector2(5f, 0f),        };        PhysicsBody object2 = world.CreateBody(bodyDefinition2);        object2.CreateShape(new CircleGeometry { radius = 3f });        // Add the bodies to the joint definition        jointDefinition.bodyA = object1;        jointDefinition.bodyB = object2;                // Add the joint to the world        world.CreateJoint(jointDefinition);            }}

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