read the force applied by the joint in order to break it dynamically

[johnny-smitherson]

The attempt is to break fixed joints when some limit in force/impulse is reached. The joints are added using the child entity method from the joint3 / despawn joint example.

In the docs on joints, under the FixedJoints section, there is this note

https://rapier.rs/docs/user_guides/bevy_plugin/joints

... (for example if you want to read the force applied by the joint in order to break it dynamically).

But I can't find any examples on this subject, or the subject of reading forces applied to entities by the joints.

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Is there some kind of event similar to these that can be queried to obtain some info on the joints?

I don't suppose the joint impulses/forces will show up either as "contact forces" or "collisions"

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In rapier3d ImpulseJoint there is a field impulses with comment The impulses applied by this joint. How would I read this value from a bevy system? The bevy ImpulseJoint holds a TypeJoint that holds a FixedJoint that holds a GenericJoint that holds a rapier3d GenericJoint that doesn't have the impulse information.

In rapier3d JointLimits there is a field for max impulse; this seems to be not used in the bevy_rapier3d joint limits. It's also not obvious how one would use reaching this limit to break the joint, without being able to read the impulse; maybe by double checking the relative positions and manually checking if the joint was overstretched?

Thanks,

[johnny-smitherson]

found on discord

fn unstick(
    mut commands: Commands,
    mut context: ResMut<RapierContext>,
    mut joints: Query<(Entity, &RapierImpulseJointHandle)>,
) {
    for (joint_entity, rapier_joint_handle) in joints.iter_mut() {
        if let Some(rapier_joint) = context.impulse_joints.get_mut(rapier_joint_handle.0) {
            for impulse in rapier_joint.impulses.column_iter() {
                let impulse_magnitude: f32 = Vec2::new(impulse.x, impulse.y).length();
                if impulse_magnitude > 0.000005 {
                    commands.entity(joint_entity).despawn();
                }
            }
        }
    }
}