Get/Set the custom Object that implements the ITransformWriteCallback to which TransformWriteEvent and TransformTweenWriteEvent will be sent. The callback will only occur if PhysicsWorld.transformWriteMode is set to PhysicsWorld.TransformWriteMode.Custom and there are PhysicsWorld.bodyUpdateEvents available. The object assigned here will be kept alive, not allowing the GC to dispose of it. To remove the object assigned here, set the callback target to NULL.