internal override void Apply(VisualElement container)
        {
            /// <sample>
            // Get a reference to the field from UXML and assign it its value.
            var uxmlField = container.Q <CurveField>("the-uxml-field");

            uxmlField.value = new AnimationCurve(new Keyframe[]
                                                 { new Keyframe(0, 0), new Keyframe(5, 8), new Keyframe(10, 4) });

            // Create a new field, disable it, and give it a style class.
            var csharpField = new CurveField("C# Field");

            csharpField.SetEnabled(false);
            csharpField.AddToClassList("some-styled-field");
            csharpField.value = uxmlField.value;
            container.Add(csharpField);

            // Mirror value of uxml field into the C# field.
            uxmlField.RegisterCallback <ChangeEvent <AnimationCurve> >((evt) =>
            {
                csharpField.value = evt.newValue;
            });
            /// </sample>
        }