This example shows how to use different interactions derived from CesiumExt.interaction.Interaction abstract class:

• CesiumExt.interaction.GetPosition: get the position based on mouse click
• CesiumExt.interaction.GetDistance: get the distance based on the vertices defined by the mouse click
• CesiumExt.interaction.GetRectangleInteraction: get the rectangle based on mouse click + drag
• CesiumExt.interaction.GetPolygonGraphics: get the Cesium PolygonGraphics with vertices defined by mouse click
• CesiumExt.interaction.GetPolylineGraphics: get the Cesium PolylineGraphics with vertices defined by mouse click.
• CesiumExt.interaction.GetPolylineVolumeGraphics: get the Cesium PolylineVolumeGraphics with vertices defined by mouse click.

Have a look at interaction.js to see how this is done.