Separating concerns in direct manipulation user interfaces

Direct-manipulation user interfaces are difficult to implement as a tapered hierarchy. Features such as drag enabling and continuous graphical feedback require frequent interaction and collaboration among a large number of objects in multiple layers. These collaborations complicate the design of the interfaces in the various layers. We present a new component-interface model called a “mode component”, whose features simplify the expression of collaboration enabling and feedback across layer boundaries. We illustrate the use of mode components through a large example