The Shapes Interaction subsystem also may send messages to DrawPad as a direct result of it receiving messages from the User Interface subsystem. The User Interface subsystem will send messages to the Shapes Interaction subsystem when the user selects menu items or edits fields within a popup dialog box. The Shapes Interaction subsystem will send messages to the user interface when a user double-clicks an on screen shape. The user interface will respond by displaying the appropriate dialog box for editing the specifications for that shape. There is no explicit concurrency in this system.
Event handling is the center of control.
The internal representation of UML shapes will have the capability to be saved and retrieved. The file format for HSG will be propriatary and based on Java serialization for object persistance. Because of the nature of Java serialization, backwards compatibility with a previous version of HSG will be non-existant each time the internal representation of shapes changes. We may fix this in future releases by using advanced serialization features to allow more precise control over how the objects get saved. In addition to saving the internal representation, HSG will also have the ability to write a Postscript file for output only.