Multiart

The Multiart client is designed as a Java applet using the Java AWT for graphics and user interaction. Being a Java applet, Multiart can be downloaded and run in a Java compliant web browser. Unfortunately, no web browser currently supports object serialization. For now, Multiart must be run using Sun's appletviewer application.

Multiart uses the Java AWT to draw its user interface and to respond to users' actions. The following is a list of the Java applet methods used by Multiart:

• public void init() - This method instantiates the Relay cache object and draws the Multiart GUI. The GUI consists of a toolbar of buttons and canvas for drawing. The toolbar has buttons for choosing a drawing mode. The four drawing modes are pointer, line, oval, and rectangle.
• public boolean mouseDown(Event evt, int x, int y) - Called when the user clicks down on the mouse, this method's action depends on the current drawing mode. When the current drawing mode is pointer, clicking the mouse on an existing figure will select the figure for updating. In all other modes, clicking the mouse marks one corner for drawing a new figure.
• public boolean mouseDrag(Event evt, int x, int y) - The mouseDrag method draws an outline of the new or updated figure as the user holds down the mouse button and drags the mouse.
• public boolean mouseUp(Event evt, int x, int y) - This method is called when a user releases the mouse button. It completes the process of updating shared state by notifying the Relay cache that an update has occurred. An object reference to the new or updated figure is passed as the argument to updateCache(Cacheable c) to enter the update into the Relay system.

As required, the Multiart client implements the java.util.Observer interface so it can be notified of changes to the Relay cache. Multiart's implementation of the Observer.update(Observable o, Object o) method simply repaints the objects in the cache to reflect any new state.

 
Figure 4.3: Snapshot of Multiart GUI