Screen Updates

Overview

The time required to update the visualization network and redraw the display(s) may vary widely - thus we should not assume that all UI changes will cause an immediate screen update. Conversely, we should not assume that all updates will be slow - for simple networks & data, it should be possible to allow for immediate updates without user intervention.

Requirements

• Allow the user to make as many changes as they want before requesting an update, regardless of how many user interface components they interact with.
• Allow the user to "reset" the UI, abandoning all of the changes they've made since the last update. A reset must restore the state(s) of UI components to match the server state.
• Support a wide variety of update policies - immediate update, update only when the user explicitly requests it, etc.
• The mechanism for handling updates should be usable by third-parties and application developers with a minimum of overhead.

Reference Implementation

Early versions of the ParaQ demo included a set of classes that accomplished some of the requirements, using Command Design Pattern:

• pqCommandDispatcherManager - singleton class that maintained a "current" command-dispatcher.
• pqCommandDispatcher - abstract interface responsible for "executing" commands.
• pqImmediateCommandDispatcher - pqCommandDispatcher implementation that immediately executes each command it receives and updates the display.
• pqTimeoutCommandDispatcher - pqCommandDispatcher implementation that caches incoming commands until a timeout expires, then executes the cached commands and updates the display.
• pqExplicitCommandDispatcher - pqCommandDispatcher implementation that caches incoming commands until the user explicitly decides to execute them.
• pqCommand - abstract interface for a "command" that can be executed at a later time.

UI components such as pqSMAdaptor do not update the visualization network directly - instead, they created derivatives of pqComand which encapsulated the state changes to be made. These instances were handed-off to the pqCommandDispatcher implementation maintained by pqCommandDispatcherManager. The pqCommandDispatcher implementation controls when to "execute" each command, and deletes the command once executed.