25. The context with a presence pattern
When we say a user 'enters a context', we can mean two things: either he takes up a role in that context, or he opens it on screen (the latter on condition of the former). The first meaning is a matter of state in Perspectives: the role instance is part of the context, filled by the user, or it is not. The second meaning, however, is a state that is much more ephemereal. We could describe it as GUI-state. But for some types of context, it would be convenient to know whether a user actually can see it at a particular moment, or not. Think of a chat room, or a board game: participants would like to know whether their chat partners or game opponents are currently available, or not.
ContextWithScreenState
model:System contains the context type ContextWithScreenState:
case ContextWithScreenState
external
property IsOnScreen (Boolean)
The value of the property IsOnScreen reflects whether, at any moment, the context is actually visible in the MyContexts application. This depends on specific code in the Screen React component.
MyContexts will actually try to set this property on every context that is opened. However, the PDR refuses to set a non-declared property on a role and fails silently to do so.
In order to actually use this facility, provide your context type with sys:ContextWithScreenState as an Aspect (and its external role with the Aspect external role).