Specification of Reactive Hardware/Software Systems - Electronic ...

4.8 Properties of Composites 91
composition, decomposition and hiding seem to be important properties of nesting.
Encapsulation is more related to structuring the problem domain into objects. Nesting
is more related to the creation of a system structure, which is a design issue. Nesting
and encapsulation play a role simultaneously.
Process objects encapsulate their data objects. Aggregates encapsulate their part objects.
They protect their encapsulated objects against direct access. All communication must
be performed via the message interface of the encapsulating object.
Clusters can perform nesting of clusters and objects. A cluster can hide its contents.
However, clusters may be rather transparent. Collaborating objects outside a cluster
may access objects in a cluster directly. Civello states that ’encapsulation can be further
constrained or relaxed by limiting or extending the visibility within and across the composite
object.’ This statement makes encapsulation to a more fuzzy concept. Our definitions
enable determining whether an object is encapsulated or nested or both. However visibility
is indeed an interesting concept. Yet we do not relate this concept to encapsulation
but to communication, identification, and various forms of clustering. The concept of
visibility is addressed in the next subsection.
4.8.3 Visibility
The topic of visibility is explained by Civello as ’A necessary (but not sufficient) condition
for an object to be part of another is that the whole object has the ability to send messages to
the part’ [Civ93]. Visibility is a metaphor that is not grasped clearly by this description.
Visibility is related here to the ability to communicate with each other. Perhaps recognition
was a better metaphor than visibility to express this. Visibility has for objects the
premise of knowing each other. Most object-oriented languages use an automated
hidden identification mechanism. Visibility is based on this mechanism. This is different
in our method. In our opinion visibility denotes that an object can perceive and recognise
another object, which means that an object knows the identifier of another object and
can directly communicate with it. A receiving object is an acquaintance of the sending
object.
Process objects know the identifiers of their data objects. Data objects also know the
data objects with which they can communicate. Process objects do not necessarily know
each other. They can simply send a message to a channel. In Section 6.4 about distribution
we will describe this communication fashion as strongly distributed, which
means that modules (process objects or clusters) communicate without the use of identifiers.
Modules that communicate with the explicit use of names communicate in a
weakly distributed fashion. Other process objects are invisible when a process object
communicates in a strongly distributed fashion. Messages are sent and received via
communication channels. A process object cannot look further than its own channels.
The concept of visibility is relevant for clusters and process objects that communicate in
a weakly distributed fashion. Data objects always communicate in a weakly distributed
fashion. Clusters are composites that are either an aggregate or not. This influences the
visibility of their internal structures.