added topics property to SystemState (fixes #374)

src/roswire/proxy/state.py

@@ -22,18 +22,30 @@ class SystemState:
         A mapping from topics to the names of subscribers to that topic.
     services: Mapping[str, Collection[str]]
         A mapping from services to the names of providers of that service.
+    nodes: AbstractSet[str]
+        The names of all known nodes running on the system.
+    topics: AbstractSet[str]
+        The names of all known topics on the system with at least one
+        publisher or one subscriber.
     """
     publishers: Mapping[str, Collection[str]]
     subscribers: Mapping[str, Collection[str]]
     services: Mapping[str, Collection[str]]
     nodes: AbstractSet[str] = attr.ib(init=False, repr=False)
+    topics: AbstractSet[str] = attr.ib(init=False, repr=False)
 
     def __attrs_post_init__(self) -> None:
         nodes: Set[str] = set()
         nodes = nodes.union(*self.publishers.values())
         nodes = nodes.union(*self.subscribers.values())
         nodes = nodes.union(*self.services.values())
+
+        topics: Set[str] = set()
+        topics = topics.union(self.publishers)
+        topics = topics.union(self.subscribers)
+
         object.__setattr__(self, 'nodes', frozenset(nodes))
+        object.__setattr__(self, 'topics', frozenset(topics))
 
 
 @attr.s(frozen=True, auto_attribs=True)