connector: wanted(port) says whether anyone watches or reads a port

backend/tests/flow/test_connector.py

@@ -306,3 +306,29 @@ def test_a_connector_publishes_bytes_as_a_media_reference(tmp_path):
     # point of a media dtype.
     MessageSpec(name="demo.frame", dtype=DType.IMAGE).check(reference)
     assert store.path(reference["digest"]).read_bytes() == b"\x89PNG..."
+
+
+def test_a_port_is_wanted_while_watched_or_read(monkeypatch):
+    """A camera fetches frames only while a screen shows them or a flow reads them."""
+    from fluksio.flow.demand import demand
+
+    node = a_sensor([])
+    # A worker process binds no controller and cannot see who watches.
+    assert node.wanted("reading")
+
+    reads: set[str] = set()
+    monkeypatch.setattr(
+        demand, "_controller", SimpleNamespace(consumed_messages=lambda: reads)
+    )
+    assert not node.wanted("reading")
+
+    reads.add("demo.reading")
+    assert node.wanted("reading")
+
+    reads.clear()
+    demand.want("a viewer", {"demo.reading"})
+    try:
+        assert node.wanted("reading")
+    finally:
+        demand.drop("a viewer")
+    assert not node.wanted("reading")