add useObserved hook

lib/hooks/observed.dart

@@ -0,0 +1,20 @@
+import 'package:flutter_hooks/flutter_hooks.dart';
+import 'package:mobx/mobx.dart';
+
+/// Observes MobX observables in [fn] and returns the built value.
+/// Behaves like a [useMemoized] with observables as a list of dependencies.
+/// The returned value can be ignored for a `useEffect(() { autorun(fn); }, [])`
+/// clone.
+T useObserved<T>(T Function() fn) {
+  final returnValue = useState(useMemoized(fn));
+
+  useEffect(() {
+    final disposer = autorun((_) {
+      returnValue.value = fn();
+    });
+
+    return disposer;
+  }, []);
+
+  return returnValue.value;
+}