Helium: Initial proto and Syncable interface definition

blimp/common/mandatory_callback.h

Index: blimp/common/mandatory_callback.h
diff --git a/blimp/common/mandatory_callback.h b/blimp/common/mandatory_callback.h
index d1871ce48665a7f48aa783d817e32281248805a4..022830936d28b9b215b65adc9c7e7e3499568627 100644
--- a/blimp/common/mandatory_callback.h
+++ b/blimp/common/mandatory_callback.h
@@ -68,6 +68,19 @@ class MandatoryCallback<ReturnType(ArgTypes...)> {
 #endif
   }
 
+  // This a overload that handles the case where there are no arguments provided
+  template <typename...>
+  ReturnType Run() {
+    DCHECK(cb_);  // Can't be run following std::move.
+
+#if DCHECK_IS_ON()
+    DCHECK(!was_run_);
+    was_run_ = true;
+#endif
+
+    cb_.Run();
+  }
+
   template <typename... RunArgs>
   ReturnType Run(RunArgs... args) {
     DCHECK(cb_);  // Can't be run following std::move.
@@ -98,6 +111,8 @@ MandatoryCallback<SignatureType> CreateMandatoryCallback(
   return MandatoryCallback<SignatureType>(callback);
 }
 
+using MandatoryClosure = MandatoryCallback<void()>;
+
 }  // namespace blimp
 
 #endif  // BLIMP_COMMON_MANDATORY_CALLBACK_H_