expose watch callbacks

Allow watch class users to explicitly invoke the watch
callback.

Since watches and callbacks share a common pool of state
all watches must complete their initialization prior to
invoking their callback methods.

Change-Id: I62ebad64da88a145f3d5006b07c01381b0eb6728
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/src/propertywatchimpl.hpp b/src/propertywatchimpl.hpp
index d0153e4..67bbaff 100644
--- a/src/propertywatchimpl.hpp
+++ b/src/propertywatchimpl.hpp
@@ -108,6 +108,16 @@
     alreadyRan = true;
 }
 
+template <typename DBusInterfaceType>
+void PropertyWatch<DBusInterfaceType>::callback()
+{
+    // Invoke callback if present.
+    if (this->alreadyRan && this->cb)
+    {
+        (*this->cb)();
+    }
+}
+
 template <typename T, typename DBusInterfaceType>
 void PropertyWatchOfType<T, DBusInterfaceType>::updateProperties(
     const std::string& busName,
@@ -144,10 +154,7 @@
         std::get<2>(item->second).get() = p.second.template get<T>();
 
         // Invoke callback if present.
-        if (this->alreadyRan && this->callback)
-        {
-            (*this->callback)();
-        }
+        this->callback();
     }
 }