Looks up the value for a property from the cache. This call does no
blocking IO.
If @proxy has an expected interface (see
#GDBusProxy:g-interface-info) and @property_name is referenced by
it, then @value is checked against the type of the property.
A reference to the #GVariant instance
that holds the value for @property_name or %NULL if the value is not in
the cache. The returned reference must be freed with g_variant_unref().
Looks up the value for a property from the cache. This call does no blocking IO.
If @proxy has an expected interface (see #GDBusProxy:g-interface-info) and @property_name is referenced by it, then @value is checked against the type of the property.