If value is not NULL, sets the cached value for the property with name property_name to the value in value.
If value is NULL, then the cached value is removed from the property cache.
If proxy has an expected interface (see g-interface-info) and property_name is referenced by it, then value is checked against the type of the property.
g_dbus_proxy_set_cached_property (proxy, "SomeProperty", g_variant_new ("(si)", "A String", 42));
Normally you will not need to use this method since proxy is tracking changes using the org.freedesktop.DBus.Properties.PropertiesChanged D-Bus signal. However, for performance reasons an object may decide to not use this signal for some properties and instead use a proprietary out-of-band mechanism to transmit changes.
As a concrete example, consider an object with a property ChatroomParticipants which is an array of strings. Instead of transmitting the same (long) array every time the property changes, it is more efficient to only transmit the delta using e.g. signals ChatroomParticipantJoined(String name) and ChatroomParticipantParted(String name).