Function content_register_serializer 

pub fn content_register_serializer<T: 'static, P: Fn(&ContentSerializer, &mut Option<T>) + 'static>(
    type_: Type,
    mime_type: &str,
    serialize: P,
)

Registers a function to serialize objects of a given type.

Since 4.20, when looking up a serializer to use, GTK will use the last registered serializer for a given mime type, so applications can override the built-in serializers.

type_

the type of objects that the function can serialize

mime_type

the mime type to serialize to

serialize

the callback

notify

destroy notify for @data