[ CCode ( cname = "GCallback" , has_target = false ) ]
public delegate Object ObjectConstructorFunc (Type type, ObjectConstructParam[] construct_properties)
The type used for callback functions in structure definitions and function signatures.
This doesn't mean that all callback functions must take no parameters and return void. The required signature of a callback function is determined by the context in which is used (e.g. the signal to which it is connected).
Use G_CALLBACK to cast the callback function to a ObjectConstructorFunc.