Using delegation
Delegation occurs when objects offload processing to other objects.

Aggregate relationship In an aggregate relationship (sometimes called a whole-part relationship ), an object (called an owner object) associates itself with a service object designed specifically for that object type.

For example, you might create a service object that handles extended row selection in DataWindow objects. In this case, your DataWindow objects contain code in the Clicked event to call the row selection object.

 To use objects in an aggregate relationship:

Create a service object (u_sort_dw in this example).


Create an instance variable (also called a reference variable) in the owner (a DataWindow control in this example):



u_sort_dw iuo_sort




Add code in the owner object to create the service object:



iuo_sort = CREATE u_sort_dw




Add code to the owner's system events or user events to call service object events or functions. This example contains the code you might place in a ue_sort user event in the DataWindow control:



IF IsValid(iuo_sort) THEN    Return iuo_sort.uf_sort() ELSE    Return -1 END IF




Add code to call the owner object's user events. For example, you might create a CommandButton or Edit>Sort menu item that calls the ue_sort user event on the DataWindow control.


Add code to the owner object's Destructor event to destroy the service object:



IF IsValid(iuo_sort) THEN    DESTROY iuo_sort END IF