Using inheritance to build user objects

When you build a user object that inherits its definition (properties, events, functions, structures, variables, controls, and scripts) from an existing user object, you save coding time. All you must do is modify the inherited definition to meet the requirements of the current application.

For example, suppose your application has a user object u_file_view that has three CommandButtons:

  • List—displays a list of files in a list

  • Open—opens the selected file and displays the file in a MultiLineEdit control

  • Close—displays a message box and then closes the window

If you want to build another user object that is exactly like the existing u_file_view except that it has a fourth CommandButton, you can use inheritance to build the new user object, and then all you need to do is add the fourth CommandButton.

To use inheritance to build a descendant user object:

  1. Click the Inherit button in the PowerBar, or select File>Inherit from the menu bar.

  2. In the Inherit From Object dialog box, select User Objects from the Objects of Type drop-down list.

  3. Select the target as well as the library or libraries you want to look in.

    Displaying user objects from many libraries

    To find a user object more easily, you can select more than one library in the Libraries list. Use Ctrl+click to toggle selected libraries and Shift+click to select a range.

  4. Select the user object you want to use to create the descendant, and click OK.

    The selected object displays in the User Object painter and the title bar indicates that the object is a descendant.

  5. Make any changes you want to the user object.

  6. Save the user object with a new name.

Using the inherited information

When you build and save a user object, PowerBuilder treats the object as a unit that includes:

  • The object (and any controls within the object if it is a custom visual user object)

  • The object's properties, events, and scripts

  • Any variables, functions, or structures declared for the object

When you use inheritance to build a new user object, everything in the ancestor user object is inherited in the direct descendant and in its descendants in turn.

Ancestor's instance variables display

If you create a user object by inheriting it from a custom class or standard class user object that has public or protected instance variables with simple datatypes, the instance variables display and can be modified in the descendant user object's Properties view.

All public instance variables with simple datatypes such as integer, boolean, character, date, string, and so on display in the descendant. Instance variables with the any or blob datatype or instance variables that are objects or arrays do not display.

What you can do in the descendant

You can do the following in a descendant user object:

  • Change the values of the properties and the variables

  • Build scripts for events that do not have scripts in the ancestor

  • Extend or override the inherited scripts

  • Add controls (in custom visual user objects)

  • Reference the ancestor's functions and events

  • Reference the ancestor's structures if the ancestor contains a public or protected instance variable of the structure datatype

  • Access ancestor properties, such as instance variables, if the scope of the property is public or protected

  • Declare variables, events, functions, and structures for the descendant

What you cannot do in the descendant

In a descendant user object, you cannot delete controls inherited from a custom visual user object. If you do not need a control in a descendant user object, you can make it invisible.

Understanding inheritance

The issues concerning inheritance with user objects are the same as the issues concerning inheritance with windows and menus. See Understanding Inheritance for more information.