The document discusses encapsulation in PowerBuilder. Encapsulation is the process of incorporating properties and methods together in an object and limiting access to an object's properties and implementation of its behavior through information hiding. PowerBuilder allows assigning access rights to user functions and instance variables to restrict access. Events are always public while operational access rights can fine-tune access to instance variables, such as making variables public but read-only.

1. Buliding Object-Oriented Applications in PowerBuilder Module 7: Encapsulation

2. Objectives Define encapsulation Describe the techniques associated with encapsulation Encapsulate properties, methods, functions, and window controls

3. Topics In troduction Information Hiding Encapsulating Properties Encapsulating Methods Window Controls and User Object Controls Encapsulation Read / Write Access

4. Encapsulation Encapsulation is the process of defining a class of objects that: Perform behavior upon request using input data Provide the results of that behavior as output data Require no external knowledge of the internal methods

5. Encapsulation 1. Clicked! 2. Send message to DataWindow control: "Scroll to next page"

6. Encapsulation — Information Hiding No direct access to manipulate properties (data) Properties accessed only via methods Object controls access to its data "Published" functional interface to make use of the object's behavior

7. Encapsulation in PowerBuilder Instance variables declared private for the object Data can be manipulated outside the object only through user-defined functions Communication with the object occurs via user-defined events or user-defined functions

8. Encapsulation — Benefits Prevents unexpected and undesirable changes from external sources No external impact from internal implementation changes

9. Encapsulating Properties — Implementation Encapsulate instance variables by defining their access as private or protected Declare "Get" and "Set" functions for access To make a property read-only, declare "Get" functions only

10. Encapsulating Properties Define access rights for instance variables: Public – Any scripts in the application (default) Private – Scripts for the object for which the variable was declared Protected – Scripts for the object and descendants of the object

11. Encapsulating Properties

12. Encapsulating Properties — Access and Scope Instance variables default to public — can make private or protected Global variables are always public Shared variables are always private PowerBuilder-defined properties are always public (for example, Window.X)

13. Encapsulating Properties Access to PowerBuilder-defined properties cannot be modified; they are always public For example: x and y are properties of a window Using Move(300, 200) sets their values and does not require knowledge of property names But, x and y values can be directly set for a window: x = 300 y = 200

14. Encapsulating Methods Some user-defined methods should not be open to public access Events are always public Object functions can be made private or protected Specify access rights in the object Function painter definition window

15. Encapsulating Methods — Function Access

16. Encapsulating Methods — Function Access Menu, window, and user-object functions can be private, protected, or public (default) All PowerBuilder functions are public — no access modifiers Globally defined user functions are always public — no access modifiers Private functions are meaningful only to the internal behavior of objects; protected functions may need to be called from descendants

17. Encapsulating Methods Benefits Implementation of behavior can be changed or enhanced without affecting "clients" Platform-dependent behavior is implemented through "plug and play" Using encapsulated behavior does not require external knowledge of implementation

18. Encapsulation — Controls Window controls or user object controls are secondary classes Secondary classes can: Be accessed only through the primary class, for example: w_cust.cb_ok.Hide( ) Access private and protected instance variables of the primary class Access private and protected functions of the primary class

19. Read / Write Access Keyword restricting ability of scripts to read / write a variable's value — called operational access Fine-tunes access rights Declared with the instance variable declaration Used for variables only, not functions

20. Read Access ProtectedRead Only scripts in object and descendants can read Only valid if access right is public public ProtectedRead integer ii_count PrivateRead Only scripts in object can read Only valid if access right is public or protected protected PrivateRead integer ii_maxcount

21. Write Access ProtectedWrite Only scripts in object and descendants can write Only valid if access right is public public ProtectedWrite int ii_count PrivateWrite Only scripts in object can write Only valid if access right is public or protected protected PrivateWrite int ii_maxcount

22. Read / Write Access Can be combined in declaration public PrivateRead PrivateWrite int ii_count Net effect: Compiler generates warning if there is an attempt to redefine access in the descendant

23. Summary Encapsulation is the process of incorporating properties and methods together in an object. Information hiding limits access to an object's properties and implementation of its behavior. PowerBuilder allows encapsulation, but it does not enforce it. Access rights can be assigned to user functions and instance variables.

24. Summary Events are always public. Operational access rights are used to fine-tune access to instance variables (for example, make instance variables public, but read-only).

25. Summary Questions

26. Lab Setup What you will need to do the lab: · Define a custom visual user class · Encapsulate methods and properties in a custom visual user class · Restrict access to properties

27. Lab Debriefing Initialize the DataWindow control reference Then use it. idw_control.ScrollToRow(1)

28. Lab Debriefing The report window uses the control uo_scroller.of_initdw( this) ue_postconstructor of Datawindow control

29. Lab Debriefing Discuss – what kinds of additional things could you have done in the of_initDW( ) Could you have been assured that these were done if the variable idw_control had not been declared private?

30. Lab Debriefing Some Examples: Debugging – you can put a breakpoint on the of_initDW() Logging – you could add code to log to a file Validation – you could add code to validate In Version 2 of your application, you could add an array of DataWindow controls If you had not declared your variable private, you could not be assured that some client was using the of_initDW() to initialize the reference.