PowerBuilder techniques
PowerBuilder provides full support for inheritance, encapsulation,
and polymorphism in both visual and nonvisual objects.
Creating reusable objects
In most cases, the person developing reusable objects is not
the same person using the objects in applications. This discussion
describes defining and creating reusable objects. It does not address
usage.
Implementing inheritance
PowerBuilder makes it easy to create descendent objects. You
implement inheritance in PowerBuilder by using a painter to inherit
from a specified ancestor object.
For examples of inheritance in visual objects,
see the w_employee window and u_employee_object in
the Code Examples sample application.
Example of ancestor service object
One example of using inheritance in custom class user objects
is creating an ancestor service object that performs basic services
and several descendent service objects. These descendent objects
perform specialized services, as well as having access to the ancestor’s services:
Figure 2-1: Ancestor service object
Example of virtual function in ancestor object
Another example of using inheritance in custom class user
objects is creating an ancestor object containing functions for
all platforms and then creating descendent objects that perform
platform-specific functions. In this case, the ancestor object contains a virtual
function (uf_change_dir in
this example) so that developers can create descendent objects using
the ancestor’s datatype.
Figure 2-2: Virtual function in ancestor object
For more on virtual functions, see “Other techniques”.
Implementing encapsulation
Encapsulation allows you to insulate your object’s
data, restricting access by declaring instance variables as private
or protected. You then write object functions to provide selective
access to the instance variables.
One approach
One approach to encapsulating processing and data is as follows:
-
Define
instance variables as public, private, or protected, depending on
the desired degree of outside access. To ensure complete encapsulation,
define instance variables as either private or protected. -
Define object functions to perform processing and
provide access to the object’s data.Table 2-1: Defining object functions To do this
Provide
this functionExample
Perform processing
uf_do_operation
uf_do_retrieve (which
retrieves rows from the database)Modify instance variables
uf_set_variablename
uf_set_style (which
modifies the is_style string variable)Read instance variables
uf_get_variablename
uf_get_style (which
returns the is_style string variable)(Optional) Read boolean instance variables
uf_is_variablename
uf_is_protected (which
returns the ib_protected boolean variable)
Another approach
Another approach to encapsulating processing and data is to
provide a single entry point, in which the developer specifies the
action to be performed:
-
Define instance
variables as private or protected, depending on the desired degree
of outside access -
Define private or protected object functions to
perform processing -
Define a single public function whose
arguments indicate the type of processing to performFigure 2-3: Defining a public function for encapsulation
For an example, see the uo_sales_order user
object in the Code Examples sample application.
Distributed components
When you generate an application server component, public
functions are available in the interface of the generated component
and you can choose to make public instance variables available.
Private and protected functions and variables are never exposed
in the interface of the generated component.
For more information, see Part 6, “Distributed Application
Techniques.”
Implementing polymorphism
Polymorphism refers to a programming language’s ability
to process objects differently depending on their datatype or class.
Polymorphism means that functions with the same name behave differently
depending on the referenced object. Although there is some discussion
over an exact definition for polymorphism, many people find it helpful
to think of it as follows:
Operational polymorphism
Separate, unrelated objects define functions with the same
name. Each function performs the appropriate processing for its object
type:
Figure 2-4: Operational polymorphism
For an example, see the u_external_functions user
object and its descendants in the Code Examples sample application.
Inclusional polymorphism
Various objects in an inheritance chain define functions with
the same name.
With inclusional polymorphism PowerBuilder determines which
version of a function to execute, based on where the current object
fits in the inheritance hierarchy. When the object is a descendant,
PowerBuilder executes the descendent version of the function, overriding
the ancestor version:
Figure 2-5: Inclusional polymorphism
For an example, see the u_employee_object user
object in the Code Examples sample application.