Creating Your Own Components
Introduction
When an existing SAS/AF software class does not
provide the behavior you desire, you can extend the functionality
of an existing class by creating a subclass. Extending a class be
as simple as creating a new frame class with a blue background or
as complex as writing your own model and view classes. By virtue of
the SCOM framework, you can take advantage of object-oriented functionality
and add to or modify any of the existing properties of a class. Subclassing
and class-wide modifications are performed in the Class Editor or
with SCL class syntax.
Creating a Simple Subclass by Overriding an Attribute
Introduction
As an example of both
the methodology and the techniques that are used to create a simple
subclass, the following example demonstrates how to build a simple
button class that closes a frame. Because you include a “Close”
button of some sort on nearly every frame you build, modifying a subclass
of the existing pushbutton_c class will save you from setting the
same attributes each time you want such a button. After creating the
class and adding it to the Components window, you simply drag your
Close Button class onto the frame; no coding is required, and you
do not have to set its properties.
Creating the Close Button Subclass
Creating a Subclass by Overriding a Method
Introduction
When you create a new
class, you must decide what unique actions you want to include in
its definition. The actions are defined by their methods. Methods
from the parent class are inherited by the subclass that you create.
Although method inheritance is one of the primary benefits of object-oriented
programming techniques, there are times when you will want to augment
a class's functionality by adding new methods or overriding inherited
methods:
-
Override an existing method to modify the behavior of the parent class in your subclass. An overridden method is one that extends a method that has the same name and signature on a parent class. In effect, overridden methods add some kind of functionality to a class, but they also call the same method on the class's parent to ensure that the method's core functionality is preserved. When overriding a method, you provide a new method definition, but you cannot change the method's name, signature, or scope. You can also overload methods to complement existing behavior. For more information on overloading, see Overloading Methods.
To demonstrate how you
can add new behavior to a class, consider the Close Button class that
was presented above. Although the Close Button class is useful, a
user of your application might accidentally click a “Close”
button. To help prevent such mistakes, you could add a confirmation
dialog box to the Close Button class. The class could have the following
behavior:
-
After deciding to augment the behavior of a class, you must decide whether to override an existing method or add a new one. In the Close Button class, the _onClick method that serves as the event handler is called whenever the button is clicked. You can override this method and have the new implementation display the dialog box. If you override a method, you should execute the same method on the parent by calling _super() in your code.
Overriding an Existing Method
-
Display the Source for the method via the pop-up menu and enter the following code in the SCL entry:
useclass sasuser.buttons.closebutton.class; onclick: method; _super(); /* call onClick method on parent */ dcl num rc; dcl list messageList = {'Close this frame?'}, char(1) sel; sel = messageBox(messageList, '?', 'YN'); if sel = 'Y' then call execcmd('end;'); rc=dellist(messageList); endmethod; enduseclass;
Note: For more information on methods
and SCL, see Implementing Methods with SCL, and Adding SAS Component Language Programs to Frames.
Testing the Overridden Method
After you have created
and saved the class, you can test the new behavior. If you created
the frame with the “Close” button from the last example,
open that frame and test it to see the new behavior. The changes to
the Close Button class are picked up automatically when the frame
is opened and the Close Button object is instantiated.
Extending a Class with New Attributes and Methods
Introduction
To add flexibility to
the Close Button class, add an attribute to the class that enables
developers who use the object in their frames to specify whether to
display the confirmation dialog box.
Editing the Existing Method to Query the New Attribute
Edit the existing overridden
_onClick method on the Close Button class by adding four lines of
code:
useclass
sasuser.buttons.closebutton.class;
onclick: method;
_super(); /* calls the parent's _onClick method */
if upcase(displayExitDialog) = 'NO' then
call execcmd('end;');
else do;
dcl num rc;
dcl list messageList = {'Close this frame?'},
char(1) sel;
sel = messageBox(messageList, '?', 'YN');
if sel = 'Y' then call execcmd('end;');
rc=dellist(messageList);
end;
endmethod;
enduseclass;