A ClassDefinition object is a PowerBuilder object that provides information about the class of another PowerBuilder object. You can examine a class in a PowerBuilder library or the class of an instantiated object. By examining the properties of its ClassDefinition object, you can get details about how a class fits in the PowerBuilder object hierarchy.
From the ClassDefinition object, you can discover:
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The variables, functions, and events defined for the class
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The class's ancestor
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The class's parent
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The class's children (nested classes)
Related objects
The ClassDefinition object is a member of a hierarchy of objects, including the TypeDefinition, VariableDefinition, and ScriptDefinition objects, that provide information about datatypes or about the variables, properties, functions, and event scripts associated with a class definition.
For more information, see the Browser or Objects and Controls.
Definitions for instantiated objects
For each object instance, a ClassDefinition property makes available a ClassDefinition object to describe its definition. The ClassDefinition object does not provide information about the object instance, such as the values of its variables. You get that information by addressing the instance directly.
Definitions for objects in libraries
An object does not have to be instantiated to get class information. For an object in a PowerBuilder library, you can call the FindClassDefinition function to get its ClassDefinition object.
Performance
Class definition objects may seem to add a lot of overhead, but the overhead is incurred only when you refer to the ClassDefinition object. The ClassDefinition object is instantiated only when you call FindClassDefinition or access the ClassDefinition property of a PowerBuilder object. Likewise, for properties of the ClassDefinition object that are themselves ClassDefinition or VariableDefinition objects, the objects are instantiated only when you refer to those properties.
The class information includes information about the relationships between objects. These definitions will help you understand what the information means.
object instance
A realization of an object. The instance exists in memory and has values assigned to its properties and variables. Object instances exist only when you run an application.
class
A definition of an object, containing the source code for creating an object instance. When you use PowerBuilder painters and save an object in a PBL, you are creating class definitions for objects. When you run your application, the class is the datatype of object instances based on that class. In PowerBuilder, the term object usually refers to an instance of the object. It sometimes refers to an object's class.
system class
A class defined by PowerBuilder. An object you define in a painter is a descendant of a system class, even when you do not explicitly choose to use inheritance for the object you define.
The object that contains the current object or is connected to the object in a way other than inheritance. This table lists classes of objects and the classes that can be the parents of those objects:
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Object |
Parent |
|---|---|
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Window |
The window that opened the window. A window might not have a parent. The parent is determined during execution and is not part of the class definition. |
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Menu item |
The menu item on the prior level in the menu. The item on the menu bar is the parent of all the items on the associated drop-down menu. |
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Control on a window |
The window. |
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Control on user object |
The user object. |
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TabPage |
The Tab control in which the TabPage is defined or in which it was opened. |
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ListViewItem or TreeViewItem |
The ListView or TreeView control. |
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Visual user object |
The window or user object on which the user object is placed. |
child
A class that is contained within another parent class. Also called a nested class. For the types of objects that have a parent and child relationship, see parent.
ancestor
A class from whose definition another object is inherited. See also descendant.
An object that is inherited from another object and that incorporates the specifics of that object: its properties, functions, events, and variables. The descendant can use these values or override them with new definitions. All objects you define in painters and store in libraries are descendants of PowerBuilder system classes.
inheritance hierarchy
An object and all its ancestors.
collapsed hierarchy
A view of an object class definition that includes information from all the ancestors in the object's inheritance tree, not just items defined at the current level of inheritance.
scalar
A simple datatype that is not an object or an array. For example, Integer, Boolean, Date, Any, and String.
instance variable and property
Built-in properties of PowerBuilder system objects are called properties, but they are treated as instance variables in the class definition information.
Most business applications do not need to use class definition information. Code that uses class definition information is written by groups that write class libraries, application frameworks, and productivity tools.
Although your application might not include any code that uses class definition information, tools that you use for design, documentation, and class libraries will. These tools examine class definitions for your objects so that they can analyze your application and provide feedback to you.
Scenarios
Class information might be used when developing:
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A custom object browser
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A tool that needs to know the objects of an application and their relationships
The purpose might be to document the application or to provide a logical way to select and work with the objects.
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A CASE tool that deconstructs PowerBuilder objects, allows the user to redesign them, and reconstructs them
To do the reconstruction, the CASE tool needs both class definition information and a knowledge of PowerBuilder object source code syntax.
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A class library in which objects need to determine the class associated with an instantiated object, or a script needs to know the ancestor of an object in order to make assumptions about available methods and variables
