Transcript Object Oriented PowerBuilder - PowerBuilder User Group

Intoduction to Custom Classes in
PowerBuilder
(Non-Visual User Objects)
William B. Heys
Senior Consultant
7/21/2015
Copyright 2000, marchFIRST, Inc. All
rights reserved.
2
Introduction to Custom Classes
 Explain how PowerBuilder is very Object-Oriented
 Help you understand Object-Oriented Terminology
 Demonstrate why OO is important
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Introduction to Custom Classes
 Show how to separate your business rules from your
interface
 Introduce you to PowerBuilder Custom Classes (NVOs)
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Stop me at any time for questions
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Object
 A software package containing ...
 Data (called attributes) and
 Processing (called operations or services) done by or to the object
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Objects
 Building blocks for Object-Oriented applications
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Class
 A description of a group of identical objects
 Template or blueprint
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Class
 Defines the data contained in the object - implemented as
instance variables
 Defines processing (methods) done by or to an object implemented as events or object functions
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Class
 During PowerBuilder development, you are really defining
classes, not objects
 Classes are stored in PowerBuilder Libraries (PBLs)
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Object
 Objects are created from classes at runtime
 An object is a specific instance of a class
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Object
 There may be many objects (instances) of a particular type
(class)
 All objects of a given class share the processing defined in
the class, but contain different values for their attributes
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Class and Objects
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Class and Objects
 Round
 Yellow
 Fuzzy
 Rubber
 Hollow
 Bounces
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Attributes
 Data elements (variables) contained in an object
 Defined once - in the class
 name, data type, initial value
 scope and access
 Each object (class instance) has its own values for its
attributes
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Implementing Attributes
 Use instance variables to define attributes in PowerBuilder
classes
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Methods
 Implement the processing performed by or to an object
 Defined in the class
 Provide access to attributes (Get and Set)
 Provide state transitions
 Implement business rules
 Do other processing
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Inheritance
 Ability to create new classes from existing classes
 Specialization, sub-classing, or sub-typing
 For Example, Fruits and Types of Fruit
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Inheritance
 The new class can utilize the methods and variables defined
in all classes above it.
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Encapsulation
 Packaging data and related processing together
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Encapsulation
 Information Hiding
 Hiding the implementation details of an object
 Keeping attributes private or protected
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Encapsulation
 Provide public interface (methods) to access or change
attribute values
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Object Communication
 Objects communicate with each other by sending messages
 A client object can send a message calling a function in the
server object
 Or by triggering an event in the server object
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Polymorphism
 Ability to hide different implementations of a method behind
a common interface.
 Different objects respond differently to the same message
 Shifts responsibility of selecting correct method from
sender (client) to receiver (server) of message
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Polymorphism
Bounce!
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Polymorphism
Bounce!
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Polymorphism
Bounce!
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Polymorphism
Bounce!
Applesauce!
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Polymorphism
Bounce!
Applesauce!
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Polymorphism
Bounce!
Applesauce!
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Scope
 The scope of a variable determines when the variable is
created and when it is destroyed
 PowerBuilder has four variable scopes
 Global
 Local
 Instance
 Shared
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Scope - Global Variables
 Exist for the entire life of the application.
 Created when the application initially starts up and
destroyed when the application shuts down.
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Scope - Global Variables
 Can be defined from the script painter within:
 Application, Menu, Window, User Object, and Function painters
 Regardless of where you define global variables, they are
stored in the PowerBuilder application object.
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Scope - Global Variables
 Public Access -The disadvantage to global variables is that
they can be accessed by all scripts in all objects in the
application
 Impact Analysis - To understand whether you can reference
or change the value of a global variable, you need to
examine all of the scripts contained in the entire application
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Scope - Global Variables
 Avoid at all cost
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Scope - Local Variables
 Defined inside a script.
 Only exist while the script is executing.
 Created just before the script begins executing, and are
destroyed as soon as the script finishes.
 Really only temporary variables
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Scope - Instance Variables
 Object-level variables
 Exist only for the life of an object (class instance)
 Created when the object is created, and destroyed when the
object is destroyed
 Allow values to be shared between scripts in the same
object
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Scope - Shared Variables
 Defined for a class
 Shared variables exist for the duration of the application
 Shared variables are created when the first instance of a
class (object type) is created, and are destroyed when the
application finishes
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Scope - Shared Variables
 Shared variables allow values to be shared between
instances of the same type of object
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Access
 PowerBuilder defines three levels of access
 Public
 Private
 Protected
 Access can be defined for instance variables and object
functions but not events
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Access - Public
 Any script anywhere in the application can access a public
variable or function as long as it is in scope.
 Global variables and global functions in PowerBuilder are
always Public
 Events are always Public.
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Access - Private
 Private variables and functions are accessible only by
scripts defined in the same object
 Shared variables are always Private
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Access - Protected
 Protected variables and functions are accessible only by
scripts defined in the same object or its descendants.
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Functions vs. Events
 Everything changed in PowerBuilder 5
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Events and Functions
 Can be triggered synchronously
 Can be posted asynchronously
 Can now pass named arguments
 Can now define return value
 Can be type checked (does it exist?) at compile time (static)
or run-time (dynamic)
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Events
 Limited ability to pass arguments prior to
PowerBuilder 5 (TriggerEvent, PostEvent)
 Message Object LongParm and WordParm
 Could not return explicit values prior to
PowerBuilder 5
 Always associated with an object
 Are always public
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Events
 Are easier to discover in the PowerBuilder script painter
 Ancestor events can be easily extended or overriden
(compile option)
 Cannot overload
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Functions
 Could only be called synchronously prior to PowerBuilder 5
 May be independent of any object (global function)
 Can be private, public, or protected (encapsulation)
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Functions
 Are more difficult to discover inherited functions
 A descendant object does not list its ancestor’s functions
 Can be overloaded (prior to
PowerBuilder 5 this required inheritance)
 Can be polymorphic (prior to PowerBuilder 5 this required
inheritance)
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Building Good Objects - Goals:
 Minimize the dependencies between objects (coupling)
 How much one object knows about another object
 How much one object can affect the behavior of another object
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Objects talk to each other
 Objects communicate by passing messages (Public
Interface)
 Clearly-defined, narrow interface
 Explicitly pass only needed information between objects
 Define simple single-purpose functions (high level of
cohesion)
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Visual Classes in PowerBuilder
 Windows
 w_frame (MDI frame)
 w_base_sheet (ancestor sheet)
 w_customer_list (application specific sheet)
 Menus
 m_main_menu
 m_sheet_menu
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Visual Classes in PowerBuilder
 Standard Visual User Objects
 u_datawindow (reusable datawindow control)
 u_cb_retrieve (reusable retrieve commandbutton)
 Custom Visual User Objects
 External Visual User Objects
 Visual Basic Controls (VBXs)
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Standard Classes (Nonvisual)
 Can now be inherited
 Error
 Message
 Transaction (SQLCA)
 Dynamic SQL Staging Area (SQLSA)
 Dynamic SQL Description Area (SQLDA)
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Custom Classes (Nonvisual)
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Custom Classes (NVOs)
 Separate processing logic from interface objects
 Implement business rules (Business Objects)
 Service Delegation
 DataWindow controllers
 Multiple Inheritance work-around
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Custom Class Example
A Customer Business Object
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Business Object Inheritance
Customer
Residential
Customer
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Commercial
Customer
Application Custom Class
 Service Delegator
 Overcomes limitation on inheriting application object
 Replacement for Global variables
 Application-wide scope vs. Global scope
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Object Manager
 Knows about all objects created from custom classes
 Does not create any objects, but may destroy them
 Each object automatically registers itself to Object Manager
when constructed
 Clean-up at application end
 Avoiding orphans and memory leaks
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Object Manager
 Each object has a primary key (optional)
 And can return value of its primary key
 Object Manager can find an object given class name and
primary key
 Returns a reference to the object if found
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Datawindow Row Selection
 Overcomes limitation on multiple inheritance
 Different custom classes provide different behavior
 Single Row Selection
 Multiple Row Toggle
 Range Selection
 No highlighting
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Datawindow Row Selection
 Related to a common ancestor selection object
 Reduces size of standard DataWindow User Object
 Create the one you need
 Only load optional code if you need it
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Why is OO so important?
 Productivity
 Increase code reuse
 Decrease unnecessary maintenance
 Getting ready for the future
 Distributed Objects
 Improved performance and scalability
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To Learn More
 Take the Powersoft course: Building Object-Oriented
PowerBuilder Applications
 Download library file NVOHOW.ZIP from the NVO section of
PBForum on Compuserve
 Try it yourself
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Special Edition Using PowerBuilder 6
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Special Edition Using PowerBuilder 6
 ISBN: 0-7897-1437-x
 Macmillan (Que) website (www.mcp.com)
 Go to Site Search, Book Information and enter ISBN
 Amazon.Com (www.amazon.com)
 Search on author: Heys, William
 SoftPro Bookstore (www.SoftPro.com)
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Books and Articles by Bill Heys
 Special Edition Using PowerBuilder 5.0.
Co-author, published by Que
June 1996
 “NVO’s How and Why”
Posted to Compuserve (Go PBForum)
NVO Library Section
Filename is NVOHOW.Zip
August 1995
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PowerBuilder Advisor Columns
 “How the PowerBuilder 5.0 Foundation Class Libraries Implement a




Service-Based Architecture.”
November-December 1996
“New Language Features in PowerBuilder 5.0.”
September-October 1996
“Managing your PowerBuilder Objects.”
May-June 1996
“Encapsulating Your Objects”
March-April 1996
“Introduction to Custom Classes”
January-February 1996 (Premier Issue)
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About the Author
CPD Professional
Certified PowerBuilder Developer
Certified for PB 5.0
Since 1994
Advanced CPI
Certified Powersoft Instructor
Since 1992
Founded Boston PowerBuilder User
Group in 1992
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You can reach me...
William B. Heys
Senior Consultant
+1 (781) 203-3171 direct
+1 (617) 513-0296 cell
bill.heys @ marchFIRST.com
bheys @ attglobal.net
[email protected]
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marchFIRST, Inc.
128 Corporate Center
70 Blanchard Rd., 2nd Flooor
Burlington, MA 01803
+1 (781) 203-3000 tel
+1 (781) 203-3050 fax
www.marchFIRST.com
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Thank You!
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