Inheritance Using PowerBuilder 5.0

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IA353Using Custom Class Objects in PowerBuilder
Fred Grau Systems Architect - VIPS fgrau_at_chesint.n
et
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Using Custom Class Objects in PowerBuilder

• Skill level Beginning to intermediate
• Agenda-OOP Basics-NVO Basics-NVO
  Usage-Service Based Architecture (SBA)

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Shameless Pitch

• Certified PowerBuilder Developer
• Former Consultant - Woodfield Technical Services,
  Inc.
• Current Position Systems Architect - VIPS
• E-mail fgrau_at_chesint.net

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IMPORTANT!!!

• If you want to leverage your PowerBuilder code
  for the Internet, you must use Custom Class
  Objects (Also called Non-Visual Objects or
  NVOs).
• If you want to use a good framework (including
  the PFC), you must learn how to use NVOs.

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OOP Basics

• Purpose To maintain code in one place.
• Avoid code duplication

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Objects

• Object - A combination of data and behaviors.
• Object consists of-Attributes What do I know
  about?-Functions What things can I do?-Events
  What can happen to me?

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Attributes

• Pre-defined attributes-window.title-window.heig
  ht-sle_1.text
• User-defined attributes-customer.name-n_cst_appm
  anager.is_userid

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Functions

• Declare methods or functions to access and
  manipulate attributes.
• Examples-w_employee.hide()-w_employee.wf_calcul
  ate_balance()-dw_1.reset()

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Events

• Predefined mapped - Directly mapped to the
  operating system. Example WindowOpen
• Predefined not mapped - Predefined in PB, but not
  mapped to an object. Example sle_1ue_changed
  (mapped to pbm_enchange)
• User defined mapped to pbm_customXX - PB 4.0 and
  before. Example ue_save (mapped to
  pbm_custom01)
• User defined not mapped to any MessageID -
  Programmer has control over the parameters and
  return types. Example ue_save (unmapped)

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Classes

• An abstraction of an object.
• The definition of an object.
• In PowerBuilder, classes are stored in PBLs.
• All windows created are in the same class of
  Window.

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Instantiation

• An object never exists until runtime.
• At runtime, a class is instantiated into an
  object.
• Example A window is opened.
• Note The same object can be instatiated more
  than once.

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Inheritance

• One class gets its definition from another
  class.
• A real-time template - When the parent (ancestor)
  object changes, so does the descendant.
• Eliminates the copy-paste method of code reuse.

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PowerBuilder Uses Inheritance

• Any Root of all objects and data
  typesPowerObject Remenber Message.PowerObjectParm
  ? NonVisualObject Transaction, Message
  GraphicObject Menu Window
  WindowObject DragObject
  Datawindow, Commandbutton, ListBox, etc.
  DrawObject Rectangle, line, etc.

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Inheriting From System Classes

• When you click New, you are inheriting from a
  PowerBuilder system class.
• Why not inherit from your own customized class
  instead?

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What Gets Inherited

• Attributes
• Events
• Functions

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How Does Inheritance Work

• Events Extending scripts-This is the default
  behavior of an event script.-When an event is
  triggered (i.e. a windows open event), the
  ancestors script is executed before the
  descendants script.

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Inherited Event Flow - Open(w_desc2)

• w_ancestoropenABC
• w_desc1openDEF
• w_desc2openGHI
• Statements executed ABC DEF GHI

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How PowerBuilder Extends a Script

• w_ancestoropenABC
• w_desc1openCALL w_ancestoropenDEF
• w_desc2openCALL w_desc1openGHI

An implicit Call to the ancestor is automatically
coded in each extended descendant.
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Overriding Scripts

• Enabled on an event-by-event basis by setting the
  Design/Override Ancestor Script menu option in
  the descendants event script.
• When an event occurs, only the descendants
  script is executed.
• If necessary, you can still call an ancestor
  script by using the CALL ltancestor_name or
  SUPERgtEvent ltevent_namegt syntax.

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Overriding Events

• w_ancestoropenABC
• w_desc1openCALL w_ancestoropenDEF
• w_desc2open OVERRIDDENGHI

Statements executed GHI
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Overriding Events - Continued

• w_ancestoropenABC
• w_desc1open OVERRIDDEN DEF
• w_desc2open CALL w_desc1openGHI

Statements executed DEF GHI
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How does Inheritance Work for Functions?

• The ancestor and descendant object each have a
  function with the same name and same argument
  list.
• If a descendant script calls the function name,
  the descendants function gets executed and the
  ancestors function is ignored.

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Overriding Functions - Call wf_calc() in w_desc2

• w_ancestor.wf_calc()ABC
• w_desc1.wf_calc()DEF
• w_desc2.wf_calc()GHI

Statements executed GHI
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Overriding Functions - Call wf_calc() in w_desc2

• w_ancestor.wf_calc()ABC
• w_desc1.wf_calc()DEF
• w_desc2.wf_calc() does not exist

Statements executed DEF
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Overloading Functions

• Ancestor and descendant object each have a
  function with the same name but a different
  argument list.
• If a descendant script calls a function, the
  function with the same argument list gets
  executed and any other functions with the same
  name are ignored.
• You can overload a function without using
  inheritance.

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PowerBuilder Example of Function Overloading

• MessageBox-MessageBox("Greeting", "Hello
  User")-MessageBox (Delete, Delete Error,
  Exclamation!)-MessageBox (Delete, Are you
  deleting?, Exclamation!, YesNoCancel!, 2)

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Polymorphism

• Provides the ability to hide different
  implementations of a method behind a common
  interface.
• The same function call performed on different
  objects produces different results.
• Shifts responsibility of correct functionality
  from the sender (client) to the receiver (server).

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Polymorphism Examples

• Example 1inv_sybase.of_get_return_code()inv_ora
  cle.of_get_return_code()inv_informix.of_get_retur
  n_code()
• Example 2basketball.bounce()football.bounce()b
  owlingball.bounce()

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How Objects Talk To Each Other

• Objects communicate to each other by passing
  messages (via a public interface).-You any pass
  the information the object needs.
• This allows you to develop single-purpose
  methods.
• In PowerBuilder you can call functions or trigger
  events.

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Ways to Pass Arguments to a Method

• By Value You pass a copy of the value to the
  method.
• By Reference You pass a reference (a pointer or
  memory location) to the method.
• By ReadOnly You pass a reference, but it cannot
  change.

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Remember

• When passing a simple data type (i.e. String,
  Int, Boolean) by value, you are passing a copy of
  the value.
• When passing a complex data type (i.e.
  DataWindow, Window, User Object) it is
  automatically passed by reference.

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Non-Visual Objects

• PowerBuilder calls them Custom Class User Objects.

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Defining Custom Class User Objects
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Non-Visual Objects

• Once defined, you can add1. Functions2.
  Events3. Attributes (e.g. Instance Variables)
• They exist only in memory.

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Instantiating Objects DECLARE

• Window w_logon lw_logon
• NVO n_cst_count lnv_count
• Declaring an object merely creates a reference
  variable (pointer). This pointer is initialized
  to null (invalid). It hasnt been created in
  memory (instantiated).
• The compiler uses this declaration to validate
  the existence of its methods and attributes.

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Instantiating Objects Instantiate

• Window Open (lw_logon)
• NVO lnv_count CREATE n_cst_count
• Instantiating an object tells PowerBuilder to
  create an object in memory and make the reference
  variable point to the object.
• NOTE You can create the declared object or a
  descendant of the declared object.
  Examples lnv_count CREATE
  n_cst_count_win16 lnv_count CREATE
  n_cst_count_win32

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Differences Between OPEN and CREATE

• OPEN is for visual objects-If the reference
  variable points to an instantiated object
  (IsValid) then a new object is NOT created.-OPEN
  returns a -1 if the object cannot be
  instantiated.
• CREATE is for Non-visual objects.-CREATE always
  creates a new object.-If the reference variable
  points to an instantiated object, then the old
  object is orphaned.

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CREATE USING

• Can now CREATE objects dynamically without hard
  coding the object names.
• Example n_cst_deptxxx are descendants of
  n_cst_dept.n_cst_dept lnv_deptString ls_dept_nvo
  Integer li_dept_numls_dept_nvo
  n_cst_dept String(li_dept_num)lnv_dept
  CREATE USING ls_dept_nvo
• If the classname referenced in CREATE USING is
  not found, a runtime error is generated.

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Instantiating Objects USE

• Window lw_logon.Hide()
• NVO lnv_count.of_get_count()
• Can use an objects Attributes Events Function
  s

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Instantiating Objects DESTROY

• Window Close (lw_logon)
• NVO Destroy lnv_count
• Destroying an object removes it from memory and
  frees up resources. The variable will still
  exist and is set to null (invalid).
• Not destroying NVOs can cause memory leaks.
• PowerBuilder 6.0 has automatic garbage
  collection which will reduce memory leaks.

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Encapsulate Your NVO

• Allows you to change the implementation of an
  object without breaking other parts of the
  system.
• One object should not directly access the
  attributes of another object. Only public
  methods (functions or events) can access these
  attributes.
• To prevent direct manipulation, declare your
  instance variables as private or protected.
• Create Get and Set functions to access the
  attributes. Examples gnv_app.of_SetUserID
  (ls_userid) ls_userid gnv_app.of_GetUserID()

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Access Modifiers

• Public (Default) Open to all other objects.
• Protected Open to the current object and its
  descendants.
• Private Open to the current object only.
• ProtectedRead Only the current object and its
  descendants can read this attribute.
• ProtectedWrite Only the current object and its
  descendants can write to this attribute.
• PrivateRead Only the current object can read
  this attribute.
• PrivateWrite Only the current object can write
  to this attribute.

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AutoInstantiation of User Objects

• NVOs can now be set to AutoInstantiate when the
  object is declared.
• Object is automatically destroyed when it goes
  out of scope.
• To AutoInstantiate-Right-click on the user
  object when it is defined.-Select the
  AutoInstantiate menu item
• CREATE causes a compile error.CREATE USING
  causes a runtime error.

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AutoInstantiated NVO Example

• Assume n_cst_employee is AutoInstantiatedn_cst_e
  mployee inv_emp // Instance or Local
  VariableInteger li_rcString ls_ssnli_rc
  inv_emp.of_edit_ssn(ls_ssn)
• Object is destroyed when it goes out of scope.

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Using AutoInstantiated NVOs

• Even with AutoInstantiation and PowerBuilders
  garbage collection, still use the CREATE and
  DESTROY.
• AutoInstantiation is very useful when using NVOs
  as structures.
• Use AutoInstantiation when an object does not
  reference any other objects.

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Some Uses for NVOs

• Hold Global variables.
• Storage for constants.
• Business rules.
• EAServer and the Web.
• Add GUI functionality.
• Database independence.
• Placeholder for external DLL functions.

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NVO Uses Hold Global variables

• Lets you easily transport globals to new
  applications.No need to duplicate globals in
  each application.
• Allows for the easy add or change of new
  attributes.
• Lets you protect against invalid changes of
  attributes through encapsulation.

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NVO Uses Storage for Constants

• Return values should be constants.
• Remember Constants do not use storage during
  runtime.
• Any changes to a constants value only has to be
  made in one place. After making the change, just
  simply regenerate the necessary objects.
• To use-Create an NVO that holds nothing but
  constants (e.g. n_cst_constants)-Create a global
  variable n_cst_constants gc-Reference
  constants using dot notation IF SQLCA.SQLCode
  gc.NOT_FOUND Then

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NVO Uses Business Rules

• Place as many edits and updates as possible in
  NVOs.
• Business rules NVOs then become reusable in your
  application.
• They also become reusable in future applications.

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NVO Uses EAServer and the Web

• NVOs can be moved to an application server
  (EAServer).
• Web pages can then be incorporated to have HTML
  interface with NVOs running on an application
  server.

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NVO Uses Add GUI Functionality

• The PFC uses a Service Based Architecture (NVOs)
  to handle Multiple Inheritance.
• Objects are loosely coupled - Less dependency on
  other objects.
• Objects (e.g. Windows and DataWindow controls)
  delegate work to the NVOs.
• Only the required services are placed into
  memory.
• PFC examplesu_dw - Ancestor datawindow control
  (Standard visual user object of type datawindow.
  n_cst_dwsrv_rowselection - Can single or
  multi-select rows.n_cst_dwsrv_sort - Sort
  service.

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NVO Uses Database Independence

• Use NVOs to encapsulate database interaction.
• Inherit a transaction object by performing the
  following-Create a Standard class user object
  (NVO) of type transaction (e.g. n_tr).-Add
  database-related methods (e.g. Commit, Rollback,
  Connect, status checking, etc.)-Edit the
  application object, edit its properties, click
  the variable types tab, and under SQLCA change
  Transaction to n_tr.-Access n_trs methods
  (e.g. SQLCA.of_commit() )

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NVO Uses Placeholder for external DLL functions.

• PFC Example

Remember The CREATE statement can instantiate
either the ancestor or one of its descendants.
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NVO Uses Placeholder for external DLL functions
(Continued).

• Create a variable n_cst_platform lnv_platform
• Call function f_SetPlatform (lnv_platform, TRUE)
  to instantiate either the 16-bit or 32-bit
  version of the external DLL NVO.
• Call the desired function with lnv_platform.
  Examplels_directory lnv_platform.of_GetSyste
  mDirectory()
• Destroy the NVO f_SetPlatform (lnv_platform,
  FALSE)

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NVO Uses Placeholder for external DLL functions
(Continued).

• f_SetPlatform(anv_platform, ab_switch)
  ScriptEnvironment lenvGetEnvironment (lenv)IF
  ab_switch TRUE // Create the NVO
  IF lenv.win16 THEN anv_platform
  CREATE n_cst_platformwin16 ELSE
  anv_platform CREATE n_cst_platformwin32
  END IFELSE // Destroy the NVO
  Destroy anv_platformEND IF

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Service Based Architecture

• Introduced to the masses in PowerBuilder 5.0 with
  the PFC (PowerBuilder Foundation Classes).

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1st Generation of Coding PowerBuilder

• No inheritance.

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2nd Generation of Coding PowerBuilder

• Inherit everything.

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Problems

• What if you want a tab with master/detail
  datawindows?
• What if you want your DataWindow to have Sort and
  Multi-row Selection capabilities?
• What if you change the GUI to now include tab
  folders?
• You need multiple-inheritance, but PowerBuilder
  doesnt provide it.

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Problem Example

• u_dw_singleselect
• u_dw_singleselect_filtersimple
• u_dw_singleselect_filtertab
• u_dw_multiselect_multi
• u_dw_multiselect_multi_filtersimple
• u_dw_multiselect_multi_filtertab
• u_dw_multiselect_extended
• u_dw_multiselect_extended_filtersimple
• u_dw_multiselect_extended_filtertab
• u_dw_filtersimple
• u_dw_filtertab

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Wrong Solution

• Create fat ancestors. -Ancestor objects have
  all the required functionality.-Results in
  fewer ancestors.-Uses IF/Case throughout to
  determine functionality.

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Wrong Solution

• Problems with fat ancestors-Slow performance.
  What if your window has multiple
  datawindows?-Maintenance (bug fixes and
  enhancements) is more difficult. Related code is
  spread throughout the object.

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3rd Generation of Coding PowerBuilder

• Use a Service Based Architecture

Client Objects
Service Objects
Services are usually NVOs
The client objects can be Windows, DataWindows,
buttons, etc
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Service Based Architecture

• Definition The delegation of client object
  common behavior to another service class.
• Common functionality is divided into smaller
  units.
• These service classes are instantiated as they
  are needed. This allows for thinner client
  classes.
• Each client class is pre-wired to use the
  services.

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SBA Benefits

• Faster loading of objects.
• Better defined objects (via encapsulation).
• Easier to maintain. Additional services are
  easily added.
• Promotes reusable code.
• Handles the multiple-inheritance problem.

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SBA Drawbacks

• Learning curve. This is a new way of thinking.
• Existing libraries (pre-PFC) will not be easy to
  convert. It is not a cost-effective short term
  solution.

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SBA Example

• You want your DataWindow to single-select rows,
  you want to sort on the column headers, and you
  want to filter the results.

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SBA (PFC) Exampleu_dw Constructor Event

• This.of_SetRowSelect (TRUE)
• inv_rowselect.of_SetStyle (inv_rowselect.SINGLE)
• This.of_SetSort (TRUE)
• inv_sort.of_SetColumnDisplayNameStyle
  (inv_sort.HEADER)
• This.of_SetFilter (TRUE)
• inv_filter.of_SetStyle (inv_filter.SIMPLE)

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Creating a New DataWindow Service in the PFC

• Create a Custom Class Object (NVO) inherited from
  n_cst_dwsrv (i.e. n_cst_dwsrv_unique).
• The datawindow name used in all scripts must be
  call idw_requestor.
• In u_dw (or the corporate layers u_dw), add an
  instance variable. Example n_cst_dwsrv_unique i
  nv_unique
• Create a new function in u_dw (i.e.
  of_setunique(ab_switch)) which will look almost
  identical to pfc_u_dw.of_setfind().
• In the destructor event, destroy the NVO to
  prevent memory leaks. Example This.of_setunique(
  FALSE)
• When you place u_dw on your window, you can now
  enable this service in the Constructor event.
  Example This.of_setunique(TRUE).
• Call scripts within the NVO as needed.
  Example dw_1.inv_unique.of_generate_number()

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Summary

• Using NVOs can provide your application and
  future applications with reusable code.
• To ensure reusability, encapsulate your NVOs.
• There are many uses for NVOs. Only some of these
  uses were described in this presentation.
• The PFC heavily uses NVOs.
• EAServer is primarily based on NVOs.
• If your want to leverage your existing
  PowerBuilder code for the Internet, place as much
  code as possible in NVOs.

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Summary

• Questions/Comments??
• E-mail fgrau_at_chesint.net
• Baltimore Area PowerBuilder Users
  Groupwww.geocities.com/bapbug