Visual%20Basic:%20An%20Object%20Oriented%20Approach

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Visual Basic An Object Oriented Approach

• 12 Creating and using ActiveX objects

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Component Object Technologies

• Components are packages of software
• Composed of one or more classes
• Can be used to provide service to programs
• Can be used flexibly
• a VB component can be used by a C program, a
  web page or an excel spreadsheet
• Current development trends emphasize the
  development and use of components
• More flexible systems
• Reusability
• Easier team development

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ActiveX

• ActiveX is Microsofts take on components
• A marketing term, covering several technologies
• Object Linking and Embedding (OLE)
• OLE Automation
• Component Object Model (COM)
• ActiveX makes it possible to create components
  that can interact regardless of location,
  language or type
• COM is the standard that ActiveX components
  conform to
• DCOM (Distributed COM) operates across local and
  wide area networks

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OLE Automation

• Use statements in a program to control objects
  within components
• Components are library files (DLLs) or separate
  executable files (EXEs)
• Controlling program can be written in almost any
  language
• VB, C, VBScript, JavaScript, Java, Perl,
  Python
• Execute efficient binaries from inefficient (but
  easy to use) scripting languages

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Use of OLE Automation

• Typical use is to build a complex Business
  application from pre-built components
• e.g an Access database, an Excel calculating
  engine and a Word reporting engine
• Quick, simple and easy to maitain

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Typical OLE Automation Code
Sub CreateCustomerDocument(CustDetails As
String) Dim WOLE As Word.Application Set WOLE
CreateObject(Word.Application)
WOLE.Documents.Add ATemplate, 0
WOLE.Selection.Goto wdGotoBookmark,
wdGotoAbsolute, _
1, Customer WOLE.Selection.Find
.Text .Replacement.Text End With
Selection.InsertAfter CustDetails
Application.Printout Set WOLE Nothing End Sub
Code uses Word to create a document (e.g. a
Customers invoice) and print it out.
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Where to get information on OLE Auto servers
(ActiveX servers)

• Documentation e.g. manuals supplied with
  libraries
• VB on-line help for servers provided by
  Microsoft (e.g. Office applications)
• Primary source The Object Browser
• Built in to VB
• Provides accurate interface information on any
  components on the system

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VBs Object Browser
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VBs Object Browser showing a user-defined class
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Creating an ActiveX server

• Almost trivial in VB
• VB creates COM components without need for extra
  code (unlike VC)
• Only changes required are type of project
  selected, and a few (very few) property settings
• Instancing components strategy for creating
  objects from its classes
• ActiveX type
• In-process (Dynamic Link Library DLL)
• Out-of-process (Executable EXE)

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Instancing

• States how objects can be created for use by the
  client application
• Private only creatable by another class in the
  same component (Server)
• PublicNotCreatable component creates objects for
  client application
• SingleUse only one object of class allowed
  client creates it
• Global SingleUse as above, but no need to create
  object reference
• MultiUse possible to create many objects from
  the component many clients can create them
• GlobalMulitUse can be used by many clients

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Use of Instancing Property

• Allows memory and processor allocations to be
  optimised depending on the type of component
• e.g. no point in a MultiUse Printing component if
  there is only one printer that can be used by one
  client at a time
• e.g. Many applications could make use of several
  Invoice objects simultaneously MultiUse
  obviously
• Careful design consideration at the stage of
  deciding on instancing setting can greatly
  improve application efficiency

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Process allocation

• Server can be in-process or out-of-process
• In-process uses the client applications memory
  allocation
• Client finds it inherently easier to work with
  because it is inside its own memory space
• Faster
• Limited to one client per instance of the
  component
• Out-of-process runs in its own memory allocation
• Clients need to communicate with it using
  operating system resources
• Slower and more resource hungry
• Several clients can make use of the same server
• Server can execute in its own right (without a
  cllient)
• E.g. Word, Excel, Access etc.

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Building an ActiveX project

• Before use, server component must be compiled
• It will be executed by another program out-with
  Visual Basic
• This can complicate debugging, since client is
  required to make component work
• Can crate VB Project groups (for in-process
  components)
• Can run multiple instances of VB (for
  out-of-process components)
• Can use the Immediate Window (to do limited test
  and debug)

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Deploying an ActiveX Component

• Must be registered on the system it will be used
  on
• VB does this automatically on the system it was
  developed on
• To move the component to another system, must
  register it explicitly
• Use RegSvr32.exe, or
• Build a setup program using the Packaging and
  Deployment Wizard registers component
  automatically
• Old components can clutter up a system
• Use RegServer32 /unregserver to remove
  registration

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Version Compatibility

• Once a component is registered
• Windows maintains its registry entry
• Can not replace it with an incompatible version
• VB allows three forms of compatibility setting
• None Windows does not check for compatibility
• Project for use while a project is under
  development
• Changes to component interfaces will result in
  registration information being updated
  accordingly to allow compatibility between
  components and clients
• Binary for use in deployed components
• changes made to the interface of one or more
  classes will result in VB updating the registry
  information to ensure that incompatible client
  programs do not try to interoperate with the
  component

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Creating a Server

• Start an ActiveX EXE or DLL Project
• Implement server classes as required
• Set appropriate instancing for each class
• Create suitable public methods and properties for
  communication with client
• Compile project to target form (EXE or DLL)
• For an ActiveX EXE, run it once on the target
  computer
• For an ActiveX DLL, use Package and Deployment
  Wizard, or execute RegSvr32.exe

RegSvr32 c\MyComponents\NewComponent.dll
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Using a Server in a VB Program

• Register the server with the program using
  Project/References menu and checking on the
  server component
• Create an instance of the component and use its
  methods