Visual Basic: An Object Oriented Approach

12 – Creating and using ActiveX objects

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

ActiveX ActiveX is Microsoft’s 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

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

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

Business Application

DocumentProduction

Calculation

Typical OLE Automation CodeSub 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 = NothingEnd Sub

Code uses Word to create a document (e.g. a Customer’s invoice) and print it out.

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

VB’s Object Browser

VB’s Object Browser showing a user-defined class

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 – component’s strategy for creating objects from its classes

ActiveX type In-process (Dynamic Link Library – DLL) Out-of-process (Executable – EXE)

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

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

Process allocation Server can be in-process or out-of-process

In-process uses the client application’s 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.

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)

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

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

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

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 it’s methods