Using Components from ASP
The majority of this book is about creating your own
components for use in your ASP applications, and in this section we'll use a
custom-built component in an ASP page.
We'll begin by looking at how to call components from ASP.
It's likely that you'll have done something like this before during your time
as an ASP developer, but you may not have completely understood the detail of
what your code was up to. Let's put that right with a quick example.
Did You Know You Were Using COM
Components?
Whether you realized it or not, the Server, Request, Response, Application, and Session objects are in fact COM components. They are known as ASP's intrinsic or built-in objects—they are hosted
in a COM server called asp.dll,
which you'll find on your machine if you have IIS or PWS installed. There are also a number of other COM
components that come with ASP, such as the Ad Rotator and the Content Rotator.
The components available to you will depend upon which version of ASP you are
running.
In ASP pages, the ASP intrinsic objects are treated a bit
differently from other COM components. The ASP intrinsic objects are there and
ready-to-use in the page—the ASP environment takes care of this for us.
When we want to use other components on our ASP pages, we
have to create them explicitly using the Server object's CreateObject() method. We also have to pass the
CLSID or the ProgID of the object to the CreateObject() method, so that it creates an object of the
appropriate type for us to use in our script (we will meet ProgIDs
later in this chapter).
Using Our First Component
Later in the chapter, we will build a component called BrickCalc.wsc. Its job is to
calculate how many bricks we would need in order to build a wall. At this
point, we'll talk about how that component is used in a simple user interface
called wroxblox.asp. This page
asks the end user to specify the size of the wall they want to build. It also allows users to specify
the type of brick (breeze block or house brick), using a drop-down list box.
This information is held in an HTML form:
<HTML>
<HEAD>
<TITLE>Wrox Blox Building Supplies</TITLE>
</HEAD>
<BODY>
<P><FONT FACE="Arial" SIZE="6">Wrox Blox
Building Supplies BrickCalc.</FONT></P>
<FORM ACTION="WroxBloxResult.asp"
METHOD="post">
<P>Length of wall in feet: <INPUT ID="WallLength"
NAME="WallLength" ></P>
<P>Height of wall in feet: <INPUT ID="WallHeight"
NAME="WallHeight" ></P>
<P>Select Type of Brick:
<SELECT ID="BrickType" NAME="BrickType"
STYLE="HEIGHT: 22px; WIDTH: 131px">
<OPTION SELECTED
VALUE="BreezeBlock">Breeze Block</OPTION>
<OPTION VALUE="HouseBrick">House
Brick</OPTION>
</SELECT>
</P>
<P>Click here to calculate the number of bricks you will
need:
<INPUT TYPE="submit" VALUE="Calculate">
</P>
</FORM>
</BODY>
</HTML>
The code for this
example and the component it uses is available from our web site at http://webdev.wrox.co.uk/books/2882. You'll also find a compiled version of the
component there. Before you use this component, however, you need to install
the Windows Script Component download that's available from http://www.microsoft.com/scripting; this will be discussed in more depth when we
see how to create the component later in the chapter.
Here is what the WroxBloxForm.asp page looks like:
When the user clicks on the Calculate
button, the values are sent to our WroxBloxResult.asp page. Before we take a look at
that page, let's just see what the component allows us to do.
The Brick Calculator Component
The Brick Calculator component BrickCalc.wsc is a Windows
Script Component (hence the .wsc file extension). Windows Script Components (WSC) is a
Microsoft technology that allows us to create COM components using VBScript or JScript. This type of component
is excellent for speedy prototyping of components, because—as we will see later
in the chapter—the interface is generated by a Wizard, and the methods are
implemented using script.
The Brick Calculator has one property, BrickType, which specifies the
type of brick. It also has one method, HowManyBricks(),which requires the WallHeight and WallLength as parameters in
order to perform its calculation. Given the type of brick and the size of the
wall, it will work out the number of bricks we need to build the wall.
Using the Brick Calculator Component
When the user clicks on the Calculate
button of the HTML form, the values are sent to our WroxBloxResult.asp page. This is the page that uses
the Brick Calculator component and returns the result of its HowManyBricks() method to the
browser. The page starts by creating an instance of the component so that we
can use it from the page:
<%
Dim objBrickCalc
Set objBrickCalc = Server.CreateObject("BrickCalc.wsc")
...
Here, we create an object variable in the first line, then
set this variable to an instance of the component we want to create. In this
case, the ProgID is BrickCalc.wsc.
Recall that the instance of the component created is called an object.
People often use the
terms component and object interchangeably, but in strict terms an object is an
instance of a component.
Now we collect the wall dimensions from the form to submit
to the object's HowManyBricks() method.
We set the BrickType property of the
object to the value of the select box, and write the dimensions of the wall
back to the browser:
...
WallLength = Request.Form("WallLength")
WallHeight = Request.Form("WallHeight")
objBrickCalc.BrickType = Request.Form("BrickType")
Response.Write "<FONT FACE=arial SIZE=6>" & _
"Wrox Blox Building Supplies BrickCalc
Result</FONT><BR><BR>"
Response.Write "Calculation for a wall " & WallHeight
& _
"
foot high and " & WallLength & " foot long."
...
Having created an instance of the component in the ASP page,
we can treat the object just as we would any other object in ASP. So we can
reference the methods our component implements using dot notation:
objectname.methodname(parameter1, parameter2, ..., parameterN)
You should be familiar with this notation already, because
it's exactly the same as the one we use when using the ASP intrinsic objects,
and all other COM components in ASP. For example, when we want to write
something back to the browser in ASP, we are using the Write() method of the Response object.
In our case, however, we have the objBrickCalc variable holding the reference to our
component, so we can access the methods of the component using:
objBrickCalc.methodname(parameter1,
parameter2, ..., parametern)
On this occasion, the component only has one method, so
we'll be using:
objBrickCalc.HowManyBricks(WallHeight, WallLength)
We want to save the return value of the method in a variable
strResult so that we can use it
in the ASP page. To do this, we just make sure that the variable we want to use
in the ASP page is set to the above line, simply by using:
...
strResult = objBrickCalc.HowManyBricks(WallHeight, WallLength)
...
Finally, we write the value of the variable strResult, which holds the
return value of the component's HowManyBricks()
method, to the browser:
...
Response.Write "<BR><BR> You will need " &
strResult & " bricks."
%>
And that's it. Here is the result:
So, we have created an instance of our Brick Calculator
component, and used it from script in our page. Obviously it's a very simple
example, and it's a task that could have been achieved using script within the
form. But it does show you how to use components from your ASP script, and how
we talk to objects via their methods and properties.
Having seen how to use components from our ASP pages, let's
take a look at some of the common types of component, and how they might fit
into our applications.
