ASP 101 - Articles

Altering the Properties of an Object

So, we have a telephone object, which defines the characteristics of any telephone. For a particular instance of the object � that is, a real physical telephone � values are associated to the properties that describe the characteristics of that one telephone.

A program that uses the instance can then retrieve the values associated to these properties. Alternatively, they can be used by a method or event to perform some action. The programmer working with the instance of the telephone object is responsible for setting the values of many properties; other properties will be set based on the results of methods being called.

Setting a Property

First, let's look at how to set a property. The four properties that we'll use here to describe our instance of the telephone object are:

q Color

q Material

q Weight

q NumberOfKeys

When the instance of our object is created, these values are left blank or set to default values. It is up to the program that creates the object to set the specific values that we want.

Try It Out � Setting Property Values

In this example, we will be configuring the properties of our object so that it represents this telephone:

1 Using your editor of choice, enter the following source code:

� Option Explicit

� Dim objTelephone

� Set objTelephone = Server.CreateObject("MyTelephone.Telephone")

� objTelephone.Color = "Blue"

� objTelephone.Material = "Thermoplastic"

� objTelephone.Weight = 22

� objTelephone.NumberOfKeys = 12

� Response.Write "Done"

� Set objTelephone = Nothing

2 Save this file, with the name SetProperties.asp, to your BegASP directory.

3 View the file in your web browser. If everything worked properly, then the browser will display the word Done.

How It Works

The first step in obtaining a reference to an object is to allocate a variable to hold the reference. The variable is created using the Dim statement:

� Dim objTelephone

You'll recall from Chapter 4 that the variables in VBScript are in fact variants.

The next step actually creates the object:

� Set objTelephone = Server.CreateObject("MyTelephone.Telephone")

This is done using the Server.CreateObject method. This is the process of instantiation, that we referred to earlier in the chapter, we discussed the concept of instances. The Server object is one of the built-in ASP objects; objects are everywhere; here we are using an object to create an object. This method has one parameter � the name of the object you want to create. The method also has a return value � it's a reference to an instance of the object.

Since the value returned by the CreateObject method is a reference to an instance of the Telephone object, we must use the Set statement to assign its reference to our variable. The Set statement is a VBScript statement that lets us store object references in variables. Since the return value is a reference to the object, we have to use the Set method to store its value for later use. We will cover the CreateObject method in more detail in Chapter 11.

Now that we have our reference to the instance of the telephone object, we can go about setting the properties. To do this, we simply use the object.property notation and set it to the value that we desire:�

� objTelephone.Color = "Blue"

� objTelephone.Material = "Thermoplastic"

� objTelephone.Weight = 22

� objTelephone.NumberOfKeys = 12

As you can see, the general syntax for this is:

object.property = value

Lastly we set the object to nothing to release the reference and free up memory:

Set objTelephone = nothing

This is general housekeeping you should perform every time you have finished with an object. Now that we have set some property values in our telephone object, we can look at how to retrieve these values.

Retrieving a Property

The last section showed how to set the values of properties of an object. Now that information is stored there, we can retrieve this information at a later time. In essence, we have an instance of an object that has some data stored in its properties. All we need to refer to this instance is the reference to the object's instance. All of the data that the object has stored inside of it comes along with the object.

Read-Only Properties

In addition to the data that we have explicitly stored in the object, there is information that the object uses to describe its state. In our telephone object, there is a property called Connected that describes whether or not a telephone is connected to the telephone exchange. In order to change the connection state of the phone, we would use a method. This method is read only for the user, so the user cannot change it themselves. The only way to change it is internally, as a result of using a method such as PlaceCall, which would change from connected from false to true.

You may wonder why we would not just change the property by hand? This is another example of encapsulation. There is more to disconnecting a phone than just changing a value of a property: the object needs to perform some actions, which the user of the object does not need to be concerned about. This functionality is encapsulated in a method, and the method is responsible for updating the value of the Connected property. This makes the Connected property a read-only property, which means that we cannot set its value, only retrieve it.

Try It Out � Retrieving Property Values

In this example, we will be retrieving the values of some of the properties of the object, and storing them in local variables.

1 Using NotePad or your editor of choice, adapt the program SetProperties.asp, from above, as follows:

� Option Explicit

� Dim objTelephone

� Set objTelephone = Server.CreateObject("MyTelephone.Telephone")

� objTelephone.Color = "Blue"

� objTelephone.Material = "Thermoplastic"

� objTelephone.Weight = 22

� objTelephone.NumberOfKeys = 12

� Dim strColor

� Dim strMaterial

� Dim intNumKeys

� Dim intWeight

� Dim blnConnected

� strColor = objTelephone.Color

� strMaterial = objTelephone.Material

� intNumKeys = objTelephone.NumberOfKeys

� intWeight = objTelephone.Weight

� blnConnected = objTelephone.IsConnected

� Response.Write "objTelephone.Color = " & strColor & " "

� Response.Write "objTelephone.Material = " & strMaterial & " "

� Response.Write "objTelephone.NumberOfKeys = " & intNumKeys & " "

� Response.Write "objTelephone.Weight = " & intWeight & " "

� Response.Write "objTelephone.IsConnected = " & blnConnected & " "

� Set objTelephone = nothing

2 Save this code in the file RetrieveProperties.asp, in the BegASPFiles directory.

3 View the page in your browser.

How It Works

First, we set the Color, Material, NumberOfKeys and Weight properties of our telephone, just as we did in the previous example. Next, we allocate some variables that will hold the values of the properties of our telephone object, using the Dim statement. We allocate one variable for each property that we are storing:

� Dim strColor

� Dim strMaterial

� Dim intNumKeys

� Dim intWeight

� Dim blnConnected

Next, we set about retrieving the property values. To do this, we use the object.property notation again � this time to retrieve the property, and then we store the property in the appropriate variable. Here's the code that does this for the Color property:

� strColor = objTelephone.Color

As you can see, the general syntax for this is:

myVariable = object.property

Then we output the results. Here's the line that does this for the Color property:

� Response.Write "objTelephone.Color = " & strColor & " "

If the value of the property is a reference to an object, then you will need to use the Set statement to assign the property value to our local variable:

Set myVariable = object.property

We have now seen how to put information into the properties of an object and retrieve that information. You might also have noticed that the IsConnected property didn't return a value. That's because it can only be set by another method, the Answer method. As we haven't called the method, the value can't be assigned. Up until the moment that the method is called, it doesn't have a value. We'll be looking at how it can be changed next. So now lets get our object to do some work for us by calling its methods.