Sample Chapter: Beginning Active Server Pages 3.0 - Retrieving a Property
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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 & "<BR>"
Response.Write "objTelephone.Material = " & strMaterial & "<BR>"
Response.Write "objTelephone.NumberOfKeys = " & intNumKeys & "<BR>"
Response.Write "objTelephone.Weight = " & intWeight & "<BR>"
Response.Write "objTelephone.IsConnected = " & blnConnected & "<BR>"
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 & "<BR>" 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.
Calling Methods of an Object The syntax for calling the method of an object is very similar to setting or retrieving a property value. There are two points that we need to be concerned about:
- If the method requires parameters, that they are passed correctly.
- If the method has a return value we must receive and capture it.
Try It Out – Calling a Basic Method To make this example a simple one, we will be calling a method that has no parameters. Also, in this example, we are not interested in its return value. We will be using the same objTelephone instance of our telephone object that we have been using in the previous examples in this chapter.
1. Using your editor of choice, enter the following source code:
<%
Option Explicit
Dim objTelephone
Dim blnIsConnected
Set objTelephone = Server.CreateObject("MyTelephone.Telephone")
Response.Write "Answering the phone...<BR>"
objTelephone.Answer()
blnIsConnected = objTelephone.IsConnected
Response.Write "The IsConnected property is " & blnIsConnected & "<P>"
Response.Write "Hanging up the phone...<BR>"
objTelephone.HangUp()
Response.Write "The IsConnected property is " & objTelephone.IsConnected & "<P>"
Set objTelephone = nothing
%> 2. Save this file, with the name MethodsExample.asp, to your BegASP directory.
3. View the file in your web browser.
How It Works In this example, we are using two of the methods that the Telephone object supports. We will also be checking one of the properties after calling the methods to see if they had any effect.
<%
Option Explicit
Dim objTelephone
Dim blnIsConnected
Set objTelephone = Server.CreateObject("MyTelephone.Telephone") The first step, as we have done in the previous examples, is to create an instance of the Telephone object using the Server.CreateObject method. The reference that this method returns will be stored in a local variable. Remember that since we are storing a reference to an object, we have to use the Set statement.
Response.Write "Answering the phone...<BR>"
objTelephone.Answer() The next step is to call the Answer method of the Telephone object. We will use the reference to the instance that we created to call the method. The preceding Response.Write line is being used to provide a visual indication that the method is being called.
blnIsConnected = objTelephone.IsConnected
Response.Write "The IsConnected property is " & blnIsConnected & "<P>" Next, we will want to check the status of the IsConnected property. This property indicates if the phone is in use or not. Since we have just answered the phone, we would assume that this property would be set to true. We will store its value in a local variable, then use that local variable in a Response.Write method to display its value.
Response.Write "Hanging up the phone...<BR>"
objTelephone.HangUp()
Response.Write "The IsConnected property is " & objTelephone.IsConnected & "<P>"
Set objTelephone = nothing
%> Finally, we will hang up the phone by calling the HangUp method of the Telephone object. Once that has completed, we will check the value of the IsConnected property again. This time, instead of storing the value of the property to a local variable before displaying it, we will directly display the value of the property. Both ways work exactly the same way. Then we can release the reference to the object.
Next, we will look at a variation of this example and see how to call a method that has a parameter.
Try It Out – Calling a Method with Parameters In this example, we will be calling a method that has parameters¾we're still not interested in the return value, just yet. Again, we will be using the objTelephone instance of our telephone object that we have been using in all the previous examples.
1. Using your editor of choice, enter the following source code:
<%
Option Explicit
Dim objTelephone
Dim strPhoneNumber
Dim blnIsConnected
Set objTelephone = Server.CreateObject("MyTelephone.Telephone")
strPhoneNumber = "615-555-8329"
Response.Write "Calling " & strPhoneNumber & "...<P>"
objTelephone.PlaceCall(strPhoneNumber)
blnIsConnected = objTelephone.IsConnected
Response.Write "The IsConnected property is " & blnIsConnected & "<P>"
Set objTelephone = nothing
%> 2. Save this file, with the name ParameterExample.asp, to your BegASP directory.
3. View the file in your web browser.
How It Works Now we're telling the telephone to execute the PlaceCall method. As we know, the PlaceCall method doesn't work alone: we need to tell the telephone who to call! We do this by specifying the telephone number as a parameter to the PlaceCall method.
<%
Option Explicit
Dim objTelephone
Dim strPhoneNumber
Dim blnIsConnected
Set objTelephone = Server.CreateObject("MyTelephone.Telephone") First, as we have done in the previous examples, we will create an instance of the Telephone object. The reference to this instance is then stored in a local variable.
strPhoneNumber = "615-555-8329"
Response.Write "Calling " & strPhoneNumber & "...<P>"
objTelephone.PlaceCall(strPhoneNumber) The telephone number that we will be calling is stored as a string. In this example, the number is hard coded. We could have just as easily used a form to supply the value. We then will display a message indicating the number that will be called. We can then pass this value to the PlaceCall method. The parameter that we supply to the PlaceCall method is included within the method's parentheses. The contents of the parentheses are known as the parameter list. The entries in the parameter list could be variables or explicit values.
One thing that you need to be careful with is the order of the parameters in the parameter list. If we were calling a method that requires multiple parameters, then the order of the parameters in the parameter list must exactly match the order that the method is expecting. So, for example, if you call the SendCardNumber method, then you must specify two parameters: the first must be the value of the NumCCN parameter, and the second must be the value of the NumPIN parameter and these parameters are separated by a single comma.
Finally, we check the value of the IsConnected property and display its value to the user and release the object reference.
blnIsConnected = objTelephone.IsConnected
Response.Write "The IsConnected property is " & blnIsConnected & "<P>"
Set objTelephone = nothing
%> We have now seen how to program with the properties and methods of objects. In our examples, we have been using an object that represents a physical entity. The remainder of this chapter will be devoted to looking at a set of objects that represent an application environment in Active Server Pages. These objects comprise the Active Server Pages object model.
Next: What is the Active Server Pages Object Model? >>
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