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Published: Monday, November 19, 2001 Chapter 6: Separating Code from Presentation ASP.Net UnleashedStephen WaltherChapter 6: Separating Code from PresentationIn This Chapter
Web developers are not necessarily good designers. Most companies divide the task of building Web sites between two teams. Normally, one team is responsible for the design content of a page, and the other team is responsible for the application logic. Maintaining this separation of tasks is difficult when both the design content and application logic are jumbled together on a single page. A carefully engineered ASP.NET page can be easily garbled after being loaded into a design program. Likewise, a beautifully designed page can quickly become mangled in the hands of an engineer. In this chapter, you learn two methods of dividing your application code from your presentation content. In other words, you learn how to keep both your design and engineering teams happy. First, you learn how to package application code into custom business components. Using a business component, you can place all your application logic into a separate Visual Basic class file. You also learn how to use business components to build multitiered Web applications. Next, you learn how to take advantage of a feature of ASP.NET pages named code behind. Using code behind, you can place all your application logic into a file and create one or more ASP.NET pages that inherit from this file. Code behind is the technology used by Visual Studio.NET to divide presentation content from application logic. Creating Business ComponentsIn this section, you learn how to create business components using Visual Basic and use the components in an ASP.NET page. Business components have a number of benefits:
Creating a Simple Business ComponentA business component is a Visual Basic class file. Whenever you create a component, you need to complete each of the following three steps:
Start by creating a simple component that randomly displays different quotations. You can call this component the quote component. First, you need to create the Visual Basic class file for the component. The quote component is contained in Listing 6.1. Listing 6.1 quote.vbImports System
Namespace myComponents
Public Class quote
Dim myRand As New Random
Public Function showQuote() As String
Select myRand.Next( 3 )
Case 0
showQuote = "Look before you leap"
Case 1
showQuote = "Necessity is the mother of invention"
Case 2
showQuote = "Life is full of risks"
End Select
End Function
End Class
End Namespace
The first line in Listing 6.1 imports the System namespace. You need to import this namespace because you use the Random class in your function, and the Random class is a member of the System namespace.
Next, you need to create your own namespace for the class file. In Listing 6.1, you created a new namespace called myComponents. You could have created a namespace with any name you pleased. You need to import this namespace when you create the ASP.NET page that uses this component. The remainder of Listing 6.1 contains the declaration for your Visual Basic class. The class has a single function, named showQuote(), that randomly returns one of three quotations. This function is exposed as a method of the quote class. After you write your component, you need to save it in a file that ends with the extension .vb. Save the file in Listing 6.1 with the name quote.vb. The next step is to compile your quote.vb file by using the vbc command-line compiler included with the .NET framework. Open a DOS prompt, navigate to the directory that contains the quote.vb file, and execute the following statement (see Figure 6.1): vbc /t:library quote.vb The /t option tells the compiler to create a DLL file rather than an EXE file. Figure 6.1
If no errors are encountered during compilation, a new file named quote.dll should appear in the same directory as the quote.vb file. You now have a compiled business component. The final step is to move the component to a directory where the Web server can find it. To use the component in your ASP.NET pages, you need to move it to a special directory named /BIN. If this directory does not already exist, you can create it.
The /BIN directory must be an immediate subdirectory of your application's root directory. By default, the /BIN directory should be located under the wwwroot directory. However, if your application is contained in a Virtual Directory, you must create the /BIN directory in the root directory of the Virtual Directory. Immediately after you copy the component to the /BIN directory, you can start using it in your ASP.NET pages. For example, the page in Listing 6.2 uses the quote component to assign a random quote to a Label control.
Listing 6.2 showquote.aspx<%@ Import Namespace="myComponents" %> <Script Runat="Server"> Sub Page_Load( s As Object, e As EventArgs ) Dim myQuote As New Quote myLabel.Text = myQuote.ShowQuote() End Sub </Script> <html> <head><title>showquote.aspx</title></head> <body> And the quote is... <br> <asp:Label ID="myLabel" Runat="Server" /> </body> </html> The first line in Listing 6.2 imports the namespace. After the namespace is imported, you can use your component just like any .NET class. In the Page_Load subroutine, you create an instance of your component. Next, you call the ShowQuote() method of the component to assign a random quotation to the Label control (see Figure 6.2). Figure 6.2 Using Properties in a ComponentYou can add properties to a component in two ways. Either you can create public variables, or you can use property accessor syntax. Adding properties by using public variables is the easiest method. For example, the component in Listing 6.3 has two variables named firstValue and secondValue. Because these variables are exposed as public variables, you can access them as properties of the object. Listing 6.3 adder.vbImports System Namespace myComponents Public Class adder Public firstValue As Integer Public secondValue As Integer Function AddValues() As Integer Return firstValue + secondValue End Function End Class End Namespace The component in Listing 6.3 is named the adder component. It simply adds together the values of whatever numbers are assigned to its properties and returns the sum in the AddValues() function. The ASP.NET page in Listing 6.4 illustrates how you can use the adder component to add the values entered into two TextBox controls. Listing 6.4 addValues.aspx<%@ Import Namespace="myComponents" %> <Script Runat="Server"> Sub add( s As Object, e As EventArgs ) Dim myAdder As New Adder myAdder.FirstValue = val1.Text.ToInt32() myAdder.SecondValue = val2.Text.ToInt32() Output.Text = myAdder.AddValues() End Sub </Script> <html> <head><title>addValues.aspx</title></head> <body> <form Runat="Server"> Value 1: <asp:TextBox ID="val1" Runat="Server" /> <p> Value 2: <asp:TextBox ID="val2" Runat="Server" /> <p> <asp:Button Text="Add!" OnClick="add" Runat="Server" /> <p> Output: <asp:Label ID="output" Runat="Server" /> </form> </body> </html> You also can expose properties in a component by using property accessor syntax. Using this syntax, you can define a Set function that executes every time you assign a value to a property and a Get function that executes every time you read a value from a property. You can then place validation logic into the property's Get and Set functions to prevent certain values from being assigned or read. The modified version of the adder component in Listing 6.5, for example, uses property accessor syntax. Listing 6.5 adder2.aspxImports System Namespace myComponents Public Class adder2 Private _firstValue As Integer Private _secondValue As Integer Public Property firstValue As Integer Get Return _firstValue End Get Set _firstValue = Value End Set End Property Public Property secondValue As Integer Get Return _secondValue End Get Set _secondValue = Value End Set End Property Function AddValues() As Integer Return _firstValue + _secondValue End Function End Class End Namespace The modified version of the adder component in Listing 6.5 works in exactly the same way as the original adder component. When you assign a new value to the firstValue property, the Set function executes and assigns the value to a private variable named _firstValue. When you read the firstValue property, the Get function executes and returns the value of the private _firstvalue variable. The advantage of using accessor functions with properties is that you can add validation logic into the functions. For example, if you never want someone to assign a value less than 0 to the firstValue property, you can declare the firstValue property like this: Public Property firstValue As Integer Get Return _firstValue End Get Set If value < 0 Then _firstValue = 0 Else _firstValue = Value End If End Set End Property This Set function checks whether the value passed to it is less than 0. If the value is, in fact, less than 0, the value 0 is assigned to the private _firstValue variable. Using a Component to Handle EventsYou can use components to move some, but not all, of the application logic away from an ASP.NET page to a separate compiled file. Imagine, for example, that you want to create a simple user registration form with an ASP.NET page. When someone completes the user registration form, you want the information to be saved in a file. You also want to use a component to encapsulate the logic for saving the form data to a file. Listing 6.6 contains a simple user registration component. This component has a subroutine named doRegister that accepts three parameters: firstname, lastname, and favColor. The component saves the values of these parameters to a file named userlist.txt.
Listing 6.6 register.vbImports System Imports System.IO Namespace myComponents Public Class register Sub doRegister( firstname As String, lastname As String, favColor As String ) Dim myPath As String = "c:\userlist.txt" Dim myFile As StreamWriter myFile = File.AppendText( myPath ) myFile.Write( "first name: " & firstname & Environment.NewLine ) myFile.Write( "last name: " & lastname & Environment.NewLine ) myFile.Write( "favorite Color: " & favColor & Environment.NewLine ) myFile.Write( "=============" & Environment.NewLine ) myFile.Close End Sub End Class End Namespace After you compile the register.vb file and copy the compiled component to the /BIN directory, you can use the component in an ASP.NET page. The page in Listing 6.7 demonstrates how you can use the register component. Listing 6.7 userRegistration.aspx<%@ Import Namespace="myComponents" %> <Script Runat="Server"> Sub doSomething( s As Object, e As EventArgs ) Dim myReg As New Register myReg.doRegister( firstname.Text, lastname.Text, favColor.Text ) End Sub </Script> <html> <head><title>userRegistration.aspx</title></head> <body> <form Runat="Server"> First Name: <br> <asp:TextBox ID="firstname" Runat="Server" /> <p> Last Name: <br> <asp:TextBox ID="lastname" Runat="Server" /> <p> Favorite Color: <br> <asp:TextBox ID="favColor" Runat="Server" /> <p> <asp:Button Text="Register!" OnClick="doSomething" Runat="Server" /> </form> When someone completes the form and clicks the button, the doSomething subroutine is executed. This subroutine creates an instance of the register component and calls its doRegister() method to save the form data to a file. Notice that you must still place some of the application logic in the userRegistration.aspx page. In the doSomething subroutine, you must pass the values entered into each of the TextBox controls to the doRegister() method. You are forced to do so because you cannot refer directly to the controls in userRegistration.aspx from the register component. Later in this chapter, in the discussion of code behind, you learn how to move all your application logic into a separate compiled file.
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