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Figure 8.4 Displaying page events
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ASP.NET introduces some new terms to the ASP developer s vocabulary: Web Forms and server controls. A Web Form is a form with a runat="server" attribute that causes it to be processed on the server. A server control is a control which also contains a runat="server" attribute. It too is processed on the server and generates HTML/JavaScript to be rendered in the browser. To the developer, server controls appear similar to the Windows Forms controls we saw in the previous chapter. Indeed, using Visual Studio .NET, you can design Web Forms using the built-in dragand-drop forms designer. Server controls expose properties, methods, and events as do Windows Forms controls.
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The anatomy of the Web Form Listing 8.5 presents a new version of our Hello application that uses a Web Form and server controls.
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Listing 8.5 The Hello Web Form
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<!-- helloform.aspx --> <%@ Page Language="C#" Debug="true" %> <script runat="server"> // greet the user... private void greetHandler(object sender, EventArgs e) { firstNameTextBox.Text = firstNameTextBox.Text.Trim(); if (firstNameTextBox.Text == "") { // no name, so no greeting... greetingLabel.Text = ""; } else { // greet the user...
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greetingLabel.Text = "Hello, " + firstNameTextBox.Text + "!"; } } </script> <html><head><title>Hello Web Form</title></head> <body> <h1>Hello Web Form</h1> <form action='helloform.aspx' method='POST' runat='server'> <asp:label text="What's Your First Name " runat="server"/> <asp:textbox id="firstNameTextBox" runat="server"/> <asp:button id="greetButton" text="Greet" OnClick="greetHandler" runat="server"/> <p> <asp:label id="greetingLabel" runat="server"/> </p> </form> </body> </html>
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Note the form declaration, <form ... runat="server"/> that identifies this example as a Web Form. Also, instead of using HTML tags for the labels, text box, and button, we use markup of the form:
<asp:control-type ... runat="server"/>
These are server controls. They expose properties, methods, and events that can be programmed by server-side code. When they are sent to the browser, they are translated into HTML for display. For example, if you select View | Source from your browser menu, you should see the generated HTML shown in listing 8.6.
Listing 8.6 Viewing the HelloForm HTML source
<!-- helloform.aspx --> <html><head><title>Hello Web Form</title></head> <body> <h1>Hello Web Form</h1> <form name="ctrl0" method="POST" action="helloform.aspx" id="ctrl0"> <input type="hidden"
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name="__VIEWSTATE" value="dDwt ... <span>What's Your First Name </span> <input name="firstNameTextBox" type="text" id="firstNameTextBox" /> <input type="submit" name="greetButton" value="Greet" id="greetButton" /> <p> <span id="greetingLabel"></span> </p> </form> </body> </html>
j47Pg==" />
ASP.NET generates plain HTML that is easily digested by down-level browsers. In fact, using ASP.NET server controls is a good way to produce pages that are compatible across the different browsers. If you launch your browser and open the application, you should see the page shown in figure 8.5.
Figure 8.5 Displaying the Hello Web Form
This is identical to the previous examples. Now, enter your name and click the
Greet button to be presented with the page shown in figure 8.6.
This time, we ve done things a little differently. When we display the greeting, we also display the form again above it. Note that the text in the text box ( Joe ) is preserved across the server roundtrip. By default, server controls are sticky, meaning that they preserve their values across HTTP requests. This is achieved by preserving control properties in a hidden form variable called __VIEWSTATE. You can confirm this by viewing the HTML source in the browser. See listing 8.6. The __VIEWSTATE field looks something like:
<input type="hidden" name="__VIEWSTATE" value="dDwt ... j47Pg==" />
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Figure 8.6 Greeting the user
The value of __VIEWSTATE is a base-64 encoded string containing the viewable state of the controls. If you decode the field using the Base64Service presented in chapter 5, you ll find the text property of the text box in there somewhere:
encoded decoded : dDwt ... j47Pg== : ... Hello, Joe! ...
A control s viewstate is the sum of its property values, and it makes it easy to preserve the contents of a text box, the checked state of a check box, the currently selected item in a list box, and so forth, across HTTP requests. Returning to listing 8.5, note the code:
<asp:button id="greetButton" text="Greet" OnClick="greetHandler" runat="server"/>
The OnClick attribute associates a server-side button click handler with the button s click event. Therefore, while the button is clicked in the client, the associated event is fired, and handled, on the server, causing an HTTP roundtrip to occur. The event handler has an identical signature to its Windows Forms equivalent:
private void greetHandler(object sender, EventArgs e) { ... }
Instead of specifying an OnClick attribute, you can, if you prefer, assign an event handler by overriding the OnLoad method and using the delegate approach seen in our Windows Forms examples:
protected override void OnLoad(EventArgs e) { base.OnLoad(e); // use delegate... greetButton.Click += new EventHandler(greetHandler); }
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