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Roundtrip and Postback

Implement navigation for the user interface: Manage data during postback events.

Web applications have a distributed execution model. When a user interacts with a Web form, the browser might respond to some of the user actions by executing client-side scripts while some other actions that require server resources must be sent to the Web server for processing. When server-side processing is involved, a typical interactive user session with a Web form consists of the following steps:

  1. User requests a Web form from the Web server.

  2. Web Server responds back with the requested Web form.

  3. User enters the data and submits the form to the Web server.

  4. Web Server processes the form and sends the result back to the user.

Step 3 is also referred to as a page postback, whereas steps 3 and 4 are collectively referred to as a roundtrip. A roundtrip involves making a complete trip over the network to the Web server and getting the response back.

The Web applications use HTTP to establish communication between the Web browser and the Web server. HTTP is disconnected in nature, which means that the life cycle of a Web page is just a single roundtrip. Every time a Web server responds to a page request, it freshly creates the resources required to create the page, sends the page to the requesting client and destroys the page resources from the server. Between two page requests, Web server and the clients are disconnected with each other, and values of page variables and controls are not preserved across the page requests.

This model of execution allows a Web server to support a large number of clients because each client request occupies the server resources only for a short duration. However, the disconnected nature of HTTP provides a major challenge to the Web developers to implement the following functionality in their applications:

  • Maintain values of controls and variables across page postbacks.

  • Distinguish the initial request of a page from the page postback.

  • Provide smart navigation features similar to that of desktop applications.

ASP.NET provides solutions to these problems built right in to its framework. As an ASP.NET developer, you only need to write a minimum amount of code to achieve these functionalities in your application. From my discussion about server controls in Chapter 2, "Controls," you already know that ASP.NET provides a set of server controls that automatically retain their value across page postbacks. You'll learn how ASP.NET actually retains the state for server controls later in this chapter in a section titled "State Management." For now, I'll talk about two properties of the Page class—IsPostBack and SmartNavigation—that provide the other two functionalities from the previous list.

The IsPostBack Property

The IsPostBack property of the Page class returns true when a page is being loaded in response to a client postback. If the page is being requested for the first time, the value of the IsPostBack property is false.

A typical case in which you would like to make use of this distinction is when you do not want the server to execute some costly initialization operations for each page postback. Instead, you would like the initializations to be performed only with the first request to the page.

Step by Step 3.1 helps you understand roundtrip and postback operations and demonstrates the use of the IsPostBack property.

STEP BY STEP

3.1 Using the IsPostBack Property

  1. Create a new Visual C# ASP.NET Web application project. Name the project 315C03.

  2. Add a new Web form to the project. Name the Web form StepByStep3_1.aspx. Change the pageLayout property of the DOCUMENT element to FlowLayout.

  3. Add a DropDownList Web server control (ddlCategories) to the form. Set its AutoPostBack to true and TabIndex to 1 (see Figure 3.1).

  4. Add a Label control (lblQuestion) to the Web form.

  5. Add a TextBox control (txtTitle) and set its AutoPostBack to true and TabIndex to 2. Add another TextBox (txtMessage) and set its TabIndex to 3 and TextMode to MultiLine (see Figure 3.1).

  6. Add a Button control (btnPost) and set its Text to Post a message. Place a Label control (lblWeblog) at the end of the form as shown in Figure 3.1.

  7. Figure3.1Figure 3.1 The design of a form that allows you to post messages to a Weblog.

  8. Switch to the code view of the Web form and add the following code to the Page_Load() event handler:

  9. private void Page_Load(
      object sender, System.EventArgs e)
    {
      if (!Page.IsPostBack)
      {
        // If page is requested for the first time
        ddlCategories.Items.Add("Web development");
        ddlCategories.Items.Add(
          "Programming Languages");
        ddlCategories.Items.Add("Certifications");
      }
      else
      {
        // On postback, change the case of 
        // the textbox's text
        txtTitle.Text = txtTitle.Text.ToUpper();
      }
      // Set the text of the label control 
      // on each page load
      lblQuestion.Text = 
       "What do you want to write about "
       + ddlCategories.SelectedItem.Text + " today?";
    }
  10. Attach the following event handler to the Click event of the Post button:

  11. private void btnPost_Click(
       object sender, System.EventArgs e)
    {
      // Format the data entered by the user and
      // append it to the existing contents of lblWeblog
      lblWeblog.Text = "<b>" + 
           ddlCategories.SelectedItem.Text
           + " :: " + txtTitle.Text + "</b> (" 
           + DateTime.Now.ToString() + ")<hr>" 
           + txtMessage.Text + "<p>" 
           + lblWeblog.Text + "</p>";
    }
  12. Set StepByStep3_1.aspx as the start page in the project.

  13. Run the project. Use the Tab key to navigate between various fields and publish a few entries to the Weblog as shown in Figure 3.2.

Figure3.2Figure 3.2 The Web form retains state for both postback as well as non-postback controls.

In Step by Step 3.1, I am using the event handler for the Load event of the Page class to check if the page is loaded by a postback operation. If that is the case, I skip executing the code for adding items to the drop-down list.

You also note that the navigation between controls is not smooth. When the form returns after a postback, it does not remember the active control. However, you have a solution to this problem with the help of the SmartNavigation property.

The SmartNavigation Property

ASP.NET has a feature called smart navigation that can greatly enhance the user experience of a Web page for users of Internet Explorer 5.0 or higher browsers. The following list summarizes the enhancements provided by smart navigation:

  • Persists element focus between postbacks—When a postback occurs, the active control on the Web page loses its focus. Anyone using a keyboard for navigation will have to press the Tab key several times to return to their original position of data entry. However, when smart navigation is enabled, the information about the active control is persisted between postbacks to the server.

  • Persists scroll position between postbacks—When a postback occurs, the browser loses the record of the scroll position of the page. This can be especially annoying in the case of large data entry forms because after postback, the form will be positioned at the beginning and user has to scroll down to find her last data entry position. However, when smart navigation is enabled, the information about the scroll position persists between postbacks to the server.

  • Eliminates page flash caused by page postback—When users navigate from one page to another, the old page is destroyed and the new one is created on a blank screen. Depending on the user's video card and display setting, this operation can cause a small flash to occur. This flash is especially noticeable during page postback operations in which the visual contents of the page do not change significantly between the two page requests. When smart navigation is enabled, ASP.NET uses a technique called double buffering to eliminate this flash.

  • Prevents each postback from being saved in the browser history—Normally, every postback to an ASP.NET page causes an entry to be created in the browser's history. This defeats the purpose of the browser's back button because instead of going back to the previous page, users are taken to the previous state of the current page. Smart navigation prevents this from happening by saving only the latest state of the current page in the browser's history.

Smart navigation is specified by the SmartNavigation property of the Page class. The default value of this property is false, which disables smart navigation for the Web form.

Smart Navigation

Smart navigation is only supported in Internet Explorer 5.0 or later browsers. Therefore, when you are targeting an application for a generic browser, this feature won't be very helpful.

In most cases, you should not set the SmartNavigation property directly in the code. Instead, you should use the SmartNavigation attribute of the Page directive. Using the SmartNavigation attribute of the Page directive automatically generates the correct code for setting the SmartNavigation property of the Page class, when the Web form is compiled to create a Page derived class.

Step by Step 3.2 enhances the Web form created in Step by Step 3.1 to use the smart navigation feature.

STEP BY STEP

3.2 Using the Smart Navigation Feature of ASP.NET

  1. Make a copy of StepByStep3_1 and save it as StepByStep3_2. Make sure that you change all references of StepByStep3_1 to StepByStep3_2 in both the ASPX as well as the CS file.

  2. Switch to the HTML view of the StepByStep3_2.aspx file and modify the Page directive to the following:

  3. <%@ Page language="c#" 
         Codebehind="StepByStep3_2.aspx.cs" 
         AutoEventWireup="false" 
         Inherits="_315C03.StepByStep3_2" 
         SmartNavigation="true"
    %>
  4. Set StepByStep3_2.aspx as the start page. Build the project and Browse to StepByStep3_2.aspx using Internet Explorer 5.0 or higher.

  5. Use the Tab key to navigate between various fields, publish a few messages, and observe the enhancements because of the smart navigation feature.

In a corporate scenario in which you are sure about the browsers used by the users, you might want to turn on smart navigation for the complete Web application instead of individual files. In that case, you can make the following changes to the web.config file:

<configuration> 
   <system.web> 
     <pages SmartNavigation="true"/> 
   </system.web> 
</configuration> 
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