Home > Articles

This chapter is from the book

Round Trip and Postback

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

  1. The user requests a Web Form from the Web server.

  2. The Web server responds back with the requested Web Form.

  3. The user enters the data and submits the form to the Web server.

  4. The 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 round trip. A round trip involves making a complete trip over the network to the Web server and getting back the response .

The Web applications use the HTTP protocol to establish communication between the Web browser and the Web server. The HTTP protocol is disconnected in nature, which means that the life cycle of a Web page is just a single round trip. 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 servers and clients are disconnected from each other. The values of page variables and controls are not preserved between 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 the HTTP protocol provides major challenges to the Web developer. It is difficult to implement the following functionality in Web 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 those of desktop applications

ASP.NET provides solutions to these problems built right into its framework. As an ASP.NET developer, you only need to write a small amount of code to implement these features in your applications. 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 preceding 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 (that is, if you've requested the page by navigating to its URL, or by following a link), the value of the IsPostBack property is False.

You can use this 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 in understanding round trip 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 Basic ASP.NET Web application project. Name the project 305C03.

  2. Add a new Web Form to the project. Name the Web Form StepByStep3-1.aspx. Change the pageLayout property of DOCUMENT element to FlowLayout.

  3. Add a DropDownList Web server control (ddlCategories) to the form. Set its AutoPostBack to True and TabIndex to 1. Add a Label control next to the DropDownList control.

  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. Add Label controls for both TextBox controls.

  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. Figure 3.1 shows a design for the completed form.

Figure 3.1Figure 3.1 A design of a form that allows you to post messages to a Web log.

  1. Switch to the code view of the Web Form and add the following code to the Page_Load() method:

    Private Sub Page_Load( _
     ByVal sender As System.Object, _
     ByVal e As System.EventArgs) Handles MyBase.Load
      If Not Page.IsPostBack Then
        ' 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 textbox
        txtTitle.Text = txtTitle.Text.ToUpper()
      End If
      ' Set the text of the label
      ' control on each page load
      lblQuestion.Text = _
       "What do you want to write about " & _
      ddlCategories.SelectedItem.Text + " today?"

    End Sub

  2. Attach the following event handler to the Click event of the Post button:

    Private Sub btnPost_Click( _
     ByVal sender As System.Object, _
     ByVal e As System.EventArgs) Handles btnPost.Click
      ' 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

    End Sub

  3. Set StepByStep3-1.aspx as the start page for the project.

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

Figure 3.2Figure 3.2 The Web Form retains state for both postback as well as non-postback controls.

In Step by Step 3.1, I use the event handler for the Load event to check whether 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 will 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, there is 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 the users of Internet Explorer 5.0 or higher browsers. The following list summarizes the enhancements provided by smart navigation:

  • Persisting element focus between postbacks When a postback occurs, the active control on the Web page loses its focus. Those using the 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 information about the active control is persisted between postbacks to the server.

  • Persisting 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 the user will have to scroll down to find his last data entry position. However, when smart navigation is enabled, the scroll position persists between postbacks to the server.

  • Eliminate 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 the page postback operations where 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 from occurring.

  • 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 to be saved in the browser's history.

WARNING

Intranet Only Because smart navigation is limited to recent Internet Explorer versions, it's generally only useful in intranet scenarios. For Internet applications you can't be sure that any given visitor will see the benefits of smart navigation.

Smart navigation is specified by the SmartNavigation property of the Page class. The default value of this property is false, thereby disabling smart navigation for the Web Form.

You can set the SmartNavigation property to true through the Page directive on a Web page. This setting generates the correct code for setting the SmartNavigation property of the Page class to true, 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 features.

STEP BY STEP 3.2 - Using the Smart Navigation Features of ASP.NET

  1. Make a copy of StepByStep3-1.aspx and save it as StepByStep3-2.aspx. Make sure that you change all references of StepByStep3_1 to StepByStep3_2 in both the ASPX and the VB files.

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

    <%@ Page Language="vb" AutoEventWireup="false"
     Codebehind="StepByStep3-2.aspx.vb"
     Inherits="_305C03.StepByStep3_2"

    SmartNavigation="true"%>

  3. Set StepByStep3-2.aspx as the start page. Build the project and browse to StepByStep3-2.aspx using Internet Explorer 5.0 or higher.

  4. 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 where you are sure about the browsers used by the users, you may 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>

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020