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:
The user requests a Web Form from the Web server.
The Web server responds back with the requested Web Form.
The user enters the data and submits the form to the Web server.
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
Create a new Visual Basic ASP.NET Web application project. Name the project 305C03.
Add a new Web Form to the project. Name the Web Form StepByStep3-1.aspx. Change the pageLayout property of DOCUMENT element to FlowLayout.
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.
Add a Label control (lblQuestion) to the Web Form.
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.
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 3.1 A design of a form that allows you to post messages to a Web log.
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?"
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
Set StepByStep3-1.aspx as the start page for the project.
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 3.2 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.
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
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.
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"
Set StepByStep3-2.aspx as the start page. Build the project and browse to StepByStep3-2.aspx using Internet Explorer 5.0 or higher.
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>