Session 8.2 Postback, ViewState How ASP.NET pages are processed

1. Session 8.2 • Postback, ViewState • How ASP.NET pages are processed

2. Review • Web site projects • ASP.NET pages have .ASPX extension • Each project has its own directory • Examples: • C:/prog11044/projects/mySite • C:/prog11044/projects/assign1 • In Visual Studio 2008, “Web Application” Projects are more complex “Web Sites” • If given the option to open a project, choose “Web Site” Wendi Jollymore, ACES

3. Review • Server controls • <asp:Label ID=“lblName” runat=“server”>Name:</asp:Label> • How ASP.NET controls are rendered in HTML form • E.g. ListBox and ListItem to Select and Option • Common properties of controls Wendi Jollymore, ACES

4. Postback • When a page is first requested, it is retrieved from the server and sent to the browser • Then the user triggers an event • E.g. clicks a button • Another request is made to process this event and return an updated version of the page • This is called POSTBACK Wendi Jollymore, ACES

5. Postback • Buttons always trigger a postback • Some other controls can trigger a postback • They don’t by default • You can change the AutoPostback property to True • E.g. if you set a check box’s AutoPostback to true, it will cause a postback when clicked • You can then add an event handler to it! Wendi Jollymore, ACES

6. AutoPostback Example • Redo Exercise 3 from ASP.NET:Introduction / Exercises • Delete the button control • Modify the program so that the colours change when a radio button is selected Wendi Jollymore, ACES

7. Postback and State • Recall: • When you view source of page in browser • Hidden field called __VIEWSTATE • This retains state information about controls between postbacks • Why? • Because HTTP is a stateless protocol! • Once your page leaves the server, it is forgotten by the server! Wendi Jollymore, ACES

8. Postback and State - Example • Make this: (names in the notes) Wendi Jollymore, ACES

9. Postback and State - Example • cmdGetName_Click() event: if (!txtName.Text.Equals("")) lblDisplayName.Text = "Hello, " + txtName.Text; Wendi Jollymore, ACES

10. Postback and State - Example • cmdGetNum_Click() event: int n = 0; if (int.TryParse(txtNumber.Text, out n)){ if (n == 7) lblDisplay.Text = "You guessed my number!"; else lblDisplay.Text = "You guessed wrong."; } else{ lblDisplay.Text = "I don't understand " + txtNumber.Text; } Wendi Jollymore, ACES

11. Postback and State - Example • What happens when page loads: • Browser requests stuff.aspx from server • Stuff.aspx located • Stuff.aspx is sent to ASPNET_ISAPI.DLL for processing • Any initial scripts/code executed • Special ASP.NET tags rendered in HTML • Text/HTML from a. and b. are put together in single HTML stream • HTML stream sent to browser Wendi Jollymore, ACES

12. Postback and State - Example • Type a “Kaluha” in txtName and click cmdGetName: • Browser sends form data with request for stuff.aspx to server • Stuff.aspx located • Stuff.aspx is sent to ASPNET_ISAPI.DLL for processing • Any initial scripts/code executed • Special ASP.NET tags rendered in HTML • From scratch!! Controls created with defaults! • cmdGetName event handler executed and HTML updated accordingly • HTML stream sent to browser Wendi Jollymore, ACES

13. Postback and State - Example • Now type 3 in the number box and press cmdGetNum button • Browser sends form data with request for stuff.aspx to server • Stuff.aspx located and sent to ASPNET_ISAPI.DLL for processing • Any initial scripts/code executed • Special ASP.NET tags rendered in HTML • From scratch!! Controls created with defaults! • cmdGetNum event handler executed and HTML updated accordingly • HTML stream sent to browser Wendi Jollymore, ACES

14. Postback and State - Example • Wait!! • On second request, lblDisplayName Text property was changed • Default value was “Waiting for your name..” • New value was “Hello, Kaluha” • On third request, stuff.aspx was created from scratch with default values • lblDisplayName.Text back to “Waiting…” • On this visit, we only updated lblDisplayGuess! Wendi Jollymore, ACES

15. Postback and State - Example • Try it – what actually happens? • Both labels have the values from their buttons’ event handlers • How does this happen? • ViewState!! • Before page is finished rendering, ViewState is updated with any state changes to controls Wendi Jollymore, ACES

16. Postback and State - Example • How it works: • On second visit to server: • ViewState updated: lblDisplayName Text contains “Hello, Kaluha” • On second visit to server: • Before cmdGetNum event handler • ViewState is examined and all controls updated accordingly • After cmdGetNum event handler • ViewState updated: lblDisplayGuess Text contains “You guessed wrong.” Wendi Jollymore, ACES

17. Postback and State - Example • Some controls remember their own state information • Text boxes, list boxes, radio buttons • Controls that have a corresponding HTML tag • <input type=“text” value=“Kaluha” /> • <input type=“radio” value=“blue” checked=“true” /> • State of these controls is part of form data (method=“POST”) Wendi Jollymore, ACES

18. Summary: How Pages are Processed • Browser sends request for aspx page to server. • Server retrieves the page and sends page to ASPNET_ISAPI.DLL • ASPX page is constructed and rendered in plain HTML, all controls being given their default values. • Initial scripts/events executed and HTML updated accordingly. • Newly rendered HTML is sent to browser • User triggers postback (e.g. button click) • Browser sends form data and request for aspx page to server • Continued…. Wendi Jollymore, ACES

19. Summary: How Pages are Processed • Server retrieves the page and sends page to ASPNET_ISAPI.DLL • ASPX page is constructed and rendered in plain HTML, all controls being given their default values. • Initial scripts/events executed and HTML updated accordingly. • ViewState field is examined and controls updated • Requested event handler is executed and HTML updated accordingly • ViewState fields is updated to reflect any changes made to relevant controls. • The stream of HTML is sent to the browser and displayed. Wendi Jollymore, ACES

20. More on ViewState • You can also use ViewState for “global” variables • You can’t have normal globals on a web page • Don’t believe me? Try it! • New Web Site: • Number of Postbacks: [lblCount] • Button cmdPost Wendi Jollymore, ACES

21. More on ViewState • cmdPost Event Handler: if (ViewState["numPosts"] == null) ViewState["numPosts"] = 0; int num = int.Parse(ViewState["numPosts"] .ToString()); ViewState["numPosts"] = ++num; lblCount.Text = num.ToString(); } Wendi Jollymore, ACES

22. Testing for Postback • Sometimes you want code to execute ONLY if this is the first time the page loads • Or vice versa • The Page is an object • It has events, methods, properties • IsPostback property • True if the current load is a postback (return trip) • False if this is the first load Wendi Jollymore, ACES

23. Testing for Postback private void Page_Load(object sender, EventArgs e) { if (!IsPostBack) { // first load lblDisplay.Text = "Hello!"); } else { // return trip lblDisplay.Text = "Welcome back!!!"); } } Wendi Jollymore, ACES

24. Homework • meh Wendi Jollymore, ACES