PROG11044 - Advanced Web Apps 5/6/2019 Session 8.2 Postback, ViewState How ASP.NET pages are processed Wendi Jollymore, ACES

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” 5/6/2019 Wendi Jollymore, ACES

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 5/6/2019 Wendi Jollymore, ACES

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 5/6/2019 Wendi Jollymore, ACES

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! 5/6/2019 Wendi Jollymore, ACES

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 5/6/2019 Wendi Jollymore, ACES

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! 5/6/2019 Wendi Jollymore, ACES

Postback and State - Example Make this: (names in the notes) 5/6/2019 Wendi Jollymore, ACES

Postback and State - Example cmdGetName_Click() event: if (!txtName.Text.Equals("")) lblDisplayName.Text = "Hello, " + txtName.Text; 5/6/2019 Wendi Jollymore, ACES

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; } 5/6/2019 Wendi Jollymore, ACES

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 5/6/2019 Wendi Jollymore, ACES

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 5/6/2019 Wendi Jollymore, ACES

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 5/6/2019 Wendi Jollymore, ACES

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! 5/6/2019 Wendi Jollymore, ACES

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 5/6/2019 Wendi Jollymore, ACES

Postback and State - Example How it works: On second visit to server: ViewState updated: lblDisplayName Text contains “Hello, Kaluha” Before cmdGetNum event handler ViewState is examined and all controls updated accordingly After cmdGetNum event handler ViewState updated: lblDisplayGuess Text contains “You guessed wrong.” 5/6/2019 Wendi Jollymore, ACES

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”) 5/6/2019 Wendi Jollymore, ACES

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…. 5/6/2019 Wendi Jollymore, ACES

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. 5/6/2019 Wendi Jollymore, ACES

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 5/6/2019 Wendi Jollymore, ACES

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(); } 5/6/2019 Wendi Jollymore, ACES

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 5/6/2019 Wendi Jollymore, ACES

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!!!"); } 5/6/2019 Wendi Jollymore, ACES

Homework meh 5/6/2019 Wendi Jollymore, ACES