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

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”
5/6/2019
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
5/6/2019
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
5/6/2019
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!
5/6/2019
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
5/6/2019
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!
5/6/2019
Wendi Jollymore, ACES

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

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

16. 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

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”)
5/6/2019
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….
5/6/2019
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.
5/6/2019
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
5/6/2019
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(); }
5/6/2019
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
5/6/2019
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!!!"); }
5/6/2019
Wendi Jollymore, ACES

24. Homework
meh
5/6/2019
Wendi Jollymore, ACES