How to Add Code and Validation Controls to a Form - How to use page and control events
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The first table in figure 2-16 presents some of the common events for working with web pages. The Init and Load events of a page occur whenever a page is requested from the server. The Init event occurs first, and it’s used by ASP.NET to restore the view state of the page and its controls. Because of that, you don’t usually create an event handler for this event. Instead, you add any initialization code to the event handler for the Load event. You’ll see how this works in the next figure.
In contrast, the PreRender event is raised after all the control events for the page have been processed. It’s the last event to occur before a page is rendered to HTML. In later chapters, you’ll see a couple of cases in which this event is useful.
The second table in this figure lists some of the common events for web server controls. When the user clicks a button, for example, the Click event of that control is raised. Then, the page is posted back to the server, the event handlers for the Init and Load events of the page are executed, if present, followed by the event handler for the Click event of the control that was clicked.
The TextChanged event occurs when the user changes the value in a text box. In most cases, you won’t code an event handler for the TextChanged event. However, you might code an event handler for the CheckedChanged event that occurs when the user clicks a radio button or checks a check box. You might also code an event handler for the SelectedIndexChanged event that occurs when the user selects an item from a list.
If you want the event handler for one of these events to be executed immediately when the event occurs, you can set the AutoPostBack property of the control to True. Then, the event handler will be executed after the Init and Load event handlers for the page. Note that if you don’t set the AutoPostBack property to True, the event is still raised, but the event handler isn’t executed until another user action causes the page to be posted to the server. Then, the event handlers for the Init and Load events of the page are executed, followed by the event handlers for the control events in the order they were raised.
In this figure, you can see the event handler for the Click event of the Clear button on the Future Value form. Note that the name for this event handler is btnClear_Click, which is the ID of the button followed by the name of the event. Remember, though, that the Handles clause actually determines what event or events the procedure responds to. In this procedure, the value in the drop-down list is reset to 50, and the text boxes and label are reset to empty strings.
Incidentally, using the Handles clause is the default method for wiring events to their event handlers. However, you can also wire an event to an event handler by naming the event handler on the event attribute of a control. Although you’ll learn how this works in chapter 6, there’s usually no reason to change from using the Handles clause.
Common ASP.NET page events
Event | Procedure name | Occurs when… |
Init | Page_Init | A page is requested from the server. This event is raised before the view state of the page controls has been restored. |
Load | Page_Load | A page is requested from the server, after all controls have been initialized and view state has been restored. This is the event you typically use to perform initialization operations such as retrieving data and initializing form controls. |
PreRender | Page_PreRender | All the control events for the page have been processed but before the HTML that will be sent back to the browser is generated. |
Common ASP.NET control events
| Event | Occurs when… |
| Click | The user clicks a button, link button, or image button control. |
| TextChanged | The user changes the value in a text box. |
| CheckedChanged | The user selects a radio button in a group of radio buttons or selects or unselects a check box. |
| SelectedIndexChanged | The user selects an item from a list box, a drop-down list, a check box list, or a radio button list. |
Code for the Click event of the btnClear button
Protected Sub btnClear_Click(ByVal sender As Object, _
ByVal e As System.EventArgs) Handles btnClear.Click
ddlMonthlyInvestment.Text = 50
txtInterestRate.Text = ""
txtYears.Text = ""
lblFutureValue.Text = ""
End Sub
Description
All of the events associated with an ASP.NET web page and its server controls are executed on the server. Because of that, the page must be posted back to the server to process any event for which you’ve coded an event handler.
When a page is posted back to the server, the Init and Load events are always raised so any event handlers for those events are run first. Next, the event handlers for any control events that were raised are executed in the ordered they were raised. When these event handlers finish, the PreRender event is raised and any event handler for that event is run.
--------------------------------------------Figure 2-16 How to use page and control events
Please check back tomorrow for the conclusion to this article series.
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 This article is an excerpt from chapter two of Murach's ASP.NET 3.5 Web Programming with VB 2008, written by Anne Boehm (Murach, 2008; ISBN: 1890774472). Check it out today at your favorite bookstore. Buy this book now.
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