Event-Handling Basics
The process of creating a Web Forms user interface involves placing controls
onto a page and then writing code to react to events that occur as users
interact with those controls. ASP.NET makes it simple for you to work with
pages, controls, and events as if the entire process were taking place on a
client computer.
Interacting with the Server
In actuality, almost all event handling takes place on the Web server, and
it''''s important that you understand what happens when you add event code
that runs in reaction to user events. More important, code that you write
in reaction to control events always runs on the server, except in one
case: When you write script code that handles events on the client, in
JavaScript or VBScript, the code does run on the client. Even the
validation controls (covered in Chapter 8, "Validation Controls") run code
on the server to validate data, even though the controls also provide
client-side script for validating on the client side.
When you place a command button on a page, you can add code to react to
that button''''s Click event. Clicking a command button control always
triggers a postback to the server, where the page''''s Load event occurs.
Then, your button''''s Click event procedure runs, allowing the procedure to
react to the user clicking the button.
Each control supplies its own set of events that it can react to. For
example, CommandButton controls provide a Click event. ListBox and
DropDownList controls supply a SelectedIndexChanged event. RadioButton
controls provide a CheckChanged event. It''''s your job as a developer to
learn which controls supply which events and write code reacting to the
appropriate events.
TIP
It''''s important to remember that none of the code you create in reaction to
events, in your page''''s code-behind file, runs on the client. All this code
runs on the server and requires a roundtrip to the server in order to run.
This is a startling realization for Visual Basic 6.0 developers, who are
used to having all event code run immediately in response to triggering an
event.
Very few controls automatically trigger a postback to the server (in other
words, very few controls'''' code runs automatically when you interact with
the controls). For example, selecting an item in a list box won''''t
automatically trigger a postback to the server and won''''t run the
SelectedIndexChanged event procedure. The next time the page does post
back, the code will run. If you want immediate feedback (more like a Visual
Basic 6.0 form), you can set the AutoPostBack property for most controls to
True. Doing this causes a change to the control''''s value to trigger a
postback to the server.
You''''ll need to set the AutoPostBack property to True for controls in which
you require immediate feedback梡erhaps selecting an item from a list
requires updating a label on the page. Alternatively, you might want to
filter a list of values based on a selection in another list. Several
examples in this chapter make use of the AutoPostBack property.
WARNING
There''''s no "free lunch" here. A roundtrip to the server is, well, a
roundtrip to the server, and that takes time. If you have every control on
your page initiate a roundtrip, by setting the AutoPostBack property to
True for all the controls, you might be sorry due to increased network
traffic and the time it takes for the page to respond to the user. With the
Web server on your local machine, it''''s hard to determine the price you''''ll
pay for postbacks, but even there, roundtrips aren''''t immediate. Consider
carefully when you actually need to post back to the server梱our users will
appreciate it.
How Event Handling Works
In order to demonstrate some of the features of event handling and the VB
.NET language, we''''ve provided a simple project named VBLanguage.sln. You''''ll
want to load this sample project so you can follow along with the
discussion. This project already includes the layout for the pages, but
you''''ll need to add the appropriate event code. The first page we''''ll
discuss, Events.aspx, is shown in Figure 7.1.
Figure 7.1. Clicking a button or changing the selection within a drop-down
list can fire an event procedure back on the server.
We''''ll start by investigating the Click event of the CommandButton control
on this page. Each server control (such as the CommandButton control)
provides a default event procedure, and double-clicking the control in
Design view will load the code editor and create the stub of the event
procedure for you. Double-clicking the Click Me command button, for
example, loads the code editor and writes this code for you:
Private Sub btnClickMe_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles btnClickMe.Click
End Sub
TIP
Here, and throughout this book, we''''ve reformatted event procedures so that
they fit within the requirements of the printed page. We''''ve added line
continuation characters (a space, followed by an underscore, and then a
carriage return/linefeed) to the end of lines that need to be broken to fit
on the printed page. The Visual Studio .NET editor doesn''''t perform these
same line breaks for you, so the code you see on the screen will be
formatted slightly different from the code you see printed here.
You should notice some important things about this procedure:
Visual Studio .NET generates a procedure name for you. In this case, the
procedure is named btnClickMe_Click. Unlike in Visual Basic 6.0, the name
of the procedure is arbitrary梩hat is, the name isn''''t used internally by
the event handling in the page framework. Visual Studio .NET generates a
name based on the name of the control and the name of the event, but that''''s
only for your convenience梩he name could be anything at all. When you
double-click the Click Me button, you''''ll see the following code:
Private Sub btnClickMe_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles btnClickMe.Click
End Sub
The event procedure provides two parameters, which you''''ll learn about in
the next sections. These parameters are highlighted here:
Private Sub btnClickMe_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles btnClickMe.Click
End Sub
The procedure ends with a Handles clause. This clause indicates to the
event handler that the page framework should run this particular procedure
in reaction to the specified event (btnClickMe.Click, in this case). This
object.event name is crucial梚f the object and its event name don''''t match
an actual object and event on the page, your code won''''t compile. We''''ve
highlighted the Handles clause here:
Private Sub btnClickMe_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles btnClickMe.Click
End Sub
The sender Parameter
The first parameter passed to every event procedure is a reference to the
object that raised the event. In most cases, this will be the control
listed in the Handles clause. However, as you''''ll see later in this chapter,
it''''s possible for one event procedure to handle more than one control''''s
events. In that case, the Handles clause will contain a comma-delimited
list of controls, and the sender parameter will indicate which of those
controls raised the event.
The e Parameter
For some events, the page framework will need to pass information to the
event-handling procedure. The page framework passes most controls'''' events
an object of type EventArgs in this parameter. This object has no useful
properties itself, but many controls'''' events use classes that inherit from
this base class. For example, if you place an ImageButton control on a
page, its Click event receives an object of type ImageClickEventArgs in
this parameter. This object has all the standard EventArgs properties, and
in addition, supplies X and Y properties so that the event procedure can
determine where, within the image, the user clicked.
TIP
Many classes inherit from the base EventArgs class. You should always
investigate, for any event procedure you write, the e parameter, to see
whether the page framework is sending your procedure useful information
based on the conditions when the event was raised.
Button Control Events
To test out event handling, you could have a label display some text in
reaction to clicking a button. On the sample page, you might have the Label
control, lblMessage, display "You clicked on a button" when you click
btnClickMe. To make that happen, modify the btnClick_Click procedure so
that it looks like this:
Private Sub btnClickMe_Click(_
ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles btnClickMe.Click
lblMessage.Text = "You clicked on a button"
End Sub
Right-click the Events.aspx page in the Solution Explorer window and select
Build and Browse from the context menu. When you click the Clic[1] [2] 下一页 |
