What is an Event?
An event is something that happens. Your birthday is an event. So is Christmas.
An event in programming terminology is when something special happens. These
events are so special that they are built in to the programming language. VB.NET
has numerous Events that you can write code for. And we’re going to explore
some of them in this section.
We’ll start with all that mysterious code for the Button’s Click Event.
Exploring the The Click Event
Buttons have the ability to be clicked on. When you click a button, the event
that is fired is the Click Event. If you were to add a new button to a form,
and then double clicked it, you would see the following code stub:
Private Sub Button1_Click(sender As
Object, e As EventArgs) Handles Button1.Click
This is a Private Subroutine. The name of the Sub is Button1_Click. The Event
itself is at the end: Button1.Click. The Handles word means that this Subroutine
can Handle the Click Event of Button1. Without the arguments in the round brackets,
the code is this:
Private Sub Button1_Click( ) Handles Button1.Click
You can have this Button1_Click Sub Handle other things, too. It can Handle
the Click Event of other Buttons, for example. Try this.
- Start a New project
- Give it the name it Events
- When you new Form appears, add two Buttons to it
- Double click Button1 to bring up the code
- At the end of the first line for the Button, add this:
Handles Button1.Click, Button2.Click
Add a message box as the code for the Button. Your code window might then look
like this (we’ve used an underscore to spread the code out over two lines):
Run your programme, and then click both of the buttons in turn.
The same message box appears, regardless of which one you clicked.
The reason it did so was because the Events that the Button1.Click Subroutine
can Handle are at the end: the Events for Button1.Click AND Button2.Click.
You can add as many Events as you want on the End. As long as the Subroutine
can Handle them, the Event will happen. For example, you could create two more
buttons, and then add the Click Event on the end of the first button:
Handles Button1.Click, Button2.Click, Button3.Click, Button4.Click
When you click any of the four button, the code inside of the Button1_Click
Subroutine will fire.
However, if you double clicked button2 to try to bring up its coding window,
you’d find that the cursor is flashing inside of the code for Button1_Click.
Because you’ve attached the Click Event of button2 to the Button1 Subroutine,
you can’t have a separate Click Event just for Button2. This Click Event
is Handled By the Subroutine called Button1_Click.
The arguments for a Button’s click event, the ones from the round brackets,
are these two:
sender As Object, e As EventArgs
Hidden from you, is the default for these two arguments, which is ByVal rather
than ByRef. So it’s really this:
ByVal sender As Object, ByVal e As EventArgs
This sets up two variable: one called sender and one called e.
Instead of sender being an integer or string variable, the type of variable
set up for sender is System.Object. This stores a reference to a control (which
button was clicked, for example).
For the e variable, this is holding an object, too – information about the event.
For a button, this information might be which Mouse Button was clicked or where
the mouse pointer was on the screen.
But because this is the Click Event, there’s not much more information available:
either the button was clicked or it wasn’t.
But you can use other Events available to the button. One of these is the MouseDown
Event. The information for the event would be which button was clicked, where
the mouse pointer was when the mouse button was held down, and something called
Delta (a count of how many notches have been rotated on a mouse wheel).
Let’s explore the MouseDown Event.
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