Event question


T

tshad

I am just getting started with events and had a couple of questions on why
they do what they do.

If you have a textbox and you want to handle an event you can just do:

this.TextBox2.TextChanged += new
EventHandler(TextBox2_TextChanged);
and then have the function.

void TextBox2_TextChanged(object sender, EventArgs e)
{
Response.Write(this.TextBox2.Text);
}

Here the event is created and tied to the control and function.

But if you do it in a user control, the function doesn't seem to be tied to
anything.

For example, in my .ascx page I could have this:
public event EventHandler BubbleTextChanged;
//protected void OnBubbleTextChanged(EventArgs e)
//{
// //if (BubbleTextChanged != null)
// //BubbleTextChanged(this, e);
//}

Where I normally wouldn't comment out the function, so it isn't tied to
anything (or doesn't seem to be). And the function seems to always need the
word "on" in front of it.

If I had 3 events (and no functions) they would show up in intellisense with
"on" in front of them:

public event EventHandler A1;

public event EventHandler B1;

public event EventHandler C1;

public event EventHandler D1;

public event EventHandler E1;

If I only had the above and nothing more, in my aspx page my intellisense
would show:

onA1
onB1
onC1
onD1
onE1

Yet in my first example, the event was TextBox2.TextChanged no
TextBox2.onTextChanged or the event that was tied to that:
TextBox2_TextChanged wasn't onTextBox2_TextChanged.

Why is it different?

Thanks,

Tom
 
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B

bruce barker

events are pretty simple.

an event is just a collection of delegates (functions to call).
if you have an object that wants to raise events, that object defines a
public event (any name you want). when the object wants to raise events it
calls the delegates stored in its event object. the event will be null if no
delegates have been added. by convention, the routine that raises the event
is called OnEventName, and is code like:

public EventHandler MyEvent;
private void OnRaiseMyEvent(EventArgs e)
{
EventHandler handler = MyEvent;
if (handler != null)
handler(this, e);
}

some point in your code when you want to fire the event, it would call
OnRaiseMyEvent.

to subscribe to an event, your code addes a delegate (with the correct
signature) to the event, via the syntax of the language. in c# its one of the
following:

myObject.MyEvent += new EventHandler(myFunction);
myObject.MyEvent += delegate(object o, EventArgs e)
{
// code goes here
}
myObject.MyEvent += (o,e) =>
{
// code goes here
}


-- bruce (sqlwork.com)
 
T

tshad

bruce barker said:
events are pretty simple.

an event is just a collection of delegates (functions to call).
if you have an object that wants to raise events, that object defines a
public event (any name you want). when the object wants to raise events it
calls the delegates stored in its event object. the event will be null if
no
delegates have been added. by convention, the routine that raises the
event
is called OnEventName, and is code like:

public EventHandler MyEvent;
private void OnRaiseMyEvent(EventArgs e)
{
EventHandler handler = MyEvent;
if (handler != null)
handler(this, e);
}

But in your example you use something different. How does MyEvent know it
is tied to OnRaiseMyEvent? The only place I can see that is in the
OnRaiseMyEvent function:

EventHandler handler = MyEvent;

Whe is that executed to connect the two?

In my code:

public event EventHandler A1;
public event EventHandler B1;
public event EventHandler C1;

Even with out setting up the onA1, onB1 and onC1 - the system seemed to use
those names as defaults since in my other page that used the control,
intellisense was using them.

In you example, you use OnRaiseMyEvent and not OnMyEvent. Does it matter?
You said it was a convention.

I am just trying to understand it here. I don't see anywhere where it says
something like: MyEvent uses OnRaiseMyEvent.
some point in your code when you want to fire the event, it would call
OnRaiseMyEvent.

In the case of a user control, I assume it would be on the User control on
the aspx page (or another ascx page).
to subscribe to an event, your code addes a delegate (with the correct
signature) to the event, via the syntax of the language. in c# its one of
the
following:

myObject.MyEvent += new EventHandler(myFunction);
myObject.MyEvent += delegate(object o, EventArgs e)

Do you use both of these? or just one?

Thanks,

Tom
 
B

bruce barker

it doesn't. MyEvent is just a collection of delegates. if exposed as as
public property, intellisence knows how to add code to create the
delegate. it names a method OnEventName and uses:

EventName += new EventHandler(OnEventName);


this is all with subscribing to an event. this is just a naming
convention that vs uses. i generally use a lambda function or create an
anonymous delegate as I think this is often cleaner if the amount of
code in the delegate is small (couple lines).

at some point the code (often a control) must actually raise the event,
so the delegate(s) will be called. this is when OnRaiseEvent is called.

if you write asp.net controls, the control has interfaces you can
implement, that will be called when it time to raise an event (like
OnChange or OnClick).

-- bruce (sqlwork.com)
 
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T

tshad

But that was my question. I am not trying to be difficult here, just trying
to understand what you HAVE to do versus what CAN you do.

Convention, means that you don't have to do something - just that everyone
agrees that this is the way we are going to do it. Required means that you
have to do it this way.

If I have a control to put on my page like so:
**********************************************
public partial class TextBox : System.Web.UI.UserControl
{
public string Text
{
get { return this.TextBox1.Text; }
set { this.TextBox1.Text = value; }
}
public bool AutoPostBack
{
get { return TextBox1.AutoPostBack; }
set { TextBox1.AutoPostBack = value; }
}

protected override void OnInit(EventArgs e)
{
this.TextBox1.TextChanged += new
EventHandler(TextBox1_TextChanged);
base.OnInit(e);
}
void TextBox1_TextChanged(object sender, EventArgs e)
{
OnBubbleTextChanged(e);
}
public event EventHandler BubbleTextChanged;
protected void OnBubbleTextChanged(EventArgs e)
{
if (BubbleTextChanged != null)
BubbleTextChanged(this, e);
}
}
*************************************

And on my .aspx page I have the control as:

<uc1:TextBox ID="TextBox1" runat="server"
OnBubbleTextChanged="TextChanged" AutoPostBack="true" />

And the .aspx.cs page is:
******************************************
namespace UserControlEvents
{
public partial class _Default : System.Web.UI.Page
{
protected void TextChanged(object sender, EventArgs e)
{
Response.Write("In .aspx page: " + this.TextBox1.Text);
}
}
}
*********************************************

This works fine.

But if I change the first set of code to:

*********************************************
void TextBox1_TextChanged(object sender, EventArgs e)
{
OnRaiseBubbleTextChanged(e);
}
public event EventHandler BubbleTextChanged;
protected void OnRaiseBubbleTextChanged(EventArgs e)
{
if (BubbleTextChanged != null)
BubbleTextChanged(this, e);
}
**********************************************

Now there is NO OnBubbleChanged function. But the page STILL works as if it
is. If you look at the text box on the .aspx page, intellisense still shows
OnBubbleChanged and when I put a break on the first line of the
OnRaiseBubbleTextChanged function it stops there ??????

If I change to the TextBox to:

<uc1:TextBox ID="TextBox1" runat="server"
OnRaiseBubbleTextChanged="TextChanged" AutoPostBack="true" />

It does a post back, but nothing happens.

Why is that?

It now goes to a non-existant method. It is forcing the "On" to precede the
event name, which is fine. But I am trying to understand why and why it
goes to the new function name.

Thanks,

Tom
 

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