[C#] Dynamic Event Handling

[zackiv31]

I have an arbitrary list of elements that I'm adding to a tableLayoutPanel (it was the first layout I came across that I could add elements to and it would be scrollable). For each element, I add the name of the element, with a TrackBar and a TextBox. (so three controls per row).

I want to link the TrackBar with the TextBox in a way where when I move the TrackBar, it changes the numeric value in the textBox to the value of the TrackBar (and ViceVersa).

I know how to add the Events for the two (+= new EventHandler(Scrolled)). The problem is all my elements are being added with the same EventName, so no matter which control I manipulate, i want the corresponding controls to change as well... I think this is OK, because I don't know how many of these elements I will have until runtime.

My question is this... inside the Scrolled method, how can I make elementScrollBar1 change elementTextBox1, and 2 -> 2, 3 -> 3, etc? The only things passed in are object sender, EventArgs events. Is what I'm trying to do possible? If so please help.

[Winston]

You need something like this:

		private void AddControls_Click(object sender, EventArgs e)
		{
			TextBox textbox = new TextBox();
			TrackBar trackbar = new TrackBar();
			trackbar.Maximum = 10;
			trackbar.Tag = textbox;

			trackbar.Scroll +=  delegate(object sender2, EventArgs e2)
								{
									TrackBar currentTrackBar = sender2 as TrackBar;
									TextBox currentTextBox = currentTrackBar.Tag as TextBox;
									currentTextBox.Text = currentTrackBar.Value.ToString();
								};
			tableLayoutPanel1.Controls.AddRange(new Control[] { textbox, trackbar });

		}

Add that into a button click event.

Hope this helps.

[zackiv31]

My controls are added dynamically when the program is run, not by a click event... I'm guessing replacing my code with this for those two controls will have the same effect? Looking at this it seems like I should do this for other trackbar/textbox combos I have... this links the two I'm guessing?

Does this also allow someone to edit the textbox and it affect the trackbar?

[zackiv31]

I figured out that it doesn't and modified my code to make it work.. I did something similar to what you showed, using the textBox.Tag = trackBar to put a reference.

Is this common practice? If I want to relate two controls, should I just reference each within the Tag variable? Is that common practice? It seems a little odd to me, even though it works as a hack.

[Brandon Live Veteran]

Why don't you create a custom control that includes the elements you need, and put the logic there? That's probably the most common practice in this case, since it's exactly why custom controls exist...

Just add a new UserControl to your project, name it something useful, and add a TextBox and Trackbar to it with the layout you want. Then set up the event handlers as you normally would inside the control, you can even use the Forms Designer for all of this.

Then in your main app, instead of dynamically generating TextBoxes and Trackbars, you add MyCustomControls that encapsulate the functionality you need. If you need to you can then expose a method or property on your custom control like GetValue() that returns the value (as indicated by both the textbox and trackbar).

[zackiv31]

I didn't know about this custom control stuff, this seems to be much easier to work with... but I have a new problem because of it

So I have a control with a bunch of controls, one of them again linking a trackbar to a textBox... when I manipulate the trackBar, it updates the textBox correctly.. I have all the event stuff inside the Control Class... but the problem is, whenever I change the trackBar I want to call my update() method from the main class (Form1)... How would one do this?

Edited January 4, 2008 by zackiv31

[azcodemonkey]

  zackiv31 said:

I didn't know about this custom control stuff, this seems to be much easier to work with... but I have a new problem because of it

So I have a control with a bunch of controls, one of them again linking a trackbar to a textBox... when I manipulate the trackBar, it updates the textBox correctly.. I have all the event stuff inside the Control Class... but the problem is, whenever I change the trackBar I want to call my update() method from the main class (Form1)... How would one do this?

Ideally, your control would expose an event that would notify any listeners that the trackbar has changed.

[zackiv31]

Is there a way to expose the event without returning the trackBar? My brain isn't working this morning, but the only way I've found was to return the trackBar and add an event in Form1... there must be a better solution...

[azcodemonkey]

You would create a delegate and an event that are public in your user control, and when you handle the trackbar change event in your user control, you fire another event indicating that it changed to the 'outside' world.

using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace cs3
{
	public delegate void ValueChangedHandler(object sender, int newValue);

	public partial class UserControl1 : UserControl
	{
		public UserControl1()
		{
			InitializeComponent();
			textBox1.Text = trackBar1.Value.ToString();
		}

		[Category("Action")]
		public event ValueChangedHandler ValueChanged;

		private void trackBar1_ValueChanged(object sender, EventArgs e)
		{
			textBox1.Text = trackBar1.Value.ToString();
			OnValueChanged(trackBar1.Value);
		}

		protected void OnValueChanged(int newValue)
		{
			if (ValueChanged != null)
				ValueChanged(this, newValue);
		}
	}
}

[zackiv31]

		[Category("Action")]
		public event ValueChangedHandler ValueChanged;

This is new to me.. but seems interesting... what exactly are the [] for? What does it represent or tell he compiler?

[azcodemonkey]

  zackiv31 said:

		[Category("Action")]
		  public event ValueChangedHandler ValueChanged;

This is new to me.. but seems interesting... what exactly are the [] for? What does it represent or tell he compiler?

It's an attribute for the designer. Add the control to your form, and then select the control. In the Properties pane of the designer, you see properties categorized. If you click on the Events button(the lightning bolt), it lists the events that your control exposes. If you didn't put the CategoryAttribute above your event, it would show up at the bottom of the list in the Misc category.

Attributes are one of the cool aspects of .NET that can be leveraged in many interesting ways. You could effectively describe your class with attributes and discover the metadata at runtime, and perform actions based on it. I've built a framework that utilized attributes to describe how properties should be validated. It was only a proof of concept, but it worked quite well. I'm now seeing that same idea show up in other frameworks, such as the Castle Project's MVC framework.

Edited January 4, 2008 by azcodemonkey

[zackiv31]

	public delegate void ValueChangedHandler(object sender);

declaring this in my Class... ok, got that... but how do I declare the definition in my main Form1? I tried it like this:

public void ValueChangedHandler(object sender) { } ... but it never gets fired... :-/

[Winston]

That's because you only declared the method, but you haven't subscribed the method to the event.

What you need to do is, for each instance of your custom control, you do this:

MyCustomControl.ValueChanged += ValueChangedHandler;

[zackiv31]

here's my userControl:

	public delegate void ValueChangedHandler(object sender);

	public partial class ListBag : UserControl
	{
		[Category("Action")]
		public event ValueChangedHandler ValueChanged;

here's the declaration in Form1:

ListBag lb = new ListBag();
					lb.ValueChanged += new ValueChangedHandler;

It doesn't recognize this, it tries to make it " += ValueChangedHandler(someNameHere)"...

[Brandon Live Veteran]

You're missing the parameter that initializes the delegate. I think you want something like:

lb.ValueChanged += new ValueChangedHandler(this.OnValueChanged);

[zackiv31]

wo wo wo... OnValueChanged is a part of my custom UserControl... therefore Form1 has no idea abou the inner workings of it...

someone confirm this? (and help still)

[Brandon Live Veteran]

But you said you wanted your Form1 to know when something changed inside the user control. That's why you need to have a method inside Form1 that gets called when the event is fired. It can be named anything you want (like OnListBagValueChanged or something).

[zackiv31]

oh ok... gotcha... u just confused me because you used the same name as the protected function insided my userControl...

I think all is well now... this Event stuff is pretty kool.

[azcodemonkey]

  zackiv31 said:

wo wo wo... OnValueChanged is a part of my custom UserControl... therefore Form1 has no idea abou the inner workings of it...

someone confirm this? (and help still)

Your form needs to have a method that matches the signature of the delegate of your control.

protected void UserCtrlValueChanged(object sender)
{
	...
}

Then you can use the += syntax to add a new handler in your form's InitializeComponent method.

userCtrlInstance.ValueChanged += new ValueChangedHandler(UserCtrlValueChanged);

You should be able to do this completely through the designer. Add the control to your form, select the control, go to the Properties panel in the IDE and select the Event(lightning bolt), and double-click the ValueChanged event in the UI. This will do all of the above correctly for you.

Edit: Brandon got it. :)

[Brandon Live Veteran]

Not that it matters much, but if that's what you delegate looks like - you don't actually need a custom one. Just use the built-in EventHandler delegate. Other than that little nit, it looks like you've got things figured out.

[zackiv31]

Could you explain that a little more Brandon? I'm always interested in the more clean/efficient way of programming these events

[Brandon Live Veteran]

Well it depends. Are you using:

public delegate void ValueChangedHandler(object sender);

or

public delegate void ValueChangedHandler(object sender, int newValue);

If the latter, then just keep it as is. If the former, then you don't need to declare that delegate at all.

protected void UserCtrlValueChanged(object sender)
{
...
}

in your main form, becomes

protected void UserCtrlValueChanged(object sender, EventArgs e)
{
...
}

And everywhere you used "ValueChangedHandler" before, you could now use just the built-in EventHandler delegate.

For example:

userCtrlInstance.ValueChanged += new ValueChangedHandler(UserCtrlValueChanged);

becomes

userCtrlInstance.ValueChanged += new EventHandler(UserCtrlValueChanged);

But if you are using a custom delegate to pass an integer with the new value, then you'll want to keep doing it the way you are. And when the event is raised, you just pass EventArgs.Empty for the second parameter.