Update value of one field based on another - save

Question

I have a template two text fields. A user creates an item based on this template and enters content into the first text field.
On item save, I want to be able to manipulate that value, somehow, and write it to the second field when the item is saved.
I have read about 3 ways of doing this - item:saved, item:saving or item save rules engine. I'm looking for an explanation in the difference of these approaches and if you had to choose which one would you pick?

Answer 1

Item Saved is an event fired after an item has been saved. I wouldn't use that because it will chain events once you want to update again the same item. There is ways to avoid that but I would rather use Item:Saving, which is fired before the item has been saved.
Rules engine, I don't know exactly how it would fit your needs.
A sample of a item saving that changes the item name prior to being saved:
public class ItemNameReplacementEventHandler
{
/// Called when [item saving].
/// </summary>
/// <param name="sender">The sender.</param>
/// <param name="args">The <see cref="System.EventArgs"/> instance containing the event data.</param>
public void OnItemSaving(object sender, EventArgs args){
var item = GetContextItem(args);
if (!ShouldChange(item))
return;
//do the replace
Regex pattern = new Regex(this.ReplaceFromRegexPattern);
string newName = pattern.Replace(item.Name, this.ReplaceToString);
item.Name = newName;
}
private static Item GetContextItem(EventArgs args)
{
return Event.ExtractParameter(args, 0) as Item;
}
}
Just change the value of a field, or in this case the item name, and it is enough.
The configuration is like this:
<events>
<event name="item:saving">
<handler patch:before="handler[#type='Sitecore.Tasks.ItemEventHandler, Sitecore.Kernel']"
type="TYPE, ASSEMBLY" method="OnItemSaving">
</handler>
</event>
</events>