Types and Templates are useful for 4 reasons:

• much like scripted_triggers and scripted_effects in normal script, using them makes your GUI script less redundant
• likewise, they also simplify changing your GUI, as modifying a Type or Template automatically translates to where they're used
• they can greatly expedite your GUI update process when vanilla releases a patch
• they're great for compatibility

A Type is a predefined widget that you can plug in.

If you want to display a button in GUI, you might use this:

button = {
   text = "YOUR_LOCK_KEY"
}

If you use the same button 3 times in various places, you'll copy/paste that 3 times in your GUI script.

And when you want to add an onclick effect to your button, you'll need to update it 3 times - but not if you use a Type.

Types can be defined in any .gui file, always outside of windows, in a top block, and you can define several types in the same block.

Be careful about the use of = and brackets there, which is unusual:

Types your_types {
   Type your_button = button {
      text = "YOUR_LOC_KEY"
   }
   Type your_widget = widget {
      <...>
   }
}

Then in your gui file, if you use

your_button = {}

It will do a basic text replacement, and work just like before. But now if you add a onclick parameter to your type, all your buttons will get that behavior, without having to modify each one of them.

You can also add things to a type. If you want a different onclick for each button, you can use:

your_button = {
   onclick = "[<...>]"
}

so this particular button will have that particular behaviour.

This example is quite simple, but to ensure that what you append to a type is inserted right where it needs to be, you can use blockoverrides.

In your type, define a block:

type your_button = button {
   text = "YOUR_LOC_KEY"
   block "button_onclick" {
      onclick = "[<...>]"
   }
}

When invoking the type, this will remove the default onclick behavior from that instance of the button:

your_button = {
   blockoverride "button_onclick" {
   }
}

while this will replace the default behavior with another one, specific to that instance of the button:

your_button = {
   blockoverride "button_onclick" {
      onclick = "[<...>]"
   }
}