I need to add a button to the vanilla GUI class GuiCreateWorld.

In order to add this button, I need to move the Survival/Hardcore/Creative button out of the way.

 

How would I go about doing this (preferably without ASM)?

You could try using GuiOpenEvent InitGuiEvent.Post to add new button to list, you could also probably access alredy created buttons and find the one you want to move and re-add it.

 

Note: Your mod shouldn't edit whole GUI, rather make button-link to new window.

 

EDIT: use GuiScreenEvent, it has access to initGui() (InitGuiEvent.Post), while open event is totally pre-init.

How would I go about giving functionality to that button?

 

 

Well, I have NO idea on how to do it properly, but I know how I did that in past.

 

Have look here:

http://www.minecraftforge.net/forum/index.php/topic,28979.0.html

 

You can pretty much do the same/similar thing. (utilize MouseEvent).

Remember about scaling.

 

Note: There might be a better way.

You can use ActionPerformedEvent for click. (Maybe ActionPerformedEvent.Post would do what you want)

You can check if it is your button by comparing the button.id