Question how do you add a UnityAction<int> to OnPointerDown in a button?

what's the proper form for both?

 

Button.OnPointerDown is not an event. It's how the Button knows that the pointer is down; not something you can subscribe to. So the exact thing you are asking is impossible.

You could write your own component implementing IPointerDownHandler and add that to the same GameObject, and make that do whatever you want.

Or you could subscribe to some actual event that already exists, like Button.onClick. The syntax for that would be

Code (CSharp):

1. button.onClick.AddListener(() => onClickDelegate(index));

You can't add onClickDelegate directly because the argument list is incompatible, so you need to wrap it in another function, which is done here using a lambda expression. Note that this captures the variable "index" in a closure, not its current value, so if you change the value of "index" after doing this you will be changing what gets called when the button is clicked.

 

This is a very important distinction if you're setting up a bunch of buttons in a loop. I've scribbled about it before:

Delegate / Action variable capture / value capture:

https://forum.unity.com/threads/dyn...s-with-parameter-passing.974346/#post-6334800

 

does
Method (index, delegate);
...
Method(int i, UnityAction<int> delegate)
{
button.onClick.AddListener(delegate(i));
}

capture the value or the variable 'index'?

 

Neither, because you didn't use any lambda expressions anywhere in that example. Also, it won't compile.

But if you replaced "delegate(i)" with "() => delegate(i)", then it would capture the variable

i

, which is local to that particular invocation of the function. Which is effectively the same as capturing the value of "index".

 

Do I need to remove the listener if the button gets destroyed at runtime? Is there a way to store this wrap function reference for future RemoveListener?

 

As far as I'm aware, UnityEvents automatically take care of clearing delegate references when destroyed.