Reference another component's property in formula

I have a basic list with selection on right control, and then I have a button.
I want to disable the button if none of the items are selected, so I thought I could modify the Disabled property of the button with a formula that checks if the count of selected items is greater than 0. But I do not know how to reference another component in a formula. I only have self for referencing the current component, the button.

What can I do?

You should create a Page variable that adds a counter every time a checkbox is clicked = true, then check the value of that Page variable > 0 in your formula

I did figure that out since there is an event when selected items change but I know the component keeps track of the selected items – I can see this as an option in the binding options elsewhere – and I should more simply handle this. But thanks for your response.