How to get the position of a UI element moving in a scroll Rect

Hello, So, I have a scrollable list of buttons and some image on each of them that I want to disable when the button is outside of the viewport (hidden by a mask). I can't figure how to get the buttons position because their RectTransform.position never changes, and their transform.position can change depending on wich resolution you are using. So, how can I get a solid position value linked to the viewport size that change when the button is moved by scroll ? Thanks in advance , Constantin