New to android need to understand “?android:attr/actionBarSize”

Question

I was going through Lars Vogel\'s tutorial on using Fragments and I came across the following code:

android:layout_marginTop=\"?android:attr/actionBarSize\"
         


        

Answer 1

android:attr

is an attribute.

In specific,

android:layout_marginTop="?android:attr/actionBarSize"

means: "the size (height) of the action bar".
? is a special character that means it's an Android internal feature.