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" 

I am not sure what ?android:attr means, especially the question mark.

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.

Answer 2

Instead of write Numerical number in value of android:layout_marginTop , you can use the height specified by actionBarSize like an Example in Android Training .

You can use ?android:attr or ?attr [ in case support library ] If you need to use any android attributes[ attr ] can be found in R.attr

For More Description about ?android:attr Question 1 ,Question 2