Can someone explain the attr?

Question

I am looking at the Honeycomb Gallery sample code (here) and I ran across the following code while trying to add action items in my own app:

Answer 1

The ?attr/menuIconCamera value means that an icon from menuIconCamera attribute of the current theme will be used.

There must be a drawable assigned to the menuIconCamera attribute somewhere in the themes.xml file. If there're two themes with different values of this attribute then actual icon will depend on a theme which is currently used.

The attrs.xml file is used to define custom attributes. Without this definition compiler will treat unknown attributes as erroneous.