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:

<item android:id="@+id/camera"
    android:title="Camera"
    android:icon="?attr/menuIconCamera"
    android:showAsAction="ifRoom" />

The ?attr is throwing me for a loop. Can someone please explain what this is doing? How is this related to a drawable? I can't seem to find any good information on Google. Also is there a listing or gallery of attributes we can use for icons instead of just menuIconCamera?

Thanks

Edit: I did some more looking around and found that attrs.xml looks like this:

<resources>
<declare-styleable name="AppTheme">
    <attr name="listDragShadowBackground" format="reference" />
    <attr name="menuIconCamera" format="reference" />
    <attr name="menuIconToggle" format="reference" />
    <attr name="menuIconShare" format="reference" />
</declare-styleable>

Unfortunately that just makes it even more confusing for me. What is this doing?

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.