What is the meaning of android:layout_height="?attr/actionBarSize"

Question

I'm trying to understand the line of code in the title. I understood that ? means that the value will be taken from the Theme. But when it with prefix attrs, I understand that it is defined somewhere in the android system. Also I saw that the selected theme in the app is a child of Theme.AppCompat.NoActionBar that doesn't determine a value for actionBarSize:

<style name="Theme.AppCompat.NoActionBar">
    <item name="windowActionBar">false</item>
    <item name="windowNoTitle">true</item>
</style>



<declare-stylable name="Theme">
<attr name="actionBarSize" format="dimension" >
        <enum name="wrap_content" value="0" />
    </attr>

I also find a theme (with the name Theme) at some GitHub resource that really puts a value to actionBarSize but this Theme doesn't seems to be a parent in any parent chain of the new android api's: Theme.xml

Answer 1

As you've found, the only two attributes that .NoActionBar provides are:

<item name="windowActionBar">false</item>
<item name="windowNoTitle">true</item>

The rest of the attributes are inherited from that style's parent. In the parent style chain for .NoActionBar is Base.V21.Theme.AppCompat.Light, which sets actionBarSize:

<item name="actionBarSize">?android:attr/actionBarSize</item>

In this case, the ?android: prefix means that it comes directly from the Android platform, which sets it to 56dp.

You can see all of this if you look at the quick documentation for your style:

---

All that aside, what android:layout_height="?attr/actionBarSize" means is that the view should be the same height as the action bar. It doesn't really matter that the final value turns out to be 56dp; the point is that, whatever the value actually is, this view will be that tall.