What is the 'app' Android XML namespace?

Here is an example of the app namespace that I've seen from a res/menu/main.xml file

<menu xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
tools:context=".MainActivity" >
<item android:id="@+id/action_settings"
    android:title="@string/action_settings"
    android:orderInCategory="100"
    app:showAsAction="never" />
</menu>

What purpose does the app namespace serve? Is it a "standard" Android XML namespace? Are the same value options available for the same attribute placed in two different namespaces (e.g. app:showAsAction and android:showAsAction).

From the [docs][1]: android:showAsAction=["ifRoom" | "never" | "withText" | "always" | "collapseActionView"]

i.e., would the line in the above example mean something else if the attribute were instead:

android:showAsAction="never"

It almost looks like it might be some sort of "subclassing" mechanism, but I can't seem to find any real documentation on the app namespace from Google/Android sources.

[1]: http://developer.android.com/guide/topics/resources/menu-resource.html "docs"

The app namespace is not specific to a library, but it is used for all attributes defined in your app, whether by your code or by libraries you import, effectively making a single global namespace for custom attributes - i.e., attributes not defined by the android system.

In this case, the appcompat-v7 library uses custom attributes mirroring the android: namespace ones to support prior versions of android (for example: android:showAsAction was only added in API11, but app:showAsAction (being provided as part of your application) works on all API levels your app does) - obviously using the android:showAsAction wouldn't work on API levels where that attribute is not defined.

You can get some explaination from this link

XML namespace

> Namespace declaration An XML namespace is declared using the reserved XML attribute xmlns or xmlns:prefix, the value of which must be a valid namespace name.

>For example, the following declaration maps the "xhtml:" prefix to the XHTML namespace:

>xmlns:xhtml="http://www.w3.org/1999/xhtml"

>Any element or attribute whose name starts with the prefix "xhtml:" is considered to be in the XHTML namespace, if it or an ancestor has the above namespace declaration.

>It is also possible to declare a default namespace. For example:

>xmlns="http://www.w3.org/1999/xhtml"

>In this case, any element without a namespace prefix is considered to be in the XHTML namespace, if it or an ancestor has the above default namespace declaration.

>If there is no default namespace declaration in scope, the namespace name has no value.[6] In that case, an element without an explicit namespace prefix is considered not to be in any namespace.

>Attributes are never subject to the default namespace. An attribute without an explicit namespace prefix is considered not to be in any namespace.