Difference between android: and app: prefix in Android XML?

Question

What is the difference and more importantly the necessity of having different prefixes in Andriod view XML?

For example,

<android.support.v7.widget.Toolbar     android:id="@+id/actionToolBar"     android:layout_width="match_parent"     android:layout_height="wrap_content"     android:contentInsetEnd="20dp"     app:contentInsetEnd="20dp"     android:elevation="3dp"   /> 

Has contentInsetEnd for both android and app.

Answer 1

android is usually used for attribute coming from Android SDK itself.

app is often used if you are using the support library.

You may also see other namespaces if you are using custom views (of your own or form a library).

Here is some extra information: http://developer.android.com/training/custom-views/create-view.html#customattr

Answer 2

app namespace is used for custom defined attributes, which are usually defined in /values/attrs.xml Here is a sample of such file

<?xml version="1.0" encoding="utf-8"?> <resources>     <declare-styleable name="SimpleTabIndicator">         <attr name="numberOfTabs" format="integer"/>         <attr name="indicatorColor" format="color"/>     </declare-styleable> </resources> 

And a sample usage would be

<com.someapp.demo.SimpleTabIndicator     android:id="@+id/tabIndicator"     android:layout_width="match_parent"     android:layout_height="2dp"     android:background="#26292E"     app:indicatorColor="#FFFDE992"     app:numberOfTabs="5"/> 

Android namespace you use for Android's widgets and UI controls.