Linear

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="match_parent"
              android:layout_height="match_parent"
              android:orientation="vertical" >
    <TextView android:id="@+id/text"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:text="Hello, I am a TextView" />
    <Button android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Hello, I am a Button" />
</LinearLayout>

where:

• LinearLayout
• TextView
• Android - Button

Data-binding layout

Data-binding layout files are slightly different and start with a root tag of layout followed by a data element and a view root element.

<?xml version="1.0" encoding="utf-8"?>
<layout 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">

    <data>
        <!-- The variable within data describes a property that may be used within this layout. -->
        <variable name="ClassName" type="com.example.ClassName" />
        <variable name="user" type="com.example.User"/>            
    </data>
     <LinearLayout
		 <FloatingActionButton
			android:id="@+id/fab"
			android:layout_width="wrap_content"
			android:layout_height="wrap_content"
			android:clickable="true"
			android:onClick="@{ClassName::onCreateFabClick}" 
			android:tint="@android:color/white"
			app:fabSize="mini"
			app:srcCompat="@drawable/ic_create_black_24dp" />
                <!-- Expressions within the layout are written in the attribute properties using the "@{}" syntax -->
		<TextView android:layout_width="wrap_content"
			android:layout_height="wrap_content"
                        android:visibility="@{user.isAdult ? View.VISIBLE : View.GONE}"
			android:text="@{user.lastName}"/>
    </LinearLayout>
</layout>

Default

All XML layout file are saved in the following directory:

res/layout/

App

App Attributes are custom attributes defined in your app, whether by your code or by libraries you import.

It's a single global namespace for custom attributes - i.e., attributes not defined by the android system.

App attributes that don't exist are silently ignored.

For example:

• android:showAsAction was only added in API11 and will work only on all API above or equal to this level.
• app:showAsAction will work on all API levels

For instance, the appcompat-v7 library uses this app custom attributes mirroring the android: namespace ones to support all prior versions of android.

<FloatingActionButton
   app:srcCompat="@drawable/ic_add_black_24dp" />

Android

The android namespace are attribute defined by the android system.

Tools

the tools namespace that enable design-time features (such as which layout to show in a fragment) or compile-time behaviors (such as which shrinking mode to apply to your XML resources).

See Tools Attributes Reference

• tools:context declares which activity this layout is associated with by default.