path: root/docs/html/tools/data-binding/guide.jd

page.title=Data Binding Guide
page.tags="databinding", "layouts"
@jd:body

<div class="wrap">
<p class="caution" style="background-color:#fffdeb;">
The <strong>Android Data Binding Library</strong> is available as a <strong>developer
preview</strong>. Expression syntax and behaviors may change prior to the full release
of the library, currently planned for Q3 2015. If you have feedback or want to report
issues, please use the <a href="https://code.google.com/p/android-developer-preview/">issue
tracker</a>. Stay tuned for more information about Data Binding and examples of how to use it. </a>
</p>
</div>
<div id="qv-wrapper">
  <div id="qv">
    <h2>
      In this document:
    </h2>

    <ol>
      <li>
        <a href="#build_environment">Build Environment</a>
      </li>

      <li>
        <a href="#data_binding_layout_files">Data Binding Layout Files</a>
        <ol>
          <li>
            <a href="#writing_expressions">Writing your first data binding
            expressions</a>
          </li>

          <li>
            <a href="#data_object">Data Object</a>
          </li>

          <li>
            <a href="#binding_data">Binding Data</a>
          </li>
        </ol>
      </li>

      <li>
        <a href="#layout_details">Layout Details</a>
        <ol>
          <li>
            <a href="#imports">Imports</a>
          </li>

          <li>
            <a href="#variables">Variables</a>
          </li>

          <li>
            <a href="#custom_binding_class_names">Custom Binding Class Names</a>
          </li>

          <li>
            <a href="#includes">Includes</a>
          </li>

          <li>
            <a href="#expression_language">Expression Language</a>
          </li>
        </ol>
      </li>

      <li>
        <a href="#data_objects">Data Objects</a>
        <ol>
          <li>
            <a href="#observable_objects">Observable Objects</a>
          </li>

          <li>
            <a href="#observablefields">ObservableFields</a>
          </li>

          <li>
            <a href="#observable_collections">Observable Collections</a>
          </li>
        </ol>
      </li>

      <li>
        <a href="#generated_binding">Generated Binding</a>
        <ol>
          <li>
            <a href="#creating">Creating</a>
          </li>

          <li>
            <a href="#views_with_ids">Views With IDs</a>
          </li>

          <li>
            <a href="#variables">Variables</a>
          </li>

          <li>
            <a href="#viewstubs">ViewStubs</a>
          </li>

          <li>
            <a href="#advanced_binding">Advanced Binding</a>
          </li>
        </ol>
      </li>

      <li>
        <a href="#attribute_setters">Attribute Setters</a>
        <ol>
          <li>
            <a href="#automatic_setters">Automatic Setters</a>
          </li>

          <li>
            <a href="#renamed_setters">Renamed Setters</a>
          </li>

          <li>
            <a href="#custom_setters">Custom Setters</a>
          </li>
        </ol>
      </li>

      <li>
        <a href="#converters">Converters</a>
        <ol>
          <li>
            <a href="#object_conversions">Object Conversions</a>
          </li>

          <li>
            <a href="#custom_conversions">Custom Conversions</a>
          </li>
        </ol>
      </li>
    </ol>
  </div><!-- qv -->
</div><!-- qv-wrapper -->


<p>
  This document explains how to use the Data Binding Library to write
  declarative layouts and minimize the glue code necessary to bind your
  application logic and layouts.
</p>

<h2 id="build_environment">
  Build Environment
</h2>

<p>
  <strong>Setting Up Work Environment:</strong>
</p>

<p>
  To set up your application to use data binding, add data binding to the class
  path of your build gradle file, right below "android".
</p>

<pre>
   dependencies {
       classpath <strong>"com.android.tools.build:gradle:1.2.3"
       </strong>classpath <strong>"com.android.databinding:dataBinder:</strong>1.0-rc0"
   }
}
</pre>
<p>
  Then make sure jcenter is in the repositories list for your sub projects.
</p>

<pre>
allprojects {
   repositories {
       jcenter()
   }
}
</pre>
<p>
  In each module you want to use data binding, apply the plugin right after
  android plugin
</p>

<pre>
apply plugin: ‘com.android.application&apos;
apply plugin: &apos;<strong>com.android.databinding</strong>&apos;
</pre>
<p>
  The data binding plugin is going to add necessary <strong>provided</strong>
  and <strong>compile configuration</strong> dependencies to your project.
</p>

<h2 id="data_binding_layout_files">
  Data Binding Layout Files
</h2>

<h3 id="writing_expressions">
  Writing your first data binding expressions
</h3>

<p>
  Data-binding layout files are slightly different and start with a root tag of
  <strong>layout</strong> followed by a <strong>data</strong> element and a
  <strong>view</strong> root element. This view element is what your root would
  be in a non-binding layout file. A sample file looks like this:
</p>

<pre>
<em>&lt;?</em><strong>xml version="1.0" encoding="utf-8"</strong><em>?&gt;
</em>&lt;<strong>layout xmlns:android="http://schemas.android.com/apk/res/android"</strong>&gt;
   &lt;<strong>data</strong>&gt;
       &lt;<strong>variable name="user" type="com.example.User"</strong>/&gt;
   &lt;/<strong>data</strong>&gt;
   &lt;<strong>LinearLayout
       android:orientation="vertical"
       android:layout_width="match_parent"
       android:layout_height="match_parent"</strong>&gt;
       &lt;<strong>TextView android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:text="&commat;{user.firstName}"</strong>/&gt;
       &lt;<strong>TextView android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:text="&commat;{user.lastName}"</strong>/&gt;
   &lt;/<strong>LinearLayout</strong>&gt;
&lt;/<strong>layout</strong>&gt;
</pre>
<p>
  The user <strong>variable</strong> within <strong>data</strong> describes a
  property that may be used within this layout.
</p>

<pre>
&lt;<strong>variable name="user" type="com.example.User"</strong>/&gt;
</pre>
<p>
  Expressions within the layout are written in the attribute properties using
  the “<code>&commat;{}</code>” syntax. Here, the TextView’s text is set to the
  firstName property of user:
</p>

<pre>
&lt;<strong>TextView android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="&commat;{user.firstName}"</strong>/&gt;
</pre>
<h3 id="data_object">
  Data Object
</h3>

<p>
  Let’s assume for now that you have a plain-old Java object (POJO) for User:
</p>

<pre>
<strong>public class </strong>User {
   <strong>public final </strong>String <strong>firstName</strong>;
   <strong>public final </strong>String <strong>lastName</strong>;
   <strong>public </strong>User(String firstName, String lastName) {
       <strong>this</strong>.<strong>firstName </strong>= firstName;
       <strong>this</strong>.<strong>lastName </strong>= lastName;
   }
}
</pre>
<p>
  This type of object has data that never changes. It is common in applications
  to have data that is read once and never changes thereafter. It is also
  possible to use a JavaBeans objects:
</p>

<pre>
<strong>public class </strong>User {
   <strong>private final </strong>String <strong>firstName</strong>;
   <strong>private final </strong>String <strong>lastName</strong>;
   <strong>public </strong>User(String firstName, String lastName) {
       <strong>this</strong>.<strong>firstName </strong>= firstName;
       <strong>this</strong>.<strong>lastName </strong>= lastName;
   }
   <strong>public </strong>String getFirstName() {
       <strong>return this</strong>.<strong>firstName</strong>;
   }
   <strong>public </strong>String getLastName() {
       <strong>return this</strong>.<strong>lastName</strong>;
   }
}
</pre>
<p>
  From the perspective of data binding, these two classes are equivalent. The
  expression <strong><code>&commat;{user.firstName}</code></strong> used for
  the TextView’s <strong><code>android:text</code></strong> attribute will
  access the <strong><code>firstName</code></strong> field in the former class
  and the <code>getFirstName()</code> method in the latter class.
</p>

<h3 id="binding_data">
  Binding Data
</h3>

<p>
  By default, a Binding class will be generated based on the name of the layout
  file, converting it to Pascal case and suffixing “Binding” to it. The above
  layout file was <code>activity_main.xml</code> so the generate class was
  <code>ActivityMainBinding</code>. This class holds all the bindings from the
  layout properties (e.g. the <code>user</code> variable) to the layout’s Views
  and knows how to assign values for the binding expressions.The easiest means
  for creating the bindings is to do it while inflating:
</p>

<pre>
&commat;Override
<strong>protected void </strong>onCreate(Bundle savedInstanceState) {
   <strong>super</strong>.onCreate(savedInstanceState);
   ActivityMainBinding binding = DataBindingUtil.<em>setContentView</em>(<strong>this</strong>, R.layout.<em><strong>main_activity</strong></em>);
   User user = <strong>new </strong>User(<strong>"Test"</strong>, <strong>"User"</strong>);
   binding.setUser(user);
}
</pre>
<p>
  You’re done! Run the application and you’ll see Test User in the UI.
  Alternatively, you can get the view via:
</p>

<pre>
MainActivityBinding binding = MainActivityBinding.<em>inflate</em>(getLayoutInflater());
</pre>
<p>
  If you are using data binding items inside a ListView or RecyclerView
  adapter, you may prefer to use:
</p>

<pre>
ListItemBinding binding = ListItemBinding.inflate(layoutInflater, viewGroup,
false);
//or
ListItemBinding binding = DataBindingUtil.<em>inflate</em>(layoutInflater, R.layout.<em><strong>list_item</strong></em>, viewGroup, <strong>false</strong>);
</pre>
<h2 id="layout_details">
  Layout Details
</h2>

<h3 id="imports">
  Imports
</h3>

<p>
  Zero or more <strong><code>import</code></strong> elements may be used inside
  the <strong><code>data</code></strong> element. These allow easy reference to
  classes inside your layout file, just like in Java.
</p>

<pre>
&lt;<strong>data</strong>&gt;
    &lt;<strong>import type="android.view.View"</strong>/&gt;
&lt;/<strong>data</strong>&gt;
</pre>
<p>
  Now, View may be used within your binding expression:
</p>

<pre>
&lt;<strong>TextView
   android:text="&commat;{user.lastName}"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"
   android:visibility="&commat;{user.isAdult ? View.VISIBLE : View.GONE}"</strong>/&gt;
</pre>
<p>
  When there are class name conflicts, one of the classes may be renamed to an
  “alias:”
</p>

<pre>
&lt;<strong>import type="android.view.View"</strong>/&gt;
&lt;<strong>import type="com.example.real.estate.View"
        alias="Vista"</strong>/&gt;
</pre>
<p>
  Now, <strong><code>Vista</code></strong> may be used to reference the
  <code>com.example.real.estate.View</code> and
  <strong><code>View</code></strong> may be used to reference
  <code>android.view.View</code> within the layout file. Imported types may be
  used as type references in variables and expressions:
</p>

<pre>
&lt;<strong>data</strong>&gt;
    &lt;<strong>import type="com.example.User"</strong>/&gt;
    &lt;<strong>import type="java.util.List"</strong>/&gt;
    &lt;<strong>variable name="user" type="User"</strong>/&gt;
    &lt;<strong>variable name="userList" type="List&amp;lt;User&gt;"</strong>/&gt;
    &lt;/<strong>data</strong>&gt;
</pre>
<p class="note">
  <strong>Note</strong>: Android Studio does not yet handle imports so the
  autocomplete for imported variables may not work in your IDE. Your
  application will still compile fine and you can work around the IDE issue by
  using fully qualified names in your variable definitions.
</p>

<pre>
&lt;<strong>TextView
   android:text="&commat;{((User)(user.connection)).lastName}"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"</strong>/&gt;
</pre>
<p>
  Imported types may also be used when referencing static fields and methods in
  expressions:
</p>

<pre>
&lt;<strong>data</strong>&gt;
    &lt;<strong>import type="com.example.MyStringUtils"</strong>/&gt;
    &lt;<strong>variable name="user" type="com.example.User"</strong>/&gt;
&lt;/<strong>data</strong>&gt;
…
&lt;<strong>TextView
   android:text="&commat;{MyStringUtils.capitalize(user.lastName)}"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"</strong>/&gt;
</pre>
<p>
  Just as in Java, <code>java.lang.*</code> is imported automatically.
</p>

<h3 id="variables">
  Variables
</h3>

<p>
  Any number of <strong><code>variable</code></strong> elements may be used
  inside the <strong><code>data</code></strong> element. Each
  <strong><code>variable</code></strong> element describes a property that may
  be set on the layout to be used in binding expressions within the layout
  file.
</p>

<pre>
&lt;<strong>data</strong>&gt;
    &lt;<strong>import type="android.graphics.drawable.Drawable"</strong>/&gt;
    &lt;<strong>variable name="user"  type="com.example.User"</strong>/&gt;
    &lt;<strong>variable name="image" type="Drawable"</strong>/&gt;
    &lt;<strong>variable name="note"  type="String"</strong>/&gt;
&lt;/<strong>data</strong>&gt;
</pre>
<p>
  The variable types are inspected at compile time, so if a variable implements
  <a href="#observable_objects">Observable</a> or is an <a href=
  "#observable_collections">observable collection</a>, that should be reflected
  in the type. If the variable is a base class or interface that does not
  implement the Observable* interface, the variables will <strong>not
  be</strong> observed!
</p>

<p>
  When there are different layout files for various configurations (e.g.
  landscape or portrait), the variables will be combined. There must not be
  conflicting variable definitions between these layout files.
</p>

<p>
  The generated binding class will have a setter and getter for each of the
  described variables. The variables will take the default Java values until
  the setter is called &mdash; <code>null</code> for reference types,
  <code>0</code> for <code>int</code>, <code>false</code> for
  <code>boolean</code>, etc.
</p>

<h3 id="custom_binding_class_names">
  Custom Binding Class Names
</h3>

<p>
  By default, a Binding class is generated based on the name of the layout
  file, starting it with upper-case, removing underscores ( _ ) and
  capitalizing the following letter and then suffixing “Binding”. This class
  will be placed in a databinding package under the module package. For
  example, the layout file <code>contact_item.xml</code> will generate
  <code>ContactItemBinding</code>. If the module package is
  <code>com.example.my.app</code>, then it will be placed in
  <code>com.example.my.app.databinding</code>.
</p>

<p>
  Binding classes may be renamed or placed in different packages by adjusting
  the <strong><code>class</code></strong> attribute of the
  <strong><code>data</code></strong> element. For example:
</p>

<pre>
&lt;<strong>data class="ContactItem"</strong>&gt;
    ...
&lt;/<strong>data</strong>&gt;
</pre>
<p>
  This generates the binding class as <code>ContactItem</code> in the
  databinding package in the module package. If the class should be generated
  in a different package within the module package, it may be prefixed with
  “.”:
</p>

<pre>
&lt;<strong>data class=".ContactItem"</strong>&gt;
    ...
&lt;/<strong>data</strong>&gt;
</pre>
<p>
  In this case, <code>ContactItem</code> is generated in the module package
  directly. Any package may be used if the full package is provided:
</p>

<pre>
&lt;<strong>data class="com.example.ContactItem"</strong>&gt;
    ...
&lt;/<strong>data</strong>&gt;
</pre>
<h3 id="includes">
  Includes
</h3>

<p>
  Variables may be passed into an included layout&apos;s binding from the
  containing layout by using the application namespace and the variable name in
  an attribute:
</p>

<pre>
<em>&lt;?</em><strong>xml version="1.0" encoding="utf-8"</strong><em>?&gt;
</em>&lt;<strong>layout xmlns:android="http://schemas.android.com/apk/res/android"
</strong>       <strong> xmlns:bind="http://schemas.android.com/apk/res-auto"</strong>&gt;
   &lt;<strong>data</strong>&gt;
       &lt;<strong>variable name="user" type="com.example.User"</strong>/&gt;
   &lt;/<strong>data</strong>&gt;
   &lt;<strong>LinearLayout
       android:orientation="vertical"
       android:layout_width="match_parent"
       android:layout_height="match_parent"</strong>&gt;
       &lt;<strong>include layout="&commat;layout/name"
           bind:user="&commat;{user}"</strong>/&gt;
       &lt;<strong>include layout="&commat;layout/contact"
           bind:user="&commat;{user}"</strong>/&gt;
   &lt;/<strong>LinearLayout</strong>&gt;
&lt;/<strong>layout</strong>&gt;
</pre>
<p>
  Here, there must be a <code>user</code> variable in both the
  <code>name.xml</code> and <code>contact.xml</code> layout files.
</p>

<h3 id="expression_language">
  Expression Language
</h3>

<h4 id="common_features">
  Common Features
</h4>

<p>
  The expression language looks a lot like a Java expression. These are the
  same:
</p>

<ul>
  <li>Mathematical <strong><code>+ - / * %</code></strong>
  </li>

  <li>String concatenation <strong><code>+</code></strong>
  </li>

  <li>
    <code>L</code>ogical <strong><code>&& ||</code></strong>
  </li>

  <li>Binary <strong><code>&</code> <code>|</code> <code>^</code></strong>
  </li>

  <li>Unary <strong><code>+ - ! ~</code></strong>
  </li>

  <li>Shift <strong><code>&gt;&gt; &gt;&gt;&gt; &lt;&lt;</code></strong>
  </li>

  <li>Comparison <strong><code>== &gt; &lt; &gt;= &lt;=</code></strong>
  </li>

  <li>
    <strong><code>instanceof</code></strong>
  </li>

  <li>Grouping <strong><code>()</code></strong>
  </li>

  <li>Literals - character, String, numeric, <strong><code>null</code></strong>
  </li>

  <li>Cast
  </li>

  <li>Method calls
  </li>

  <li>Field access
  </li>

  <li>Array access <strong><code>[]</code></strong>
  </li>

  <li>Ternary operator <strong><code>?:</code></strong>
  </li>
</ul>

<p>
  Examples:
</p>

<pre>
<strong>android:text="&commat;{String.valueOf(index + 1)}"
android:visibility="&commat;{age &amp;lt; 13 ? View.GONE : View.VISIBLE}"
android:transitionName=&apos;&commat;{"image_" + id}&apos;</strong>
</pre>
<h4 id="missing_operations">
  Missing Operations
</h4>

<p>
  A few operations are missing from the expression syntax that you can use in
  Java.
</p>

<ul>
  <li>
    <strong><code>this</code></strong>
  </li>

  <li>
    <strong><code>super</code></strong>
  </li>

  <li>
    <strong><code>new</code></strong>
  </li>

  <li>Explicit generic invocation
  </li>
</ul>

<h4 id="null_coalescing_operator">
  Null Coalescing Operator
</h4>

<p>
  The null coalescing operator (<strong><code>??</code></strong>) chooses the
  left operand if it is not null or the right if it is null.
</p>

<pre>
<strong>android:text="&commat;{user.displayName ?? user.lastName}"</strong>
</pre>
<p>
  This is functionally equivalent to:
</p>

<pre>
<strong>android:text="&commat;{user.displayName != null ? user.displayName : user.lastName}"</strong>
</pre>
<h4 id="property_reference">
  Property Reference
</h4>

<p>
  The first was already discussed in the <a href=
  "#writing_your_first_data_binding_expressions">Writing your first data
  binding expressions</a> above: short form JavaBean references. When an
  expression references a property on a class, it uses the same format for
  fields, getters, and ObservableFields.
</p>

<pre>
<strong>android:text="&commat;{user.lastName}"</strong>
</pre>
<h4>
  Avoiding NullPointerException
</h4>

<p>
  Generated data binding code automatically checks for nulls and avoid null
  pointer exceptions. For example, in the expression
  <code>&commat;{user.name}</code>, if <code>user</code> is null,
  <code>user.name</code> will be assigned its default value (null). If you were
  referencing <code>user.age</code>, where age is an <code>int</code>, then it
  would default to 0.
</p>

<h4 id="collections">
  Collections
</h4>

<p>
  Common collections: arrays, lists, sparse lists, and maps, may be accessed
  using the <code>[]</code> operator for convenience.
</p>

<pre>
&lt;<strong>data</strong>&gt;
    &lt;<strong>import type="android.util.SparseArray"</strong>/&gt;
    &lt;<strong>import type="java.util.Map"</strong>/&gt;
    &lt;<strong>import type="java.util.List"</strong>/&gt;
    &lt;<strong>variable name="list" type="List&lt;String&gt;"</strong>/&gt;
    &lt;<strong>variable name="sparse" type="SparseArray&amp;lt;String&gt;"</strong>/&gt;
    &lt;<strong>variable name="map" type="Map&amp;lt;String, String&gt;"</strong>/&gt;
    &lt;<strong>variable name="index" type="int"</strong>/&gt;
    &lt;<strong>variable name="key" type="String"</strong>/&gt;
&lt;/<strong>data</strong>&gt;
…
<strong>android:text="&commat;{list[index]}"
</strong>…
<strong>android:text="&commat;{sparse[index]}"
</strong>…
<strong>android:text="&commat;{map[key]}"
</strong>
</pre>
<h4 id="string_literals">
  String Literals
</h4>

<p>
  When using single quotes around the attribute value, it is easy to use double
  quotes in the expression:
</p>

<pre>
<strong>android:text=&apos;&commat;{map["firstName"]}&apos;</strong>
</pre>
<p>
  It is also possible to use double quotes to surround the attribute value.
  When doing so, String literals should either use the &amp;quot; or back quote
  (`).
</p>

<pre>
<strong>android:text="&commat;{map[`firstName`}"
android:text="&commat;{map[&amp;quot;firstName&amp;quot;]}"</strong>
</pre>
<h4 id="resources">
  Resources
</h4>

<p>
  It is possible to access resources as part of expressions using the normal
  syntax:
</p>

<pre>
<strong>android:padding="&commat;{large? &commat;dimen/largePadding : &commat;dimen/smallPadding}"</strong>
</pre>
<p>
  Format strings and plurals may be evaluated by providing parameters:
</p>

<pre>
<strong>android:text="&commat;{&commat;string/nameFormat(firstName, lastName)}"
android:text="&commat;{&commat;plurals/banana(bananaCount)}"</strong>
</pre>
<p>
  When a plural takes multiple parameters, all parameters should be passed:
</p>

<pre>

  Have an orange
  Have %d oranges

android:text="<strong>&commat{&commatplurals/orange(orangeCount, orangeCount)}</strong>"
</pre>
<p>
  Some resources require explicit type evaluation.
</p>

<table>
  <tr>
    <th>
      Type
    </th>
    <th>
      Normal Reference
    </th>
    <th>
      Expression Reference
    </th>
  </tr>

  <tr>
    <td>
      <pre>
String[]
</pre>
    </td>
    <td>
      &commat;array
    </td>
    <td>
      &commat;stringArray
    </td>
  </tr>

  <tr>
    <td>
      int[]
    </td>
    <td>
      &commat;array
    </td>
    <td>
      &commat;intArray
    </td>
  </tr>

  <tr>
    <td>
      TypedArray
    </td>
    <td>
      &commat;array
    </td>
    <td>
      &commat;typedArray
    </td>
  </tr>

  <tr>
    <td>
      Animator
    </td>
    <td>
      &commat;animator
    </td>
    <td>
      &commat;animator
    </td>
  </tr>

  <tr>
    <td>
      StateListAnimator
    </td>
    <td>
      &commat;animator
    </td>
    <td>
      &commat;stateListAnimator
    </td>
  </tr>

  <tr>
    <td>
      color <code>int</code>
    </td>
    <td>
      <pre>
&commat;color
</pre>
    </td>
    <td>
      &commat;color
    </td>
  </tr>

  <tr>
    <td>
      ColorStateList
    </td>
    <td>
      &commat;color
    </td>
    <td>
      &commat;colorStateList
    </td>
  </tr>
</table>

<h2 id="data_objects">
  Data Objects
</h2>

<p>
  Any plain old Java object (POJO) may be used for data binding, but modifying
  a POJO will not cause the UI to update. The real power of data binding can be
  used by giving your data objects the ability to notify when data changes.
  There are three different data change notification mechanisms,
  <code>Observable</code> objects, <code>ObservableField</code>s, and
  <code>observable collections</code>.
</p>

<p>
  When one of these observable data object is bound to the UI and a property of
  the data object changes, the UI will be updated automatically.
</p>

<h3 id="observable_objects">
  Observable Objects
</h3>

<p>
  A class implementing <code>android.databinding.Observable</code> interface
  will allow the binding to attach a single listener to a bound object to
  listen for changes of all properties on that object.
</p>

<p>
  The <code>Observable</code> interface has a mechanism to add and remove
  listeners, but notifying is up to the developer. To make development easier,
  a base class, <code>BaseObservable,</code> was created to implement the
  listener registration mechanism. The data class implementer is still
  responsible for notifying when the properties change. This is done by
  assigning a <code>Bindable</code> annotation to the getter and notifying in
  the setter.
</p>

<pre>
<strong>private static class </strong>User <strong>extends </strong>BaseObservable {
   <strong>private </strong>String <strong>firstName</strong>;
   <strong>private </strong>String <strong>lastName</strong>;
   &commat;Bindable
   <strong>public </strong>String getFirstName() {
       <strong>return this</strong>.<strong>firstName</strong>;
   }
   &commat;Bindable
   <strong>public </strong>String getFirstName() {
       <strong>return this</strong>.<strong>lastName</strong>;
   }
   <strong>public void </strong>setFirstName(String firstName) {
       <strong>this</strong>.<strong>firstName </strong>= firstName;
       notifyPropertyChanged(BR.firstName);
   }
   <strong>public void </strong>setLastName(String lastName) {
       <strong>this</strong>.<strong>lastName </strong>= lastName;
       notifyPropertyChanged(BR.lastName);
   }
}
</pre>
<p>
  The <code>Bindable</code> annotation generates an entry in the BR class file
  during compilation. The BR class file will be generated in the module
  package.If the base class for data classes cannot be changed, the
  <code>Observable</code> interface may be implemented using the convenient
  <code>PropertyChangeRegistry</code> to store and notify listeners
  efficiently.
</p>

<h3 id="observablefields">
  ObservableFields
</h3>

<p>
  A little work is involved in creating Observable classes, so developers who
  want to save time or have few properties may use ObservableFields.
  ObservableFields are self-contained observable objects that have a single
  field. There are versions for all primitive types and one for reference
  types. To use, create a public final field in the data class:
</p>

<pre>
<strong>private static class </strong>User <strong>extends </strong>BaseObservable {
   <strong>public final </strong>ObservableField&lt;String&gt; <strong>firstName </strong>=
       <strong>new </strong>ObservableField&lt;&gt;();
   <strong>public final </strong>ObservableField&lt;String&gt; <strong>lastName </strong>=
       <strong>new </strong>ObservableField&lt;&gt;();
   <strong>public final </strong>ObservableInt <strong>age </strong>= <strong>new </strong>ObservableInt();
}
</pre>
<p>
  That&apos;s it! To access the value, use the set and get accessor methods:
</p>

<pre>
user.<strong>firstName</strong>.set(<strong>"Google"</strong>);
<strong>int </strong>age = user.<strong>age</strong>.get();
</pre>
<h3 id="observable_collections">
  Observable Collections
</h3>

<p>
  Some applications use more dynamic structures to hold data. Observable
  collections allow keyed access to these data objects.ObservableArrayMap is
  useful when the key is a reference type, such as String.
</p>

<pre>
ObservableArrayMap&lt;String, Object&gt; user = <strong>new </strong>ObservableArrayMap&lt;&gt;();
user.put(<strong>"firstName"</strong>, <strong>"Google"</strong>);
user.put(<strong>"lastName"</strong>, <strong>"Inc."</strong>);
user.put(<strong>"age"</strong>, 17);
</pre>
<p>
  In the layout, the map may be accessed through the String keys:
</p>

<pre>
&lt;<strong>data</strong>&gt;
    &lt;<strong>import type="android.databinding.ObservableMap"</strong>/&gt;
    &lt;<strong>variable name="user" type="ObservableMap&amp;lt;String, Object&gt;"</strong>/&gt;
&lt;/<strong>data</strong>&gt;
…
&lt;<strong>TextView
   android:text=&apos;&commat;{user["lastName"]}&apos;
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"</strong>/&gt;
&lt;<strong>TextView
   android:text=&apos;&commat;{String.valueOf(1 + (Integer)user["age"])}&apos;
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"</strong>/&gt;
</pre>
<p>
  ObservableArrayList is useful when the key is an integer:
</p>

<pre>
ObservableArrayList&lt;Object&gt; user = <strong>new </strong>ObservableArrayList&lt;&gt;();
user.add(<strong>"Google"</strong>);
user.add(<strong>"Inc."</strong>);
user.add(17);
</pre>
<p>
  In the layout, the list may be accessed through the indices:
</p>

<pre>
&lt;<strong>data</strong>&gt;
    &lt;<strong>import type="android.databinding.ObservableList"</strong>/&gt;
    &lt;<strong>import type="com.example.my.app.Fields"</strong>/&gt;
    &lt;<strong>variable name="user" type="ObservableList&amp;lt;Object&gt;"</strong>/&gt;
&lt;/<strong>data</strong>&gt;
…
&lt;<strong>TextView
   android:text=&apos;&commat;{user[Fields.LAST_NAME]}&apos;
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"</strong>/&gt;
&lt;<strong>TextView
   android:text=&apos;&commat;{String.valueOf(1 + (Integer)user[Fields.AGE])}&apos;
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"</strong>/&gt;
</pre>
<h2 id="generated_binding">
  Generated Binding
</h2>

<p>
  The generated binding class links the layout variables with the Views within
  the layout. As discussed earlier, the name and package of the Binding may be
  <a href="#custom_binding_class_names">customized</a>. The Generated binding
  classes all extend <code>android.databinding.ViewDataBinding</code>.
</p>

<h3 id="creating">
  Creating
</h3>

<p>
  The binding should be created soon after inflation to ensure that the View
  hierarchy is not disturbed prior to binding to the Views with expressions
  within the layout. There are a few ways to bind to a layout. The most common
  is to use the static methods on the Binding class.The inflate method inflates
  the View hierarchy and binds to it all it one step. There is a simpler
  version that only takes a <code>LayoutInflater</code> and one that takes a
  <code>ViewGroup</code> as well:
</p>

<pre>
MyLayoutBinding binding = MyLayoutBinding.<em>inflate</em>(<strong>layoutInflater</strong>);
MyLayoutBinding binding = MyLayoutBinding.<em>inflate</em>(LayoutInflater, viewGroup, false);
</pre>
<p>
  If the layout was inflated using a different mechanism, it may be bound
  separately:
</p>

<pre>
MyLayoutBinding binding = MyLayoutBinding.<em>bind</em>(viewRoot);
</pre>
<p>
  Sometimes the binding cannot be known in advance. In such cases, the binding
  can be created using the DataBindingUtil class:
</p>

<pre>
ViewDataBinding binding = DataBindingUtil.<em>inflate</em>(LayoutInflater, layoutId,
    parent, attachToParent);
ViewDataBinding binding = DataBindingUtil.<em>bindTo</em>(viewRoot, layoutId);
</pre>
<h3 id="views_with_ids">
  Views With IDs
</h3>

<p>
  A public final field will be generated for each View with an ID in the
  layout. The binding does a single pass on the View hierarchy, extracting the
  Views with IDs. This mechanism can be faster than calling findViewById for
  several Views. For example:
</p>

<pre>
&lt;<strong>layout xmlns:android="http://schemas.android.com/apk/res/android"</strong>&gt;
   &lt;<strong>data</strong>&gt;
       &lt;<strong>variable name="user" type="com.example.User"</strong>/&gt;
   &lt;/<strong>data</strong>&gt;
   &lt;<strong>LinearLayout
       android:orientation="vertical"
       android:layout_width="match_parent"
       android:layout_height="match_parent"</strong>&gt;
       &lt;<strong>TextView android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:text="&commat;{user.firstName}"</strong>
   <strong>android:id="&commat;+id/firstName"</strong>/&gt;
       &lt;<strong>TextView android:layout_width="wrap_content"
           android:layout_height="wrap_content"
           android:text="&commat;{user.lastName}"</strong>
  <strong>android:id="&commat;+id/lastName"</strong>/&gt;
   &lt;/<strong>LinearLayout</strong>&gt;
&lt;/<strong>layout</strong>&gt;
</pre>
<p>
  Will generate a binding class with:
</p>

<pre>
<strong>public final </strong>TextView <strong>firstName</strong>;
<strong>public final </strong>TextView <strong>lastName</strong>;
</pre>
<p>
  IDs are not nearly as necessary as without data binding, but there are still
  some instances where access to Views are still necessary from code.
</p>

<h3 id="variables2">
  Variables
</h3>

<p>
  Each variable will be given accessor methods.
</p>

<pre>
&lt;<strong>data</strong>&gt;
    &lt;<strong>import type="android.graphics.drawable.Drawable"</strong>/&gt;
    &lt;<strong>variable name="user"  type="com.example.User"</strong>/&gt;
    &lt;<strong>variable name="image" type="Drawable"</strong>/&gt;
    &lt;<strong>variable name="note"  type="String"</strong>/&gt;
&lt;/<strong>data</strong>&gt;
</pre>
<p>
  will generate setters and getters in the binding:
</p>

<pre>
<strong>public abstract </strong>com.example.User getUser();
<strong>public abstract void </strong>setUser(com.example.User user);
<strong>public abstract </strong>Drawable getImage();
<strong>public abstract void </strong>setImage(Drawable image);
<strong>public abstract </strong>String getNote();
<strong>public abstract void </strong>setNote(String note);
</pre>
<h3 id="viewstubs">
  ViewStubs
</h3>

<p>
  ViewStubs are a little different from normal Views. They start off invisible
  and when they either are made visible or are explicitly told to inflate, they
  replace themselves in the layout by inflating another layout.
</p>

<p>
  Because the ViewStub essentially disappears from the View hierarchy, the View
  in the binding object must also disappear to allow collection. Because the
  Views are final, a ViewStubProxy object takes the place of the ViewStub,
  giving the developer access to the ViewStub when it exists and also access to
  the inflated View hierarchy when the ViewStub has been inflated.
</p>

<p>
  When inflating another layout, a binding must be established for the new
  layout. Therefore, the ViewStubProxy must listen to the ViewStub&apos;s
  OnInflateListener and establish the binding at that time. Since only one can
  exist, the ViewStubProxy allows the developer to set an OnInflateListener on
  it that it will call after establishing the binding.
</p>

<h3 id="advanced_binding">
  Advanced Binding
</h3>

<h4 id="dynamic_variables">
  Dynamic Variables
</h4>

<p>
  At times, the specific binding class won&apos;t be known. For example, a
  RecyclerView Adapter operating against arbitrary layouts won&apos;t know the
  specific binding class. It still must assign the binding value during the
  onBindViewHolder.
</p>

<p>
  In this example, all layouts that the RecyclerView binds to have an "item"
  variable. The BindingHolder has a getBinding method returning the
  <code>ViewDataBinding</code> base.
</p>

<pre>
<strong>public void </strong>onBindViewHolder(BindingHolder holder, <strong>int </strong>position) {
   <strong>final </strong>T item = <strong>mItems</strong>.get(position);
   holder.getBinding().setVariable(BR.item, item);
   holder.getBinding().executePendingBindings();
}
</pre>
<h4 id="immediate_binding">
  Immediate Binding
</h4>

<p>
  When a variable or observable changes, the binding will be scheduled to
  change before the next frame. There are times, however, when binding must be
  executed immediately. To force execution, use the
  <code>executePendingBindings()</code> method.
</p>

<h4>
  Background Thread
</h4>

<p>
  You can change your data model in a background thread as long as it is not a
  collection. Data binding will localize each variable / field while evaluating
  to avoid any concurrency issues.
</p>

<h2 id="attribute_setters">
  Attribute Setters
</h2>

<p>
  Whenever a bound value changes, the generated binding class must call a
  setter method on the View with the binding expression. The data binding
  framework has ways to customize which method to call to set the value.
</p>

<h3 id="automatic_setters">
  Automatic Setters
</h3>
For an attribute, data binding tries to find the method setAttribute. The
namespace for the attribute does not matter, only the attribute name itself.
<p>
  For example, an expression associated with TextView&apos;s attribute
  <strong><code>android:text</code></strong> will look for a setText(String).
  If the expression returns an int, data binding will search for a setText(int)
  method. Be careful to have the expression return the correct type, casting if
  necessary. Note that data binding will work even if no attribute exists with
  the given name. You can then easily "create" attributes for any setter by
  using data binding. For example, support DrawerLayout doesn&apos;t have any
  attributes, but plenty of setters. You can use the automatic setters to use
  one of these.
</p>

<pre>
&lt;android.support.v4.widget.<strong>DrawerLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:scrimColor="&commat;{&commat;color/scrim}"
    app:drawerListener="&commat;{fragment.drawerListener}"/&gt;</strong>
</pre>
<h3 id="renamed_setters">
  Renamed Setters
</h3>

<p>
  Some attributes have setters that don&apos;t match by name. For these
  methods, an attribute may be associated with the setter through
  BindingMethods annotation. This must be associated with a class and contains
  BindingMethod annotations, one for each renamed method. For example, the
  <strong><code>android:tint</code></strong> attribute is really associated
  with setImageTintList, not setTint.
</p>

<pre>
&commat;BindingMethods({
       &commat;BindingMethod(type = <strong>"android.widget.ImageView"</strong>,
                      attribute = <strong>"android:tint"</strong>,
                      method = <strong>"setImageTintList"</strong>),
})
</pre>
<p>
  It is unlikely that developers will need to rename setters; the android
  framework attributes have already been implemented.
</p>

<h3 id="custom_setters">
  Custom Setters
</h3>

<p>
  Some attributes need custom binding logic. For example, there is no
  associated setter for the <strong><code>android:paddingLeft</code></strong>
  attribute. Instead, <code>setPadding(left, top, right, bottom)</code> exists.
  A static binding adapter method with the <code>BindingAdapter</code>
  annotation allows the developer to customize how a setter for an attribute is
  called.
</p>

<p>
  The android attributes have already had <code>BindingAdapter</code>s created.
  For example, here is the one for <code>paddingLeft</code>:
</p>

<pre>

&commat;BindingAdapter(<strong>"android:paddingLeft"</strong>)
<strong>public static void </strong>setPaddingLeft(View view, <strong>int </strong>padding) {
   view.setPadding(padding,
                   view.getPaddingTop(),
                   view.getPaddingRight(),
                   view.getPaddingBottom());
}
</pre>
<p>
  Binding adapters are useful for other types of customization. For example, a
  custom loader can be called off-thread to load an image.
</p>

<p>
  Developer-created binding adapters will override the data binding default
  adapters when there is a conflict.
</p>

<p>
  You can also have adapters that receive multiple parameters.
</p>

<pre>
&commat;BindingAdapter({<strong>"bind:imageUrl"</strong>, <strong>"bind:error"</strong>})
<strong>public static void </strong>loadImage(ImageView view, String url, Drawable error) {
   Picasso.<em>with</em>(view.getContext()).load(url).error(error).into(view);
}
</pre>
<pre>
&lt;ImageView app:imageUrl=“&commat;{venue.imageUrl}”
app:error=“&commat;{&commat;drawable/venueError}”/&gt;
</pre>

<p>
  This adapter will be called if both <strong>imageUrl</strong> and
  <strong>error</strong> are used for an ImageView and <em>imageUrl</em> is a
  string and <em>error</em> is a drawable.
</p>

<ul>
  <li>Custom namespaces are ignored during matching.
  </li>

  <li>You can also write adapters for android namespace.
  </li>
</ul>

<h2 id="converters">
  Converters
</h2>

<h3 id="object_conversions">
  Object Conversions
</h3>

<p>
  When an Object is returned from a binding expression, a setter will be chosen
  from the automatic, renamed, and custom setters. The Object will be cast to a
  parameter type of the chosen setter.
</p>

<p>
  This is a convenience for those using ObservableMaps to hold data. for
  example:
</p>

<pre>
&lt;<strong>TextView
   android:text=&apos;&commat;{userMap["lastName"]}&apos;
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"</strong>/&gt;
</pre>

<p>
The <code>userMap</code> returns an Object and that Object will be automatically cast to
  parameter type found in the setter <code>setText(CharSequence)</code>. When there
  may be confusion about the parameter type, the developer will need
  to cast in the expression.
</p>

<h3 id="custom_conversions">Custom Conversions</h3>

<p>
  Sometimes conversions should be automatic between specific types. For
  example, when setting the background:
</p>

<pre>
&lt;<strong>View
   android:background="&commat;{isError ? &commat;color/red : &commat;color/white}"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"</strong>/&gt;
</pre>
<p>
  Here, the background takes a <code>Drawable</code>, but the color is an
  integer. Whenever a <code>Drawable</code> is expected and an integer is
  returned, the <code>int</code> should be converted to a
  <code>ColorDrawable</code>. This conversion is done using a static method
  with a BindingConversion annotation:
</p>

<pre>
&commat;BindingConversion
<strong>public static </strong>ColorDrawable convertColorToDrawable(<strong>int </strong>color) {
   <strong>return new </strong>ColorDrawable(color);
}
</pre>
<p>
  Note that conversions only happen at the setter level, so it is <strong>not
  allowed</strong> to mix types like this:
</p>

<pre>
&lt;<strong>View
   android:background="&commat;{isError ? &commat;drawable/error : &commat;color/white}"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"</strong>/&gt;
</pre>