docs: Updated support library overview

    <h2>In this document</h2>

    <ol>

      <li><a href="#overview">Overview</a></li>

      <li><a href="#revisions">Revisions</a></li>

      <li><a href="#backward">Backward Compatibility</a></li>

      <li><a href="#layout-patterns">Support for General Layout Patterns</a></li>

      <li><a href="#form-factors">Support for Different Form Factors</a></li>

      <li><a href="#utils">General Utilities</a></li>

      <li><a href="#revisions">Support Library Revision History</a></li>

    </ol>

    <h2>See also</h2>

  </div>

</div>

<p>The Android Support Library package is a set of code libraries that provide

  backward-compatible versions of Android framework APIs as well as features that are only available

  through the library APIs. Each Support Library is backward-compatible to a specific Android API

  level. This design means that your applications can use the libraries' features and still be

  compatible with devices running Android 1.6 (API level 4) and up.</p>

<p>

  The Android Support Library offers a number of features that are not built

  into the framework. These libraries offer backward-compatible versions of

  new features, provide useful UI elements that are not included in the

  framework, and provide a range of utilities that apps can draw on.

</p>

<h2 id="overview">Overview</h2>

<p>This guide provides information about what features are enabled by the Support Libraries,

  how to use them in your development environment and information about library releases.</p>

<p>

  In many cases, a feature may be valuable to many app developers, but not

  appropriate to include in the Android framework itself. For example, an app

  might only need a feature for specialized use cases, such as to smooth the

  transition between different versions of the Android system.

</p>

<p>

  To address these situations, the Android SDK includes several libraries

  collectively called the <em>Android Support Library</em>. App developers

  can include any of these libraries if they want to incorporate the

  library functionality into their apps.

</p>

<h2 id="overview">Overview</h2>

<p>

  Support libraries provide a range of different features:

</p>

<ul>

  <li>

    <a href="#backward">Backward-compatible</a> versions of framework

    components.

  </li>

  <li>UI elements to implement the recommended Android <a href=

  "#layout-patterns">layout patterns</a>.

  </li>

  <li>Support for different <a href="#form-factors">form factors</a>.

  </li>

  <li>Miscellaneous <a href="#utils">utility</a> functions.

  </li>

</ul>

<h2 id="backward">Backward Compatibility</h2>

<div class="figure" style="width:300px">

  <img src="{@docRoot}images/tools/support-library/appbar-kitkat.png"

    srcset="{@docRoot}images/tools/support-library/appbar-kitkat.png 1x,

        {@docRoot}images/tools/support-library/appbar-kitkat_2x.png 2x"

    alt="" width="300">

  <p class="img-caption">

    <strong>Figure 1.</strong> Because this app uses support library UI

    elements its interface incorporates material design principles, even though

    it is running on Android 4.4, which does not include native support for

    material design.

  </p>

</div>

<p>

  Support libraries allow apps running on older versions of the Android

  platform to support features made available on newer versions of the

  platform. For example, an app running on a version of Android lower than 5.0

  (API level 21) that relies on framework classes cannot display

  material-design elements, as that version of the Android framework doesn't

  support material design. However, if the app incorporates the Support

  Library's <a href="{@docRoot}tools/support-library/features.html">appcompat

  library</a>, the app has access to many of the features available in API

  level 21, including support for material design. As a result, your app can

  deliver a more consistent experience across a broader range of platform

  versions.

</p>

<p>

  In some cases, the support library version of a class depends as much as

  possible on the functionality that the framework provides. In these cases,

  if an app calls one of the support class's methods, the support library's

  behavior depends on what version of Android the app is running on. If the

  framework provides the necessary functionality, the support library calls on

  the framework to perform the task. If the app is running on an earlier

  version of Android, and the framework doesn't expose the needed

  functionality, the support library may try to provide the functionality

  itself, or may act as a no-op. In either case, the app generally doesn't

  need to check what version of Android it's running on; instead, the app

  can rely on the support library to do those checks and choose appropriate

  behavior. Generally, classes whose names end in

  <code>…Compat</code> (like {@link android.support.v4.app.ActivityCompat})

  behave this way.

</p>

<p>

  In other cases, the support library class provides a complete, standalone

  version of a framework class that does not rely on the availability of

  any framework APIs. These

  methods provide consistent behavior across all supported platforms.

</p>

<p>

  In either case, the app does not need to check the system version at run

  time. The app can rely on the support library class to do the appropriate

  system checks, and modify its behavior as necessary.

</p>

<h2 id="layout-patterns">Support for General Layout Patterns</h2>

<p>

  Support libraries provide user interface elements not offered by

  the Android framework. For example, the Android Support Library offers additional

  layout classes, like {@link android.support.v4.widget.DrawerLayout}. These

  classes follow recommended Android design practices; for example, the Design

  Library follows the principles of material design in a way that works across

  many versions of Android.

</p>

<p>

  By using these support library classes, you can avoid having to reinvent the

  wheel; if your app has a particular user interface requirement, you can draw

  on existing code, which provides a user interface that users will already be

  familiar with. These elements also help you build an app that looks and feels

  like a part of the Android ecosystem. For example, many apps need to display

  arbitrarily long lists of elements, and need to be able to quickly and

  efficiently reuse those elements as the list changes; this might be a list of

  emails, contacts, music albums, and so on. Those apps can use the support

  library {@link android.support.v7.widget.RecyclerView} widget to display the

  list. This saves the app developer from having to develop the list from

  scratch, and also ensures that the user will see a list that looks and

  behaves like lists in other apps.

</p>

<h2 id="form-factors">Support for Different Form Factors</h2>

<p>

  The Android SDK provides libraries for a number of different form factors,

  such as TV and wearables. An app can depend on the appropriate support

  library to provide functionality across a wide range of platform versions,

  and can provide content on external screens, speakers, and other destination

  devices.

</p>

<h2 id="utils">General Utilities</h2>

<p>

  The Android Support Library provides backward-compatible utility functions.

  Apps can use these utility functions to provide an appropriate user

  experience across a wide range of Android system versions. For example,

  support library permission methods behave appropriately depending on what

  platform version your app is running on. If the platform supports the

  runtime permissions model, these methods request the appropriate permission

  from the user; on platform versions that do not support the runtime

  permissions model, the methods check whether the appropriate permission was

  granted at install time.

</p>

<p>Including the Support Libraries in your Android project is considered a best practice for

  application developers, depending on the range of platform versions your app is targeting

  and the APIs that it uses. Using the features the libraries provide can help you improve the look

  of your application, increase performance and broaden the reach of your application to more users.

  If you use the Android

  <a href="{@docRoot}tools/projects/templates.html">code template</a> tools, you will notice that

  all the Android application templates include one or more of the Support Libraries by default.</p>

<p>The Support Libraries each target a base Android API level and each provides a different set

  of features. In order to effectively use the libraries, it is important to consider what features

  you want to support and understand what features are supported by each library at what Android

  API level. To get started, review the

  <a href="{@docRoot}tools/support-library/features.html">Support Library Features</a> guide.

  After that, go to the

  <a href="{@docRoot}tools/support-library/setup.html">Support Library Setup</a> topic to

  learn how to incorporate the Support Libraries into your application. For more details

  about Support Library APIs, see the {@link android.support.v4.app android.support}

  packages in the API reference.</p>

<h2 id="revisions">Revisions</h2>

<h2 id="revisions">Support Library Revision History</h2>

<p>This section provides details about the Support Library package releases.</p>