<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>
-<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>
+<h2 id="overview">Overview</h2>
+<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>
-<h2 id="overview">Overview</h2>
+<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>
+
+<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>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>
+<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>
+
+<h2 id="revisions">Support Library Revision History</h2>
<p>This section provides details about the Support Library package releases.</p>
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