docs: add information about known issues in resource matching

  <ul>

    <li>Different types of resources belong in different subdirectories of {@code res/}</li>

    <li>Alternative resources provide configuration-specific resource files</li>

    <li>Always include default resources so your app does not depend on specific configurations</li>

  </ul>

  <h2>In this document</h2>

  <ol>

        <li><a href="#AliasResources">Creating alias resources</a></li>

      </ol>

    </li>

    <li><a href="#Compatibility">Providing the Best Device Compatibility with Resources</a></li>

    <li><a href="#BestMatch">How Android Finds the Best-matching Resource</a></li>

    <li><a href="#KnownIssues">Known Issues</a></li>

  </ol>

  <h2>See also</h2>

<p>You should always externalize application resources such as images and strings from your

code, so that you can maintain them independently. You can also provide alternative resources for

specific device configurations, by grouping them in specially-named resource directories. Android

will then automatically apply the appropriate resource based on the current configuration. For

then applies the appropriate resource based on the current configuration. For

instance, you might want to provide a different UI layout depending on the screen size.</p>

<p>Once you save your resources external to your application code, you can access them

<p>Once you externalize your application resources, you can access them

using resource IDs that are generated in your project's {@code R} class. How to use

resources in your application is discussed in <a href="accessing-resources.html">Accessing

Resources</a>.</p>

Resources</a>. This document shows you how to group your resources in your Android project and

provide alternative resources for specific device configurations.</p>

<h2 id="ResourceTypes">Grouping Resource Types</h2>

            strings.xml  </span>

</pre>

<p>The {@code res/} directory contains all the resources (in subdirectories): an image resource, two

layout resources, and a string resource file. The resource directory names are important and are

described in table 1.</p>

<p>As you can see in this example, the {@code res/} directory contains all the resources (in

subdirectories): an image resource, two layout resources, and a string resource file. The resource

directory names are important and are described in table 1.</p>

<p class="table-caption" id="table1"><strong>Table 1.</strong> Resource directories

supported inside project {@code res/} directory.</p>

        <li>Bitmap files</li>

        <li>Nine-Patches (re-sizable bitmaps)</li>

        <li>State lists</li>

        <li>Color drawables</li>

        <li>Shapes</li>

        <li>Animation drawables</li>

        <li>Other drawables</li>

      </ul>

      <p>See <a href="drawable-resource.html">Drawable Resources</a>.</p>

    </td>

  </tr>

</table>

<p class="note"><strong>Note:</strong> You should never save resource files directly inside the

{@code res/} directory.</p>

<p class="caution"><strong>Caution:</strong> Never save resource files directly inside the

{@code res/} directory&mdash;it will cause a compiler error.</p>

<p>For more information about certain types of resources, see the <a

href="available-resources.html">Resource Types</a> documentation.</p>

<p>How to access resources in the {@code res/} subdirectories is discussed in <a

href="accessing-resources.html">Accessing Resources</a>.

</p>

<p>The resources that you save in the subdirectories defined in table 1 are your "default"

resources. That is, these resources define the default design and content for your application.

However, different types of Android-powered devices might call for different types of resources.

For example, if a device has a larger than normal screen, then you should provide

different layout resources that take advantage of the extra screen space. Or, if a device has a

different language setting, then you should provide different string resources that translate the

text in your user interface. To provide these different resources for different device

configurations, you need to provide "alternative" resources, in addition to your default

resources.</p>

<h2 id="AlternativeResources">Providing Alternative Resources</h2>

<p>Almost every application should provide alternative resources to support specific device

configurations. For instance, you should include alternative drawable resources for different

screen densities and alternative string resources for different languages. At runtime, Android

automatically detects the current device configuration and loads the appropriate

resources.</p>

detects the current device configuration and loads the appropriate

resources for your application.</p>

<p>To specify configuration-specific alternatives for a set of resources:</p>

<ol>

<em>&lt;resources_name&gt;</em>-<em>&lt;config_qualifier&gt;</em>}.

    <ul>

      <li><em>{@code &lt;resources_name&gt;}</em> is the directory name of the corresponding default

resources.</li>

resources (defined in table 1).</li>

      <li><em>{@code &lt;config_qualifier&gt;}</em> is a name that specifies a configuration

for which these resources are to be used.</li>

for which these resources are to be used (defined in table 2).</li>

    </ul>

    <p>You can append more than one <em>{@code &lt;config_qualifier&gt;}</em>. Separate each

one with a dash.</p>

  </li>

  <li>Save your alternative resources in this new directory. The resource files must be named

exactly the same as the default resource files.</li>

  <li>Save the respective alternative resources in this new directory. The resource files must be

named exactly the same as the default resource files.</li>

</ol>

<p>For example, here are some default and alternative resources:</p>

</pre>

<p>The {@code hdpi} qualifier indicates that the resources in that directory are for devices with a

high-density screen. While the images in each drawable directory are sized for a specific screen

density, the filenames are

high-density screen. The images in each of these drawable directories are sized for a specific

screen density, but the filenames are exactly

the same. This way, the resource ID that you use to reference the {@code icon.png} or {@code

background.png} image is always the same, but Android selects the

version of that drawable that best matches the current device configuration.</p>

version of each resource that best matches the current device, by comparing the device

configuration information with the qualifiers in the alternative resource directory name.</p>

<p>Android supports several configuration qualifiers and you can

add multiple qualifiers to one directory name, by separating each qualifier with a dash. Table 2

lists the valid configuration qualifiers, in order of precedence—if you use multiple

qualifiers, they must be added to the directory name in the order they are listed in the

table.</p>

qualifiers for one resource directory, they must be added to the directory name in the order they

are listed in the table.</p>

<p class="note"><strong>Note:</strong> Some resource qualifiers were added after Android 1.0, so not

all versions of Android support all the qualifiers listed in table 2. New qualifiers

indicate the version in which they were added. To avoid any issues, always include a set of default

resources for resources that your application uses. For more information, see the section about <a

href="#Compatibility">Providing the Best Device Compatibility with Resources</a>.</p>

<p class="table-caption" id="table2"><strong>Table 2.</strong> Alternative resource qualifier

names.</p>

        <th>Values</th>

        <th>Description</th>

    </tr>

    <tr>

    <tr id="MccQualifier">

      <td>MCC and MNC</td>

      <td>Examples:<br/>

        <code>mcc310</code><br/>

and mobile network code, respectively.</p>

      </td>

    </tr>

    <tr>

    <tr id="LocaleQualifier">

      <td>Language and region</td>

      <td>Examples:<br/>

        <code>en</code><br/>

href="runtime-changes.html">Handling Runtime Changes</a> for information about

how this can affect your application during runtime.</p>

        <p>See <a href="localization.html">Localization</a> for a complete guide to localizing

your application for other langauges.</p>

your application for other languages.</p>

        <p>Also see the {@link android.content.res.Configuration#locale} configuration field, which

indicates the current locale.</p>

      </td>

    </tr>

    <tr>

    <tr id="ScreenSizeQualifier">

      <td>Screen size</td>

      <td>

        <code>small</code><br/>

        available space in both width and height than an HVGA display.

        Examples are VGA and WVGA medium density screens.</li>

        </ul>

        <p><em>Added in API Level 4.</em></p>

        <p>See <a href="{@docRoot}guide/practices/screens_support.html">Supporting Multiple

Screens</a> for more information.</p>

        <p>Also see the {@link android.content.res.Configuration#screenLayout} configuration field,

or large.</p>

      </td>

    </tr>

    <tr>

    <tr id="ScreenLongQualifier">

      <td>Wider/taller screens</td>

      <td>

        <code>long</code><br/>

          <li>{@code long}: Long screens, such as WQVGA, WVGA, FWVGA</li>

          <li>{@code notlong}: Not long screens, such as QVGA, HVGA, and VGA</li>

        </ul>

        <p><em>Added in API Level 4.</em></p>

        <p>This is based purely on the aspect ratio of the screen (a "long" screen is wider). This

is not related to the screen orientation.</p>

        <p>Also see the {@link android.content.res.Configuration#screenLayout} configuration field,

which indicates whether the screen is long.</p>

      </td>

    </tr>

    <tr>

    <tr id="OrientationQualifier">

      <td>Screen orientation</td>

      <td>

        <code>port</code><br/>

which indicates the current device orientation.</p>

      </td>

    </tr>

    <tr>

    <tr id="DockQualifier">

      <td>Dock mode</td>

      <td>

        <code>car</code><br/>

        </ul>

        <p><em>Added in API Level 8.</em></p>

        <p>This can change during the life of your application if the user places the device in a

dock. You can eneable or disable this mode using {@link

dock. You can enable or disable this mode using {@link

android.app.UiModeManager}. See <a href="runtime-changes.html">Handling Runtime Changes</a> for

information about how this affects your application during runtime.</p>

      </td>

    </tr>

    <tr>

    <tr id="NightQualifier">

      <td>Night mode</td>

      <td>

        <code>night</code><br/>

        </ul>

        <p><em>Added in API Level 8.</em></p>

        <p>This can change during the life of your application if night mode is left in

auto mode (default), in which case the mode changes based on the time of day.  You can eneable

auto mode (default), in which case the mode changes based on the time of day.  You can enable

or disable this mode using {@link android.app.UiModeManager}. See <a

href="runtime-changes.html">Handling Runtime Changes</a> for information about how this affects your

application during runtime.</p>

      </td>

    </tr>

    <tr>

    <tr id="DensityQualifier">

      <td>Screen pixel density (dpi)</td>

      <td>

        <code>ldpi</code><br/>

          <li>{@code nodpi}: This can be used for bitmap resources that you do not want to be scaled

to match the device density.</li>

        </ul>

        <p><em>Added in API Level 4.</em></p>

        <p>There is thus a 4:3 scaling factor between each density, so a 9x9 bitmap

         in ldpi is 12x12 in mdpi and 16x16 in hdpi.</p>

        <p>When Android selects which resource files to use,

your bitmaps.</p>

       </td>

    </tr>

    <tr>

    <tr id="TouchscreenQualifier">

      <td>Touchscreen type</td>

      <td>

        <code>notouch</code><br/>

which indicates the type of touchscreen on the device.</p>

      </td>

    </tr>

    <tr>

    <tr id="KeyboardAvailQualifier">

      <td>Keyboard availability</td>

      <td>

        <code>keysexposed</code><br/>

keyboard and and the visibility of any kind of keyboard (including software), respectively.</p>

      </td>

    </tr>

    <tr>

    <tr id="ImeQualifier">

      <td>Primary text input method</td>

      <td>

        <code>nokeys</code><br/>

      <td>

        <ul class="nolist">

          <li>{@code nokeys}: Device has no hardware keys for text input.</li>

          <li>{@code qwert}: Device has a hardware qwerty keyboard, whether it's visible to the user

          <li>{@code qwerty}: Device has a hardware qwerty keyboard, whether it's visible to the

user

or not.</li>

          <li>{@code 12key}: Device has a hardware 12-key keyboard, whether it's visible to the user

or not.</li>

which indicates the primary text input method available.</p>

      </td>

    </tr>

    <tr>

    <tr id="NavAvailQualifier">

      <td>Navigation key availability</td>

      <td>

        <code>navexposed</code><br/>

field, which indicates whether navigation keys are hidden.</p>

      </td>

    </tr>

    <tr>

    <tr id="TouchQualifier">

      <td>Primary non-touch navigation method</td>

      <td>

        <code>nonav</code><br/>

      </td>

    </tr>

-->

    <tr>

      <td>API Level</td>

    <tr id="VersionQualifier">

      <td>System Version (API Level)</td>

      <td>Examples:<br/>

        <code>v3</code><br/>

        <code>v4</code><br/>

        <code>v5</code><br/>

        <code>v6</code><br/>

        <code>v7</code><br/>

        etc.</td>

      <td>

        <p>The API Level supported by the device, for example <code>v1</code> for API Level 1

(Android 1.0) or <code>v5</code> for API Level 5 (Android 2.0). See the <a

        <p>The API Level supported by the device. For example, <code>v1</code> for API Level

1 (devices with Android 1.0 or higher) and <code>v4</code> for API Level 4 (devices with Android

1.6 or higher). See the <a

href="{@docRoot}guide/appendix/api-levels.html">Android API Levels</a> document for more information

about these values.</p>

        <p class="caution"><strong>Caution:</strong> Android 1.5 and 1.6 only match resources

with this qualifier when it exactly matches the system version. See the section below about <a

href="#KnownIssues">Known Issues</a> for more information.</p>

      </td>

    </tr>

</table>

<h2 id="Compatibility">Providing the Best Device Compatibility with Resources</h2>

<p>In order for your application to support multiple device configurations, it's very important that

you always provide default resources for each type of resource that your application uses.</p>

<p>For example, if your application supports several languages, always include a {@code

values/} directory (in which your strings are saved) <em>without</em> a <a

href="#LocalQualifier">language and region qualifier</a>. If you instead put all your string files

in directories that have a language and region qualifier, then your application will crash when run

on a device set to a language that your strings do not support. But, as long as you provide default

{@code values/} resources, then your application will run properly (even if the user doesn't

understand that language&mdash;it's better than crashing).</p>

<p>Likewise, if you provide different layout resources based on the screen orientation, you should

pick one orientation as your default. For example, instead of providing layout resources in {@code

layout-land/} for landscape and {@code layout-port/} for portrait, leave one as the default, such as

{@code layout/} for landscape and {@code layout-port/} for portrait.</p>

<p>Providing default resources is important not only because your application might run on a

configuration you had not anticipated, but also because new versions of Android sometimes add

resource qualifiers that older versions do not support. If you use a new resource qualifier,

but maintain code compatibility with older versions of Android, then when an older version of

Android runs your application, it will crash if you do not provide default resources, because it

cannot use the resources named with the new qualifier. For example, if your <a

href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#min">{@code

minSdkVersion}</a> is set to 4, and you qualify all of your drawable resources using <a

href="NightQualifier">night mode</a> ({@code night} or {@code notnight}, which were added in API

Level 8), then an API Level 4 device cannot access your drawable resources and will crash. In this

case, you probably want {@code notnight} to be your default resources, so you should exclude that

qualifier so your drawable resources are in either {@code drawable/} or {@code drawable-night/}.</p>

<p>The bottom line is: For every type of resource your application uses,

provide a set of default resources that allow your application to perform well, then create

variations of those resources for specific device configurations.</p>

<p>There is one exception to this rule: If your application's <a

href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#min">{@code minSdkVersion}</a> is 4 or

greater, you <em>do not</em> need default drawable resources when you provide alternative drawable

resources with the <a href="#DensityQualifier">screen density</a> qualifier. Even without default

drawable resources, Android can find the best match among the alternative screen densities and scale

the bitmaps as necessary. However, for the best experience on all types of devices, you should

provide alternative drawables for all three types of density. (If your <a

href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#min">{@code minSdkVersion}</a> is

<em>less than</em> 4, see the section below about <a href="#KnownIssues">known issues</a> for

information about how to support multiple screen densities.)</p>

<h2 id="BestMatch">How Android Finds the Best-matching Resource</h2>

href="accessing-resources.html">Accessing Resources</a>.</p>

<h2 id="KnownIssues">Known Issues</h2>

<p>The following are known issues in terms of how Android finds the best-matching resource on

certain versions of Android.</p>

<h3>Android 1.5 (and lower)</h3>

<h4>Density and screen size qualifiers are not supported</h4>

<p>Android 1.5 (and lower) does not support the following resource qualifers:</p>

<dl>

  <dt><a href="#DensityQualifier">Density</a></dt>

    <dd>{@code ldpi}, {@code mdpi}, {@code ldpi}, and {@code nodpi}</dd>

  <dt><a href="#ScreenSizeQualifier">Screen size</a></dt>

    <dd>{@code small}, {@code normal}, and {@code large}</dd>

  <dt><a href="#ScreenLongQualifier">Screen length</a></dt>

    <dd>{@code long} and {@code notlong}</dd>

</dl>

<p>These resource qualifiers were introduced in Android 1.6, so Android 1.5 (API Level 3) and lower

does not support them. If your application supports Android 1.5 and includes drawable resources for

each density type ({@code drawable-ldpi/}, {@code drawable-mdpi/}, and {@code drawable-ldpi/}), then

an Android 1.5 device might use any one of them, because it doesn't support the density qualifier,

will ignore it, and will use which ever otherwise-matching drawable resource it finds first. This

caveat applies the same to screen size and screen length resources.<p>

<p><b>The fix:</b> Ensure that your <a href="{@docRoot}sdk/tools-notes.html">SDK Tools</a> version

is r6 or greater and provide default drawable resources. For example, to support Android 1.5

while providing resources for all screen densities, include a set of drawable resources that does

not use the screen density qualifier. In fact, because all Android 1.5 devices have a medium-density

screen, you can remove the {@code mdpi} qualifer and put all medium density images in {@code

drawable/} (instead of {@code drawable-mdpi/}). For the screen size, put your {@code normal} size

resources in the default resource directory, and for the screen length, put your {@code notlong}

resources in the default resource directory (because all Android 1.5 devices have medium-density,

normal, not-long screens).</p>

<p>You need SDK Tools, Revision 6 (or greater), because it includes a new packaging tool that

automatically applies an appropriate <a href="#VersionQualifier">version qualifier</a> to any

resource directory named with a qualifier that did not exist in Android 1.0. For example, because

the density qualifier was introduced in Android 1.6 (API Level 4), when the packaging tool

encounters a resource directory using the density qualifier, it adds {@code v4} to the directory

name to ensure that older versions do not use those resources (only API Level 4 and higher support

that qualifier). Thus, by putting your medium-density resources in a directory <em>without</em> the

{@code mdpi} qualifier, they are still accessible by Android 1.5, and any device that supports the

density qualifer and has a medium-density screen also uses these resources because they are the best

match for the device (instead of using the {@code ldpi} or {@code hdpi} resources).</p>

<p class="note"><strong>Note:</strong> Later versions of Android, such as API Level 8,

introduce other resource qualifiers that older version do not support. To provide the best

compatibility, you should always include a set of default resources for each type of resource

that your application uses. See the section about <a href="#Compatibility">Providing the Best Device

Compatibility with Resources</a> for more information.</p>

<h3>Android 1.5 and 1.6</h3>

<h4>Version qualifier performs exact match, instead of best match</h4>

<p>The correct behavior is for the system to match resources marked with a <a

href="#VersionQualifier">version qualifier</a> equal

to or less than the system version on the device, but on Android 1.5 and 1.6, (API Level 3 and 4),

there is a bug that causes the system to match resources marked with the version qualifier

only when it exactly matches the version on the device.</p>

<p><b>The fix:</b> To provide version-specific resources, abide by this behavior. However, because

this bug is fixed in versions of Android available after 1.6, if

you need to differentiate resources between Android 1.5, 1.6, and later versions, then you only need

to apply the version qualifier to the 1.6 resources and one to match all later versions. Thus, this

is effectively a non-issue.</p>

<p>For example, if you want drawable resources that are different on each Android 1.5, 1.6,

and 2.0.1 (and later), create three drawable directories: {@code drawable/} (for 1.5 and lower),

{@code drawable-v4} (for 1.6), and {@code drawable-v6} (for 2.0.1 and later—version 2.0, v5,

is no longer available).</p>