The AppCompat support library enables the use of the ActionBar and Material Design specific implementations such as Toolbar for older devices down to Android v2.1. Currently, new projects created through Android Studio incorporate this library by default. You can confirm by looking at the build.gradle file to see the AppCompat library being set:

android {
    compileSdkVersion 23
    buildToolsVersion "23.0.0"
}

dependencies {
    compile 'com.android.support:appcompat-v7:23.1.0'
}

Note that the AppCompat library has an implicit dependency on the support-v4 library. The support-v4 declaration however does not necessarily need to be declared.

Also notice that once you upgrade to AppCompat v7 v23, you will also be forced to update your Build Tools and compileSDKVersion to API 23 too.

There is a current bug that precludes you from compiling to lower versions. Once you are using this API version 23 or higher, be aware that the Apache HTTP Client library has been removed. Workarounds are discussed in this guide.

If you need to wish to downgrade from API 23, you need to follow more steps besides simply uninstalling the SDK as documented in this bug report:

1. Remove the Build Tools 23 from the SDK Manager.

2. Find the appcompat-v7 SDK folder and delete the entire 23.0.0.0 folder.

   Mac OS users:
   /Users/[username]/Library/Android/sdk/extras/android/m2repository/com/android/support/appcompat-v7

   PC users: C:\Documents and Settings\<user>\AppData\Local\Android\sdk\extras\android\m2repository\com\android\support\appcompat-v7

3. Inside this same folder edit maven-metadata.xml and delete the one line <version>23.0.0</version>:

dependencies { compile 'com.android.support:appcompat-v7:22.2.1' }

5. Clean the project and rebuild.


## Search and replacing changes

Older projects may not include this library, so migrating requires changing the theme references and many of the main imports described in this [blog post](http://android-developers.blogspot.com/2014/10/appcompat-v21-material-design-for-pre.html).  Because the support class declarations are not compatible with the standard Android ones, you need to make sure you are using the imports from the support library entirely.  Otherwise your app is likely to crash.

The simplest is often to do a search-and-replace to start changing the following statements to start using the support libraries.

#### Activities Changes

* `import android.app.Activity` -> `import android.support.v7.app.AppCompatActivity`
* `extends Activity` -> `extends AppCompatActivity`

#### Fragment Changes

* `extends FragmentActivity` -> `extends AppCompatActivity`
* `import android.support.v4.app.FragmentActivity` -> `import android.support.v7.app.AppCompatActivity` 
* `import android.app.Fragment` -> `import android.support.v4.app.Fragment`
* `getFragmentManager()` -> `getSupportFragmentManager()`

#### ActionBar Changes

* `import android.app.ActionBar` -> `import android.support.v7.app.ActionBar`
* `getActionBar()` -> `getSupportActionBar()`

#### AlertDialog Changes

Your AlertDialogs should import from the AppCompat support library instead, which takes advantage of the new Material Design theme.

<img src="https://blog.xamarin.com/wp-content/uploads/2015/05/alertdialogs.png"/>

* `import android.app.AlertDialog` -> `import android.support.v7.app.AlertDialog`

#### Theme XML Changes

If you were migrating from the Holo theme, your new theme  would inherit from `Theme.AppCompat` instead of `android:Theme.Holo.Light`:

```xml
-    <style name="AppTheme" parent="android:Theme.Holo.Light.DarkActionBar">
+    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">

If you wish to have a style that disables the ActionBar in certain Activity screens but still wish to use many of the ones you custom defined, you can inherit from AppTheme and apply the same styles declared in Theme.AppCompat.NoActionBar:

    <style name="AppTheme.NoActionBar">
        <item name="windowActionBar">false</item>
        <item name="windowNoTitle">true</item>
    </style>

If you see AppCompat does not support the current theme features, it's likely the windowNoTitle setting has not been set or is set to false. There is more strict enforcement on what this value must be set on newer AppCompat libraries. See this Stack Overflow article for more context.

For your menu/ layout files, add an app: namespace . For menu items, the showAsAction must be from the app namespace instead of android namespace. It is considered a custom attribute of the support library and will no otherwise be processed correctly without making this change.

-<menu xmlns:android="http://schemas.android.com/apk/res/android">
+<menu xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto">
 <item android:id="@+id/myMenuItem"
       android:title="@string/select"
-      android:showAsAction="ifRoom"
+      app:showAsAction="ifRoom"

If you are using widgets such as the SearchView, you must also use android.support.v7.widget.SearchView instead of android.widget.SearchView. Note that the app namespace must also be used.

+<menu xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto">
 
     <item android:id="@+id/contentSearch"
           android:orderInCategory="2"
           android:title="@string/search"
-          android:showAsAction="ifRoom"
-          android:actionViewClass="android.widget.SearchView">
+          app:showAsAction="ifRoom"
+          app:actionViewClass="android.support.v7.widget.SearchView">

The MenuItemCompat helper class has a few static methods that should be used in lieu of MenuItem:

  @Override
    public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
        inflater.inflate(R.menu.my_menu, menu);
        mSearchView = (SearchView) MenuItemCompat.getActionView(menu.findItem(R.id.contentSearch));

In addition, setting the targetSdkVersion to the latest SDK version ensures that the AppCompat library will attempt to apply the Material Design assuming the device itself can support it. The support library will still check to see if the minimum SDK version is being used on the device.

android {
    targetSdkVersion 23

The AppCompat library has issues with Samsung v4.2.2 devices. See this issue for more details.

• http://www.androidauthority.com/goodbye-menu-button-hello-action-bar-48312/
• http://android-developers.blogspot.in/2013/08/actionbarcompat-and-io-2013-app-source.html
• http://android-developers.blogspot.com/2014/10/appcompat-v21-material-design-for-pre.html
• https://chris.banes.me/2015/04/22/support-libraries-v22-1-0/