API Level: 17
Android 4.2 (
is an update to the Jelly Bean release that offers new features for users and app
developers. This document provides an introduction to the most notable and
useful new APIs for developers.
As an app developer, you should download the Android 4.2 system image and SDK platform from the SDK Manager as soon as possible. If you don’t have a device running Android 4.2 on which to test your app, use the Android 4.2 system image to test your app on the Android emulator. Then build your apps against the Android 4.2 platform to begin using the latest APIs.
To better optimize your app for devices running Android 4.2,
you should set your
"17", install it on an Android 4.2 system image,
test it, then publish an update with this change.
can use APIs in Android 4.2 while also supporting older versions by adding
conditions to your code that check for the system API level before executing
APIs not supported by your
To learn more about
maintaining backward-compatibility, read Creating Backward-Compatible
More information about how API levels work is available in What is API Level?
If you have previously published an app for Android, be aware of the following changes that might affect your app’s behavior:
android:exportedattribute is now
“false". If it’s important that other apps be able to access your content provider, you must now explicitly set
This change takes effect only if you set either
android:minSdkVersion to 17 or higher. Otherwise, the default value is still
even when running on Android 4.2 and higher.
ACCESS_COARSE_LOCATIONpermission but does not request the
To meet the privacy expectations of users when your app requests permission for coarse location (and not fine location), the system will not provide a user location estimate that’s more accurate than a city block.
Settings.Systemare now read-only. If your app attempts to write changes to settings defined in
Settings.Systemthat have moved to
Settings.Global, the write operation will silently fail when running on Android 4.2 and higher.
Even if your value for
android:minSdkVersion is lower than 17, your app is not able to modify the settings that have
Settings.Global when running on Android 4.2 and higher.
targetSdkVersionto 17 or higher, you must now add the
WebViewwhen running on Android 4.2 or higher. If you set the
targetSdkVersionto 16 or lower, the annotation is not required, but we recommend that you update your target version and add the annotation for additional security.
Daydream is a new interactive screensaver mode for Android devices. It activates automatically when the device is inserted into a dock or when the device is left idle while plugged in to a charger (instead of turning the screen off). Daydream displays one dream at a time, which may be a purely visual, passive display that dismisses upon touch, or may be interactive and responsive to the full suite of input events. Your dreams run in your app’s process and have full access to the Android UI toolkit, including views, layouts, and animations, so they are more flexible and powerful than either live wallpapers or app widgets.
You can create a dream for Daydream by implementing a subclass of
DreamService APIs are
designed to be similar to those of
Activity. To specify the UI for your
dream, pass a layout resource ID or
setContentView() at any point after you have
a window, such as from the
DreamService class provides other important lifecycle callback
methods on top of the base
Service APIs, such as
You cannot initiate a
DreamService from your
app—it is launched automatically by the system.
If your dream is interactive, you can start an activity from the dream to send the user into
your app’s full UI for more detail or control. You can use
finish() to end the dream so the user can see the
To make your daydream available to the system, declare your
DreamService with a
in your manifest file. You must then include an intent filter with the action
"android.service.dreams.DreamService". For example:
<service android:name=".MyDream" android:exported="true" android:icon="@drawable/dream_icon" android:label="@string/dream_label" > <intent-filter> <action android:name="android.service.dreams.DreamService" /> <category android:name="android.intent.category.DEFAULT" /> </intent-filter> </service>
There are some other useful methods in
to be aware of:
setInteractive(boolean)controls whether the dream receives input events or exits immediately upon user input. If the dream is interactive, the user may use the Back or Home buttons to exit the dream or you can call
finish()to stop the dream.
setFullscreen()to hide the status bar.
setScreenBright(true)allows you to instead set the display at its usual brightness.
For more information, see the
Android now allows your app to display unique content on additional screens that are connected
to the user’s device over either a wired connection or Wi-Fi.
To create unique content for a secondary display, extend the
class and implement the
onCreate() callback. Within
onCreate(), specify your UI for the secondary display
As an extension of the
Dialog class, the
Presentation class provides the region in which your app can display a unique UI on the
To detect secondary displays where you can display your
use either the
APIs. While the
DisplayManager APIs allow you to enumerate
multiple displays that may be connected at once, you should usually use
MediaRouter instead to quickly access the system’s default display for
To get the default display for your presentation, call
MediaRouter.getSelectedRoute() and pass it
ROUTE_TYPE_LIVE_VIDEO. This returns a
MediaRouter.RouteInfo object that describes the system’s currently selected route
for video presentations. If the
MediaRouter.RouteInfo is not null, call
getPresentationDisplay() to get the
Display representing the connected display.
To detect at runtime when a new display has been connected, create an instance of
MediaRouter.SimpleCallback in which you implement the
onRoutePresentationDisplayChanged() callback method, which the system will call when a new
presentation display is connected. Then register the
MediaRouter.SimpleCallback by passing it to
MediaRouter.addCallback() along with the
ROUTE_TYPE_LIVE_VIDEO route type. When you receive a call to
onRoutePresentationDisplayChanged(), simply call
MediaRouter.getSelectedRoute() as mentioned above.
To further optimize the UI in your
secondary screens, you can apply
a different theme by specifying the
android:presentationTheme attribute in the
<style> that you’ve
applied to your application or activity.
Keep in mind that screens connected to the user’s device often have a larger screen size and
likely a different screen density. Because the screen characteristics may different, you should
provide resources that are optimized specifically for such larger displays. If you need
to request additional resources from your
.getResources() to get the
Resources object corresponding to the display. This provides
the appropriate resources from your app that are best suited for the
secondary display's screen size and density.
For more information and some code samples, see the
Android now allows users to add app widgets to the lock screen. To make your App Widget available for use on the
lock screen, add the
android:widgetCategory attribute to your XML file that specifies the
AppWidgetProviderInfo. This attribute supports two values:
keyguard. By default, the attribute is set to
home_screen so users can add your
app widget to the Home screen. If you want your app widget to be also available on the lock
screen, add the
<appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android" ... android:widgetCategory="keyguard|home_screen"> </appwidget-provider>
You should also specify an initial layout for your app widget when on the lock screen with
android:initialKeyguardLayout attribute. This works the same way as the
android:initialLayout, in that it provides
a layout that can appear immediately until your app widget is initialized and able to update the
For more information about building app widgets for the lock screen, including to properly size your app widget when on the lock screen, see the App Widgets guide.
Android now allows multiple user spaces on shareable devices such as tablets. Each user on a device has his or her own set of accounts, apps, system settings, files, and any other user-associated data.
As an app developer, there’s nothing different you need to do in order for your app to work properly with multiple users on a single device. Regardless of how many users may exist on a device, the data your app saves for a given user is kept separate from the data your app saves for other users. The system keeps track of which user data belongs to the user process in which your app is running and provides your app access to only that user’s data and does not allow access to other users’ data.
Whenever your app saves user preferences, creates a database, or writes a file to the user’s internal or external storage space, that data is accessible only while running as that user.
To be certain that your app behaves properly in a multi-user environment, do not refer to your internal app directory or external storage location using hard-coded paths and instead always use the appropriate APIs:
No matter which of these APIs you use to save data for a given user, the data will not be accessible while running as a different user. From your app’s point of view, each user is running on a completely separate device.
If your app wants to identify unique users such as to gather analytics or create other account
associations, you should follow the recommended practices for identifying
unique installations. By creating a new
UUID when your app starts for the
first time, you’re certain to obtain a unique ID for tracking each user, regardless of how many
users install your app on a single device. Alternatively, you can save a local token fetched from
your server or use the registrations ID provided by Google Cloud Messaging.
Beware that if your app requests one of the hardware device identifiers (such as the WiFi MAC
address or the
SERIAL number), they will provide the same value for each
user because these identifiers are tied to the hardware and not the user. Not to mention the other
problems these identifiers introduce as discussed in the Identifying
App Installations blog post.
The system settings have been updated to support multiple users with the addition of
Settings.Global. This collection of settings is similar to
Settings.Secure settings because they are read-only, but applies globally across
all user spaces on the device.
Several existing settings were relocated here from either
Settings.Secure. If your app is
currently making changes to settings previously defined in
AIRPLANE_MODE_ON), then you should expect that
doing so will no longer work on a device running Android 4.2 or higher if those settings were
Settings.Global. You can continue to read settings that are in
Settings.Global, but because the settings are no longer considered safe
for apps to change, attempting to do so will fail silently and the system will write a warning to
the system log when running your app on Android 4.2 or higher.
Android now offers several APIs that allow you to build user interfaces that gracefully transform layout orientation to support languages that use right-to-left (RTL) UIs and reading direction, such as Arabic and Hebrew.
To begin supporting RTL layouts in your app, set the
android:supportsRtl attribute to the
<application> element in your manifest file
and set it
“true". Once you enable this, the system will enable various RTL APIs to
display your app with RTL layouts. For instance, the action bar will show the icon and title
on the right side and action buttons on the left, and any layouts you’ve created with the
View classes will also be reversed.
If you need to further optimize the appearance of your app when displayed with an RTL layout, there are two basic levels of optimization:
RelativeLayout class also provides the corresponding layout
attributes to replace left/right positions, such as
android:layout_toStartOf instead of
ldrtlresource qualifier (
ldrtlstands for layout-direction-right-to-left}). For example, you can save your default layout files in
res/layout/and your RTL optimized layouts in
ldrtl qualifier is great for drawable resources, so that you can provide
graphics that are oriented in the direction corresponding to the reading direction.
Various other APIs are available across the framework to support RTL layouts, such as in
View class so that you can implement the proper behaviors for custom
views and in
Configuration to query the current layout direction.
Note: If you are using SQlite and have tables or column names that are
“number only," be
String.format(String, Object...) can lead to errors where the numbers
have been converted to their Arabic equivalents if your device has been set to the Arabic locale.
You must use
String.format(Locale,String,Object...) to ensure numbers are
preserved as ASCII. Also use
String.format("%d", int) instead of using
You can now embed fragments inside fragments. This is useful for a variety of situations in
which you want to place dynamic and re-usable UI components into a UI component that is itself
dynamic and re-usable. For example, if you use
create fragments that swipe left and right and consume a majority of the screen space, you can
now insert fragments into each fragment page.
To nest a fragment, simply call
Fragment in which you want to add a fragment. This returns a
FragmentManager that you can use like you normally do from the top-level activity
to create fragment transactions. For example, here’s some code that adds a fragment from within
Fragment videoFragment = new VideoPlayerFragment(); FragmentTransaction transaction = getChildFragmentManager().beginTransaction(); transaction.add(R.id.video_fragment, videoFragment).commit();
From within a nested fragment, you can get a reference to the parent fragment by calling
The Android Support Library also now supports nested fragments, so you can implement nested fragment designs on Android 1.6 and higher.
Note: You cannot inflate a layout into a fragment when that layout
<fragment>. Nested fragments are only supported when added to a
Renderscript computation functionality has been enhanced with the following features:
You can use Renderscript's built-in script intrinsics that implement common operations for you such as:
Per-channel lookup table
Converting an Android YUV buffer to RGB
To use a script intrinsic, call the static
create() method of each instrinsic
to create an instance of the script. You then call the available
methods of each script intrinsic to set any necessary inputs and options.
Finally, call the
method to execute the script.
ScriptGroups allow you to chain together related Renderscript
scripts and execute them with one call.
ScriptGroup.Builder to add all of the scripts to the group
addKernel(). Once you
add all the scripts, create the connections between the
scripts by calling
When you are done adding the connections, call
to create the script group. Before executing the script group, specify the input
Allocation and initial script to run with the
setInput(Script.KernelID, Allocation) method and provide the output
Allocation where the result will be written to and final script to
setOutput(). Finally, call
execute() to run the script group.
Filterscript defines constraints on the existing Renderscript APIs that allow the resulting code to run
on a wider variety of processors (CPUs, GPUs, and DSPs). To create Filterscript files, create
files instead of
.rs files, and specify
#pragma rs_fp_relaxed to
tell the Renderscript runtime your scripts do not require strict IEEE 754-2008 floating point precision.
This precision allows flush-to-zero for denorms and round-towards-zero. In addition, your Filterscript
scripts must not use 32-bit built-in types and must specify a custom root function by using the
__attribute__((kernel)) attribute because Filterscript does not support pointers, which
the default signature of the
root() function defines.
Note: Although Filterscript support is in the platform, developer support will be available in SDK Tools Release 21.0.1.
For a detailed view of all API changes in Android 4.2, see the API Differences Report.