Android API Guides 之 App Components（16） - App Widgets

App Widgets

IN THIS DOCUMENT

1. The Basics
2. Declaring an App Widget in the Manifest
3. Adding the AppWidgetProviderInfo Metadata
4. Creating the App Widget Layout
5. Using the AppWidgetProvider Class
   1. Receiving App Widget broadcast Intents
6. Creating an App Widget Configuration Activity
   1. Updating the App Widget from the Configuration Activity
7. Setting a Preview Image
8. Enabling App Widgets on the Lockscreen
   1. Sizing guidelines
9. Using App Widgets with Collections
   1. Sample application
   2. Implementing app widgets with collections
   3. Keeping Collection Data Fresh

KEY CLASSES

1. AppWidgetProvider

2. AppWidgetProviderInfo

3. AppWidgetManager

App Widgets are miniature application views that can be embedded in other applications (such as the Home screen) and receive periodic updates. These views are referred to as Widgets in the user interface, and you can publish one with an App Widget provider. An application component that is able to hold other App Widgets is called an App Widget host. The screenshot below shows the Music App Widget.

This document describes how to publish an App Widget using an App Widget provider. For a discussion of creating your own

AppWidgetHost

 to host app widgets, see App Widget Host.

Widget Design

For information about how to design your app widget, read theWidgets design guide.

The Basics

To create an App Widget, you need the following:

AppWidgetProviderInfo

 object

Describes the metadata for an App Widget, such as the App Widget's layout, update frequency, and the AppWidgetProvider class. This should be defined in XML.

AppWidgetProvider

 class implementation

Defines the basic methods that allow you to programmatically interface with the App Widget, based on broadcast events. Through it, you will receive broadcasts when the App Widget is updated, enabled, disabled and deleted.

View layout

Defines the initial layout for the App Widget, defined in XML.

Additionally, you can implement an App Widget configuration Activity. This is an optional 

Activity

 that launches when the user adds your App Widget and allows him or her to modify App Widget settings at create-time.

The following sections describe how to set up each of these components.

Declaring an App Widget in the Manifest

First, declare the 

AppWidgetProvider

 class in your application's 

AndroidManifest.xml

 file. For example:

<receiver android:name="ExampleAppWidgetProvider" >
    <intent-filter>
        <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />
    </intent-filter>
    <meta-data android:name="android.appwidget.provider"
               android:resource="@xml/example_appwidget_info" />
</receiver>      

The 

<receiver>

 element requires the 

android:name

 attribute, which specifies the 

AppWidgetProvider

 used by the App Widget.

The 

<intent-filter>

 element must include an 

<action>

 element with the 

android:name

 attribute. This attribute specifies that the 

AppWidgetProvider

 accepts the 

ACTION_APPWIDGET_UPDATE

 broadcast. This is the only broadcast that you must explicitly declare. The 

AppWidgetManager

 automatically sends all other App Widget broadcasts to the AppWidgetProvider as necessary.

The 

<meta-data>

 element specifies the 

AppWidgetProviderInfo

 resource and requires the following attributes:

• android:name

   - Specifies the metadata name. Use 

  android.appwidget.provider

   to identify the data as the 

  AppWidgetProviderInfo

   descriptor.

• android:resource

   - Specifies the 

  AppWidgetProviderInfo

   resource location.

Adding the AppWidgetProviderInfo Metadata

The 

AppWidgetProviderInfo

 defines the essential qualities of an App Widget, such as its minimum layout dimensions, its initial layout resource, how often to update the App Widget, and (optionally) a configuration Activity to launch at create-time. Define the AppWidgetProviderInfo object in an XML resource using a single

<appwidget-provider>

 element and save it in the project's 

res/xml/

 folder.

For example:

<appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android"
    android:minWidth="40dp"
    android:minHeight="40dp"
    android:updatePeriodMillis="86400000"
    android:previewImage="@drawable/preview"
    android:initialLayout="@layout/example_appwidget"
    android:configure="com.example.android.ExampleAppWidgetConfigure" 
    android:resizeMode="horizontal|vertical"
    android:widgetCategory="home_screen|keyguard"
    android:initialKeyguardLayout="@layout/example_keyguard">
</appwidget-provider>      

Here's a summary of the 

<appwidget-provider>

 attributes:

• The values for the 

  minWidth

   and 

  minHeight

   attributes specify the minimum amount of space the App Widget consumes by default. The default Home screen positions App Widgets in its window based on a grid of cells that have a defined height and width. If the values for an App Widget's minimum width or height don't match the dimensions of the cells, then the App Widget dimensions round up to the nearest cell size.

  See the App Widget Design Guidelines for more information on sizing your App Widgets.

  Note: To make your app widget portable across devices, your app widget's minimum size should never be larger than 4 x 4 cells.

• The 

  minResizeWidth

   and 

  minResizeHeight

   attributes specify the App Widget's absolute minimum size. These values should specify the size below which the App Widget would be illegible or otherwise unusable. Using these attributes allows the user to resize the widget to a size that may be smaller than the default widget size defined by the 

  minWidth

   and 

  minHeight

   attributes. Introduced in Android 3.1.

  See the App Widget Design Guidelines for more information on sizing your App Widgets.

• The 

  updatePeriodMillis

   attribute defines how often the App Widget framework should request an update from the 

  AppWidgetProvider

   by calling the 

  onUpdate()

   callback method. The actual update is not guaranteed to occur exactly on time with this value and we suggest updating as infrequently as possible—perhaps no more than once an hour to conserve the battery. You might also allow the user to adjust the frequency in a configuration—some people might want a stock ticker to update every 15 minutes, or maybe only four times a day.

  Note: If the device is asleep when it is time for an update (as defined by 

  updatePeriodMillis

  ), then the device will wake up in order to perform the update. If you don't update more than once per hour, this probably won't cause significant problems for the battery life. If, however, you need to update more frequently and/or you do not need to update while the device is asleep, then you can instead perform updates based on an alarm that will not wake the device. To do so, set an alarm with an Intent that your AppWidgetProvider receives, using the

  AlarmManager

  . Set the alarm type to either 

  ELAPSED_REALTIME

   or

  RTC

  , which will only deliver the alarm when the device is awake. Then set 

  updatePeriodMillis

   to zero (

  "0"

  ).
• The 

  initialLayout

   attribute points to the layout resource that defines the App Widget layout.
• The 

  configure

   attribute defines the 

  Activity

   to launch when the user adds the App Widget, in order for him or her to configure App Widget properties. This is optional (read Creating an App Widget Configuration Activity below).
• The 

  previewImage

   attribute specifies a preview of what the app widget will look like after it's configured, which the user sees when selecting the app widget. If not supplied, the user instead sees your application's launcher icon. This field corresponds to the 

  android:previewImage

   attribute in the 

  <receiver>

   element in the 

  AndroidManifest.xml

   file. For more discussion of using 

  previewImage

  , see Setting a Preview Image. Introduced in Android 3.0.
• The 

  autoAdvanceViewId

   attribute specifies the view ID of the app widget subview that should be auto-advanced by the widget's host. Introduced in Android 3.0.
• The 

  resizeMode

   attribute specifies the rules by which a widget can be resized. You use this attribute to make homescreen widgets resizeable—horizontally, vertically, or on both axes. Users touch-hold a widget to show its resize handles, then drag the horizontal and/or vertical handles to change the size on the layout grid. Values for the 

  resizeMode

   attribute include "horizontal", "vertical", and "none". To declare a widget as resizeable horizontally and vertically, supply the value "horizontal|vertical". Introduced in Android 3.1.
• The 

  minResizeHeight

   attribute specifies the minimum height (in dps) to which the widget can be resized. This field has no effect if it is greater than 

  minHeight

   or if vertical resizing isn't enabled (see 

  resizeMode

  ). Introduced in Android 4.0.
• The 

  minResizeWidth 

  attribute specifies the minimum width (in dps) to which the widget can be resized. This field has no effect if it is greater than 

  minWidth

   or if horizontal resizing isn't enabled (see 

  resizeMode

  ). Introduced in Android 4.0.
• The 

  widgetCategory

   attribute declares whether your App Widget can be displayed on the home screen, the lock screen (keyguard), or both. Values for this attribute include "home_screen" and "keyguard". A widget that is displayed on both needs to ensure that it follows the design guidelines for both widget classes. For more information, see Enabling App Widgets on the Lockscreen. The default value is "home_screen". Introduced in Android 4.2.
• The 

  initialKeyguardLayout

   attribute points to the layout resource that defines the lock screen App Widget layout. 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 layout. Introduced in Android 4.2.

See the 

AppWidgetProviderInfo

 class for more information on the attributes accepted by the 

<appwidget-provider>

 element.

Creating the App Widget Layout

You must define an initial layout for your App Widget in XML and save it in the project's 

res/layout/

 directory. You can design your App Widget using the View objects listed below, but before you begin designing your App Widget, please read and understand the App Widget Design Guidelines.

Creating the App Widget layout is simple if you're familiar with Layouts. However, you must be aware that App Widget layouts are based on 

RemoteViews

, which do not support every kind of layout or view widget.

A RemoteViews object (and, consequently, an App Widget) can support the following layout classes:

• FrameLayout

• LinearLayout

• RelativeLayout

• GridLayout

And the following widget classes:

• AnalogClock

• Button

• Chronometer

• ImageButton

• ImageView

• ProgressBar

• TextView

• ViewFlipper

• ListView

• GridView

• StackView

• AdapterViewFlipper

Descendants of these classes are not supported.

RemoteViews also supports 

ViewStub

, which is an invisible, zero-sized View you can use to lazily inflate layout resources at runtime.

Adding margins to App Widgets

Widgets should not generally extend to screen edges and should not visually be flush with other widgets, so you should add margins on all sides around your widget frame.

As of Android 4.0, app widgets are automatically given padding between the widget frame and the app widget's bounding box to provide better alignment with other widgets and icons on the user's home screen. To take advantage of this strongly recommended behavior, set your application's targetSdkVersion to 14 or greater.

It's easy to write a single layout that has custom margins applied for earlier versions of the platform, and has no extra margins for Android 4.0 and greater:

1. Set your application's 

   targetSdkVersion

    to 14 or greater.
2. Create a layout such as the one below, that references a dimension resource for its margins:

   <FrameLayout
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     android:padding="@dimen/widget_margin">
   
     <LinearLayout
       android:layout_width="match_parent"
       android:layout_height="match_parent"
       android:orientation="horizontal"
       android:background="@drawable/my_widget_background">
       …
     </LinearLayout>
   
   </FrameLayout>      

3. Create two dimensions resources, one in 

   res/values/

    to provide the pre-Android 4.0 custom margins, and one in 

   res/values-v14/

    to provide no extra padding for Android 4.0 widgets:

   res/values/dimens.xml:

   <dimen name="widget_margin">8dp</dimen>      

   res/values-v14/dimens.xml:

   <dimen name="widget_margin">0dp</dimen>      

Another option is to simply build extra margins into your nine-patch background assets by default, and provide different nine-patches with no margins for API level 14 or later.

Using the AppWidgetProvider Class

You must declare your AppWidgetProvider class implementation as a broadcast receiver using the 

<receiver>

element in the AndroidManifest (seeDeclaring an App Widget in the Manifest above).

The 

AppWidgetProvider

 class extends BroadcastReceiver as a convenience class to handle the App Widget broadcasts. The AppWidgetProvider receives only the event broadcasts that are relevant to the App Widget, such as when the App Widget is updated, deleted, enabled, and disabled. When these broadcast events occur, the AppWidgetProvider receives the following method calls:

onUpdate()

This is called to update the App Widget at intervals defined by the 

updatePeriodMillis

 attribute in the AppWidgetProviderInfo (see  Adding the AppWidgetProviderInfo Metadata above). This method is also called when the user adds the App Widget, so it should perform the essential setup, such as define event handlers for Views and start a temporary 

Service

, if necessary. However, if you have declared a configuration Activity,  this method is not called when the user adds the App Widget, but is called for the subsequent updates. It is the responsibility of the configuration Activity to perform the first update when configuration is done. (See  Creating an App Widget Configuration Activity below.)

onAppWidgetOptionsChanged()

This is called when the widget is first placed and any time the widget is resized. You can use this callback to show or hide content based on the widget's size ranges. You get the size ranges by calling

getAppWidgetOptions()

, which returns a 

Bundle

 that includes the following:

• OPTION_APPWIDGET_MIN_WIDTH

  —Contains the lower bound on the current width, in dp units, of a widget instance.

• OPTION_APPWIDGET_MIN_HEIGHT

  —Contains the lower bound on the current height, in dp units, of a widget instance.

• OPTION_APPWIDGET_MAX_WIDTH

  —Contains the upper bound on the current width, in dp units, of a widget instance.

• OPTION_APPWIDGET_MAX_HEIGHT

  —Contains the upper bound on the current width, in dp units, of a widget instance.

This callback was introduced in API Level 16 (Android 4.1). If you implement this callback, make sure that your app doesn't depend on it since it won't be called on older devices.

onDeleted(Context, int[])

This is called every time an App Widget is deleted from the App Widget host.

onEnabled(Context)

This is called when an instance the App Widget is created for the first time. For example, if the user adds two instances of your App Widget, this is only called the first time. If you need to open a new database or perform other setup that only needs to occur once for all App Widget instances, then this is a good place to do it.

onDisabled(Context)

This is called when the last instance of your App Widget is deleted from the App Widget host. This is where you should clean up any work done in 

onEnabled(Context)

, such as delete a temporary database.

onReceive(Context, Intent)

This is called for every broadcast and before each of the above callback methods. You normally don't need to implement this method because the default AppWidgetProvider implementation filters all App Widget broadcasts and calls the above methods as appropriate.

The most important AppWidgetProvider callback is 

onUpdate()

 because it is called when each App Widget is added to a host (unless you use a configuration Activity). If your App Widget accepts any user interaction events, then you need to register the event handlers in this callback. If your App Widget doesn't create temporary files or databases, or perform other work that requires clean-up, then 

onUpdate()

 may be the only callback method you need to define. For example, if you want an App Widget with a button that launches an Activity when clicked, you could use the following implementation of AppWidgetProvider:

public class ExampleAppWidgetProvider extends AppWidgetProvider {

    public void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds) {
        final int N = appWidgetIds.length;

        // Perform this loop procedure for each App Widget that belongs to this provider
        for (int i=0; i<N; i++) {
            int appWidgetId = appWidgetIds[i];

            // Create an Intent to launch ExampleActivity
            Intent intent = new Intent(context, ExampleActivity.class);
            PendingIntent pendingIntent = PendingIntent.getActivity(context, 0, intent, 0);

            // Get the layout for the App Widget and attach an on-click listener
            // to the button
            RemoteViews views = new RemoteViews(context.getPackageName(), R.layout.appwidget_provider_layout);
            views.setOnClickPendingIntent(R.id.button, pendingIntent);

            // Tell the AppWidgetManager to perform an update on the current app widget
            appWidgetManager.updateAppWidget(appWidgetId, views);
        }
    }
}      

This AppWidgetProvider defines only the 

onUpdate()

 method for the purpose of defining a 

PendingIntent

that launches an 

Activity

 and attaching it to the App Widget's button with 

setOnClickPendingIntent(int, PendingIntent)

. Notice that it includes a loop that iterates through each entry in 

appWidgetIds

, which is an array of IDs that identify each App Widget created by this provider. In this way, if the user creates more than one instance of the App Widget, then they are all updated simultaneously. However, only one 

updatePeriodMillis

schedule will be managed for all instances of the App Widget. For example, if the update schedule is defined to be every two hours, and a second instance of the App Widget is added one hour after the first one, then they will both be updated on the period defined by the first one and the second update period will be ignored (they'll both be updated every two hours, not every hour).

Note: Because 

AppWidgetProvider

 is an extension of 

BroadcastReceiver

, your process is not guaranteed to keep running after the callback methods return (see 

BroadcastReceiver

 for information about the broadcast lifecycle). If your App Widget setup process can take several seconds (perhaps while performing web requests) and you require that your process continues, consider starting a 

Service

 in the 

onUpdate()

method. From within the Service, you can perform your own updates to the App Widget without worrying about the AppWidgetProvider closing down due to an Application Not Responding (ANR) error. See theWiktionary sample's AppWidgetProvider for an example of an App Widget running a 

Service

.

Also see the ExampleAppWidgetProvider.java sample class.

Receiving App Widget broadcast Intents

AppWidgetProvider

 is just a convenience class. If you would like to receive the App Widget broadcasts directly, you can implement your own 

BroadcastReceiver

 or override the 

onReceive(Context, Intent)

callback. The Intents you need to care about are as follows:

• ACTION_APPWIDGET_UPDATE

• ACTION_APPWIDGET_DELETED

• ACTION_APPWIDGET_ENABLED

• ACTION_APPWIDGET_DISABLED

• ACTION_APPWIDGET_OPTIONS_CHANGED

Creating an App Widget Configuration Activity

If you would like the user to configure settings when he or she adds a new App Widget, you can create an App Widget configuration Activity. This 

Activity

 will be automatically launched by the App Widget host and allows the user to configure available settings for the App Widget at create-time, such as the App Widget color, size, update period or other functionality settings.

The configuration Activity should be declared as a normal Activity in the Android manifest file. However, it will be launched by the App Widget host with the 

ACTION_APPWIDGET_CONFIGURE

 action, so the Activity needs to accept this Intent. For example:

<activity android:name=".ExampleAppWidgetConfigure">
    <intent-filter>
        <action android:name="android.appwidget.action.APPWIDGET_CONFIGURE"/>
    </intent-filter>
</activity>      

Also, the Activity must be declared in the AppWidgetProviderInfo XML file, with the 

android:configure

attribute (see Adding the AppWidgetProviderInfo Metadata above). For example, the configuration Activity can be declared like this:

<appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android"
    ...
    android:configure="com.example.android.ExampleAppWidgetConfigure" 
    ... >
</appwidget-provider>      

Notice that the Activity is declared with a fully-qualified namespace, because it will be referenced from outside your package scope.

That's all you need to get started with a configuration Activity. Now all you need is the actual Activity. There are, however, two important things to remember when you implement the Activity:

• The App Widget host calls the configuration Activity and the configuration Activity should always return a result. The result should include the App Widget ID passed by the Intent that launched the Activity (saved in the Intent extras as 

  EXTRA_APPWIDGET_ID

  ).
• The 

  onUpdate()

   method will not be called when the App Widget is created (the system will not send the ACTION_APPWIDGET_UPDATE broadcast when a configuration Activity is launched). It is the responsibility of the configuration Activity to request an update from the AppWidgetManager when the App Widget is first created. However, 

  onUpdate()

   will be called for subsequent updates—it is only skipped the first time.

See the code snippets in the following section for an example of how to return a result from the configuration and update the App Widget.

Updating the App Widget from the configuration Activity

When an App Widget uses a configuration Activity, it is the responsibility of the Activity to update the App Widget when configuration is complete. You can do so by requesting an update directly from the

AppWidgetManager

.

Here's a summary of the procedure to properly update the App Widget and close the configuration Activity:

1. First, get the App Widget ID from the Intent that launched the Activity:

   Intent intent = getIntent();
   Bundle extras = intent.getExtras();
   if (extras != null) {
       mAppWidgetId = extras.getInt(
               AppWidgetManager.EXTRA_APPWIDGET_ID, 
               AppWidgetManager.INVALID_APPWIDGET_ID);
   }      

2. Perform your App Widget configuration.
3. When the configuration is complete, get an instance of the AppWidgetManager by calling

   getInstance(Context)

   :

   AppWidgetManager appWidgetManager = AppWidgetManager.getInstance(context);      

4. Update the App Widget with a 

   RemoteViews

    layout by calling 

   updateAppWidget(int, RemoteViews)

   :

   RemoteViews views = new RemoteViews(context.getPackageName(),
   R.layout.example_appwidget);
   appWidgetManager.updateAppWidget(mAppWidgetId, views);      

5. Finally, create the return Intent, set it with the Activity result, and finish the Activity:

   Intent resultValue = new Intent();
   resultValue.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, mAppWidgetId);
   setResult(RESULT_OK, resultValue);
   finish();      

Tip: When your configuration Activity first opens, set the Activity result to RESULT_CANCELED. This way, if the user backs-out of the Activity before reaching the end, the App Widget host is notified that the configuration was cancelled and the App Widget will not be added.

See the ExampleAppWidgetConfigure.java sample class in ApiDemos for an example.

Setting a Preview Image

Android 3.0 introduces the 

previewImage

 field, which specifies a preview of what the app widget looks like. This preview is shown to the user from the widget picker. If this field is not supplied, the app widget's icon is used for the preview.

This is how you specify this setting in XML:

<appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android"
  ...
  android:previewImage="@drawable/preview">
</appwidget-provider>      

To help create a preview image for your app widget (to specify in the 

previewImage

 field), the Android emulator includes an application called "Widget Preview." To create a preview image, launch this application, select the app widget for your application and set it up how you'd like your preview image to appear, then save it and place it in your application's drawable resources.

Enabling App Widgets on the Lockscreen

Android 4.2 introduces the ability for users to add widgets to the lock screen. To indicate that your app widget is available for use on the lock screen, declare the 

android:widgetCategory

 attribute in the XML file that specifies your 

AppWidgetProviderInfo

. This attribute supports two values: "home_screen" and "keyguard". An app widget can declare support for one or both.

By default, every app widget supports placement on the Home screen, so "home_screen" is the default value for the 

android:widgetCategory

 attribute. If you want your app widget to be available for the lock screen, add the "keyguard" value:

<appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android"
   ...
   android:widgetCategory="keyguard|home_screen">
</appwidget-provider>      

If you declare a widget to be displayable on both keyguard (lockscreen) and home, it's likely that you'll want to customize the widget depending on where it is displayed. For example, you might create a separate layout file for keyguard vs. home. The next step is to detect the widget category at runtime and respond accordingly. You can detect whether your widget is on the lockscreen or home screen by calling 

getAppWidgetOptions()

 to get the widget's options as a 

Bundle

. The returned bundle will include the key

OPTION_APPWIDGET_HOST_CATEGORY

, whose value will be one of 

WIDGET_CATEGORY_HOME_SCREEN

 or

WIDGET_CATEGORY_KEYGUARD

. This value is determined by the host into which the widget is bound. In the

AppWidgetProvider

, you can then check the widget's category, for example:

AppWidgetManager appWidgetManager;
int widgetId;
Bundle myOptions = appWidgetManager.getAppWidgetOptions (widgetId);

// Get the value of OPTION_APPWIDGET_HOST_CATEGORY
int category = myOptions.getInt(AppWidgetManager.OPTION_APPWIDGET_HOST_CATEGORY, -1);

// If the value is WIDGET_CATEGORY_KEYGUARD, it's a lockscreen widget
boolean isKeyguard = category == AppWidgetProviderInfo.WIDGET_CATEGORY_KEYGUARD;      

Once you know the widget's category, you can optionally load a different base layout, set different properties, and so on. For example:

int baseLayout = isKeyguard ? R.layout.keyguard_widget_layout : R.layout.widget_layout;      

You should also specify an initial layout for your app widget when on the lock screen with the

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 layout.

Sizing guidelines

When a widget is hosted on the lockscreen, the framework ignores the 

minWidth

, 

minHeight

,

minResizeWidth

, and 

minResizeHeight

 fields. If a widget is also a home screen widget, these parameters are still needed as they're still used on home, but they will be ignored for purposes of the lockscreen.

The width of a lockscreen widget always fills the provided space. For the height of a lockscreen widget, you have the following options:

• If the widget does not mark itself as vertically resizable (

  android:resizeMode="vertical"

  ), then the widget height will always be "small":
  • On a phone in portrait mode, "small" is defined as the space remaining when an unlock UI is being displayed.
  • On tablets and landscape phones, "small" is set on a per-device basis.
• If the widget marks itself as vertically resizable, then the widget height shows up as "small" on portrait phones displaying an unlock UI. In all other cases, the widget sizes to fill the available height.

Using App Widgets with Collections

Android 3.0 introduces app widgets with collections. These kinds of App Widgets use the

RemoteViewsService

 to display collections that are backed by remote data, such as from a content provider. The data provided by the 

RemoteViewsService

 is presented in the app widget using one of the following view types, which we’ll refer to as “collection views:”

ListView

A view that shows items in a vertically scrolling list. For an example, see the Gmail app widget.

GridView

A view that shows items in two-dimensional scrolling grid. For an example, see the Bookmarks app widget.

StackView

A stacked card view (kind of like a rolodex), where the user can flick the front card up/down to see the previous/next card, respectively. Examples include the YouTube and Books app widgets. 

AdapterViewFlipper

An adapter-backed simple 

ViewAnimator

 that animates between two or more views. Only one child is shown at a time.

As stated above, these collection views display collections backed by remote data. This means that they use an

Adapter

 to bind their user interface to their data. An 

Adapter

 binds individual items from a set of data into individual 

View

 objects. Because these collection views are backed by adapters, the Android framework must include extra architecture to support their use in app widgets. In the context of an app widget, the 

Adapter

 is replaced by a 

RemoteViewsFactory

, which is simply a thin wrapper around the 

Adapter

 interface. When requested for a specific item in the collection, the 

RemoteViewsFactory

 creates and returns the item for the collection as a 

RemoteViews

 object. In order to include a collection view in your app widget, you must implement 

RemoteViewsService

 and 

RemoteViewsFactory

.

RemoteViewsService

 is a service that allows a remote adapter to request 

RemoteViews

 objects.

RemoteViewsFactory

 is an interface for an adapter between a collection view (such as 

ListView

, 

GridView

, and so on) and the underlying data for that view. From the StackView Widget sample, here is an example of the boilerplate code you use to implement this service and interface:

public class StackWidgetService extends RemoteViewsService {
    @Override
    public RemoteViewsFactory onGetViewFactory(Intent intent) {
        return new StackRemoteViewsFactory(this.getApplicationContext(), intent);
    }
}

class StackRemoteViewsFactory implements RemoteViewsService.RemoteViewsFactory {

//... include adapter-like methods here. See the StackView Widget sample.

}      

Sample application

The code excerpts in this section are drawn from the StackView Widget sample:

This sample consists of a stack of 10 views, which display the values 

"0!"

 through 

"9!"

 The sample app widget has these primary behaviors:

• The user can vertically fling the top view in the app widget to display the next or previous view. This is a built-in StackView behavior.
• Without any user interaction, the app widget automatically advances through its views in sequence, like a slide show. This is due to the setting 

  android:autoAdvanceViewId="@id/stack_view"

   in the

  res/xml/stackwidgetinfo.xml

   file. This setting applies to the view ID, which in this case is the view ID of the stack view.
• If the user touches the top view, the app widget displays the 

  Toast

   message "Touched view n," where n is the index (position) of the touched view. For more discussion of how this is implemented, see Adding behavior to individual items.

Implementing app widgets with collections

To implement an app widget with collections, you follow the same basic steps you would use to implement any app widget. The following sections describe the additional steps you need to perform to implement an app widget with collections.

Manifest for app widgets with collections

In addition to the requirements listed in Declaring an app widget in the Manifest, to make it possible for app widgets with collections to bind to your 

RemoteViewsService

, you must declare the service in your manifest file with the permission 

BIND_REMOTEVIEWS

. This prevents other applications from freely accessing your app widget's data. For example, when creating an App Widget that uses 

RemoteViewsService

 to populate a collection view, the manifest entry may look like this:

<service android:name="MyWidgetService"
...
android:permission="android.permission.BIND_REMOTEVIEWS" />      

The line 

android:name="MyWidgetService"

 refers to your subclass of 

RemoteViewsService

.

Layout for app widgets with collections

The main requirement for your app widget layout XML file is that it include one of the collection views:

ListView

, 

GridView

, 

StackView

, or 

AdapterViewFlipper

. Here is the 

widget_layout.xml

 for the StackView Widget sample:

<?xml version="1.0" encoding="utf-8"?>

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <StackView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/stack_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:loopViews="true" />
    <TextView xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/empty_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:background="@drawable/widget_item_background"
        android:textColor="#ffffff"
        android:textStyle="bold"
        android:text="@string/empty_view_text"
        android:textSize="20sp" />
</FrameLayout>      

Note that empty views must be siblings of the collection view for which the empty view represents empty state.

In addition to the layout file for your entire app widget, you must create another layout file that defines the layout for each item in the collection (for example, a layout for each book in a collection of books). For example, the StackView Widget sample only has one layout file, 

widget_item.xml

, since all items use the same layout. But the WeatherListWidget sample has two layout files: 

dark_widget_item.xml

 and

light_widget_item.xml

.

AppWidgetProvider class for app widgets with collections

As with a regular app widget, the bulk of your code in your 

AppWidgetProvider

 subclass typically goes in

onUpdate()

. The major difference in your implementation for 

onUpdate()

 when creating an app widget with collections is that you must call 

setRemoteAdapter()

. This tells the collection view where to get its data. The

RemoteViewsService

 can then return your implementation of 

RemoteViewsFactory

, and the widget can serve up the appropriate data. When you call this method, you must pass an intent that points to your implementation of 

RemoteViewsService

 and the app widget ID that specifies the app widget to update.

For example, here's how the StackView Widget sample implements the 

onUpdate()

 callback method to set the

RemoteViewsService

 as the remote adapter for the app widget collection:

public void onUpdate(Context context, AppWidgetManager appWidgetManager,
int[] appWidgetIds) {
    // update each of the app widgets with the remote adapter
    for (int i = 0; i < appWidgetIds.length; ++i) {
        
        // Set up the intent that starts the StackViewService, which will
        // provide the views for this collection.
        Intent intent = new Intent(context, StackWidgetService.class);
        // Add the app widget ID to the intent extras.
        intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, appWidgetIds[i]);
        intent.setData(Uri.parse(intent.toUri(Intent.URI_INTENT_SCHEME)));
        // Instantiate the RemoteViews object for the app widget layout.
        RemoteViews rv = new RemoteViews(context.getPackageName(), R.layout.widget_layout);
        // Set up the RemoteViews object to use a RemoteViews adapter. 
        // This adapter connects
        // to a RemoteViewsService  through the specified intent.
        // This is how you populate the data.
        rv.setRemoteAdapter(appWidgetIds[i], R.id.stack_view, intent);
        
        // The empty view is displayed when the collection has no items. 
        // It should be in the same layout used to instantiate the RemoteViews
        // object above.
        rv.setEmptyView(R.id.stack_view, R.id.empty_view);

        //
        // Do additional processing specific to this app widget...
        //
        
        appWidgetManager.updateAppWidget(appWidgetIds[i], rv);   
    }
    super.onUpdate(context, appWidgetManager, appWidgetIds);
}      

RemoteViewsService class

Persisting data

You can’t rely on a single instance of your service, or any data it contains, to persist. You should therefore not store any data in your

RemoteViewsService

 (unless it is static). If you want your app widget’s data to persist, the best approach is to use a 

ContentProvider

 whose data persists beyond the process lifecycle.

As described above, your 

RemoteViewsService

 subclass provides the 

RemoteViewsFactory

 used to populate the remote collection view.

Specifically, you need to perform these steps:

1. Subclass 

   RemoteViewsService

   . 

   RemoteViewsService

    is the service through which a remote adapter can request

   RemoteViews

   .
2. In your 

   RemoteViewsService

    subclass, include a class that implements the 

   RemoteViewsFactory

    interface.

   RemoteViewsFactory

    is an interface for an adapter between a remote collection view (such as 

   ListView

   , 

   GridView

   , and so on) and the underlying data for that view. Your implementation is responsible for making a 

   RemoteViews

    object for each item in the data set. This interface is a thin wrapper around 

   Adapter

   .

The primary contents of the 

RemoteViewsService

 implementation is its 

RemoteViewsFactory

, described below.

RemoteViewsFactory interface

Your custom class that implements the 

RemoteViewsFactory

 interface provides the app widget with the data for the items in its collection. To do this, it combines your app widget item XML layout file with a source of data. This source of data could be anything from a database to a simple array. In the StackView Widget sample, the data source is an array of 

WidgetItems

. The 

RemoteViewsFactory

 functions as an adapter to glue the data to the remote collection view.

The two most important methods you need to implement for your 

RemoteViewsFactory

 subclass are

onCreate()

 and 

getViewAt()

 .

The system calls 

onCreate()

 when creating your factory for the first time. This is where you set up any connections and/or cursors to your data source. For example, the StackView Widget sample uses 

onCreate()

to initialize an array of 

WidgetItem

 objects. When your app widget is active, the system accesses these objects using their index position in the array and the text they contain is displayed

Here is an excerpt from the StackView Widget sample's 

RemoteViewsFactory

 implementation that shows portions of the 

onCreate()

 method:

class StackRemoteViewsFactory implements
RemoteViewsService.RemoteViewsFactory {
    private static final int mCount = 10;
    private List<WidgetItem> mWidgetItems = new ArrayList<WidgetItem>();
    private Context mContext;
    private int mAppWidgetId;

    public StackRemoteViewsFactory(Context context, Intent intent) {
        mContext = context;
        mAppWidgetId = intent.getIntExtra(AppWidgetManager.EXTRA_APPWIDGET_ID,
                AppWidgetManager.INVALID_APPWIDGET_ID);
    }

    public void onCreate() {
        // In onCreate() you setup any connections / cursors to your data source. Heavy lifting,
        // for example downloading or creating content etc, should be deferred to onDataSetChanged()
        // or getViewAt(). Taking more than 20 seconds in this call will result in an ANR.
        for (int i = 0; i < mCount; i++) {
            mWidgetItems.add(new WidgetItem(i + "!"));
        }
        ...
    }
...      

The 

RemoteViewsFactory

 method 

getViewAt()

 returns a 

RemoteViews

 object corresponding to the data at the specified 

position

 in the data set. Here is an excerpt from the StackView Widget sample's

RemoteViewsFactory

 implementation:

public RemoteViews getViewAt(int position) {
   
    // Construct a remote views item based on the app widget item XML file, 
    // and set the text based on the position.
    RemoteViews rv = new RemoteViews(mContext.getPackageName(), R.layout.widget_item);
    rv.setTextViewText(R.id.widget_item, mWidgetItems.get(position).text);

    ...
    // Return the remote views object.
    return rv;
}      

Adding behavior to individual items

The above sections show you how to bind your data to your app widget collection. But what if you want to add dynamic behavior to the individual items in your collection view?

As described in Using the AppWidgetProvider Class, you normally use 

setOnClickPendingIntent()

 to set an object's click behavior—such as to cause a button to launch an 

Activity

. But this approach is not allowed for child views in an individual collection item (to clarify, you could use 

setOnClickPendingIntent()

 to set up a global button in the Gmail app widget that launches the app, for example, but not on the individual list items). Instead, to add click behavior to individual items in a collection, you use 

setOnClickFillInIntent()

. This entails setting up up a pending intent template for your collection view, and then setting a fill-in intent on each item in the collection via your 

RemoteViewsFactory

.

This section uses the StackView Widget sample to describe how to add behavior to individual items. In theStackView Widget sample, if the user touches the top view, the app widget displays the 

Toast

 message "Touched view n," where n is the index (position) of the touched view. This is how it works:

• The 

  StackWidgetProvider

   (an 

  AppWidgetProvider

   subclass) creates a pending intent that has a custom action called 

  TOAST_ACTION

  .
• When the user touches a view, the intent is fired and it broadcasts 

  TOAST_ACTION

  .
• This broadcast is intercepted by the 

  StackWidgetProvider

  's 

  onReceive()

   method, and the app widget displays the 

  Toast

   message for the touched view. The data for the collection items is provided by the

  RemoteViewsFactory

  , via the 

  RemoteViewsService

  .

Note: The StackView Widget sample uses a broadcast, but typically an app widget would simply launch an activity in a scenario like this one.

Setting up the pending intent template

The 

StackWidgetProvider

 (

AppWidgetProvider

 subclass) sets up a pending intent. Individuals items of a collection cannot set up their own pending intents. Instead, the collection as a whole sets up a pending intent template, and the individual items set a fill-in intent to create unique behavior on an item-by-item basis.

This class also receives the broadcast that is sent when the user touches a view. It processes this event in its

onReceive()

 method. If the intent's action is 

TOAST_ACTION

, the app widget displays a 

Toast

 message for the current view.

public class StackWidgetProvider extends AppWidgetProvider {
    public static final String TOAST_ACTION = "com.example.android.stackwidget.TOAST_ACTION";
    public static final String EXTRA_ITEM = "com.example.android.stackwidget.EXTRA_ITEM";

    ...

    // Called when the BroadcastReceiver receives an Intent broadcast.
    // Checks to see whether the intent's action is TOAST_ACTION. If it is, the app widget 
    // displays a Toast message for the current item.
    @Override
    public void onReceive(Context context, Intent intent) {
        AppWidgetManager mgr = AppWidgetManager.getInstance(context);
        if (intent.getAction().equals(TOAST_ACTION)) {
            int appWidgetId = intent.getIntExtra(AppWidgetManager.EXTRA_APPWIDGET_ID,
                AppWidgetManager.INVALID_APPWIDGET_ID);
            int viewIndex = intent.getIntExtra(EXTRA_ITEM, 0);
            Toast.makeText(context, "Touched view " + viewIndex, Toast.LENGTH_SHORT).show();
        }
        super.onReceive(context, intent);
    }
    
    @Override
    public void onUpdate(Context context, AppWidgetManager appWidgetManager, int[] appWidgetIds) {
        // update each of the app widgets with the remote adapter
        for (int i = 0; i < appWidgetIds.length; ++i) {
    
            // Sets up the intent that points to the StackViewService that will
            // provide the views for this collection.
            Intent intent = new Intent(context, StackWidgetService.class);
            intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, appWidgetIds[i]);
            // When intents are compared, the extras are ignored, so we need to embed the extras
            // into the data so that the extras will not be ignored.
            intent.setData(Uri.parse(intent.toUri(Intent.URI_INTENT_SCHEME)));
            RemoteViews rv = new RemoteViews(context.getPackageName(), R.layout.widget_layout);
            rv.setRemoteAdapter(appWidgetIds[i], R.id.stack_view, intent);
    
            // The empty view is displayed when the collection has no items. It should be a sibling
            // of the collection view.
            rv.setEmptyView(R.id.stack_view, R.id.empty_view);

            // This section makes it possible for items to have individualized behavior.
            // It does this by setting up a pending intent template. Individuals items of a collection
            // cannot set up their own pending intents. Instead, the collection as a whole sets
            // up a pending intent template, and the individual items set a fillInIntent
            // to create unique behavior on an item-by-item basis.
            Intent toastIntent = new Intent(context, StackWidgetProvider.class);
            // Set the action for the intent.
            // When the user touches a particular view, it will have the effect of
            // broadcasting TOAST_ACTION.
            toastIntent.setAction(StackWidgetProvider.TOAST_ACTION);
            toastIntent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, appWidgetIds[i]);
            intent.setData(Uri.parse(intent.toUri(Intent.URI_INTENT_SCHEME)));
            PendingIntent toastPendingIntent = PendingIntent.getBroadcast(context, 0, toastIntent,
                PendingIntent.FLAG_UPDATE_CURRENT);
            rv.setPendingIntentTemplate(R.id.stack_view, toastPendingIntent);
            
            appWidgetManager.updateAppWidget(appWidgetIds[i], rv);
        }
    super.onUpdate(context, appWidgetManager, appWidgetIds);
    }
}      

Setting the fill-in Intent

Your 

RemoteViewsFactory

 must set a fill-in intent on each item in the collection. This makes it possible to distinguish the individual on-click action of a given item. The fill-in intent is then combined with the

PendingIntent

 template in order to determine the final intent that will be executed when the item is clicked.

public class StackWidgetService extends RemoteViewsService {
    @Override
    public RemoteViewsFactory onGetViewFactory(Intent intent) {
        return new StackRemoteViewsFactory(this.getApplicationContext(), intent);
    }
}

class StackRemoteViewsFactory implements RemoteViewsService.RemoteViewsFactory {
    private static final int mCount = 10;
    private List<WidgetItem> mWidgetItems = new ArrayList<WidgetItem>();
    private Context mContext;
    private int mAppWidgetId;

    public StackRemoteViewsFactory(Context context, Intent intent) {
        mContext = context;
        mAppWidgetId = intent.getIntExtra(AppWidgetManager.EXTRA_APPWIDGET_ID,
                AppWidgetManager.INVALID_APPWIDGET_ID);
    }

    // Initialize the data set.
        public void onCreate() {
            // In onCreate() you set up any connections / cursors to your data source. Heavy lifting,
            // for example downloading or creating content etc, should be deferred to onDataSetChanged()
            // or getViewAt(). Taking more than 20 seconds in this call will result in an ANR.
            for (int i = 0; i < mCount; i++) {
                mWidgetItems.add(new WidgetItem(i + "!"));
            }
           ...
        }
        ...
    
        // Given the position (index) of a WidgetItem in the array, use the item's text value in 
        // combination with the app widget item XML file to construct a RemoteViews object.
        public RemoteViews getViewAt(int position) {
            // position will always range from 0 to getCount() - 1.
    
            // Construct a RemoteViews item based on the app widget item XML file, and set the
            // text based on the position.
            RemoteViews rv = new RemoteViews(mContext.getPackageName(), R.layout.widget_item);
            rv.setTextViewText(R.id.widget_item, mWidgetItems.get(position).text);
    
            // Next, set a fill-intent, which will be used to fill in the pending intent template
            // that is set on the collection view in StackWidgetProvider.
            Bundle extras = new Bundle();
            extras.putInt(StackWidgetProvider.EXTRA_ITEM, position);
            Intent fillInIntent = new Intent();
            fillInIntent.putExtras(extras);
            // Make it possible to distinguish the individual on-click
            // action of a given item
            rv.setOnClickFillInIntent(R.id.widget_item, fillInIntent);
        
            ...
        
            // Return the RemoteViews object.
            return rv;
        }
    ...
    }      

Keeping Collection Data Fresh

The following figure illustrates the flow that occurs in an app widget that uses collections when updates occur. It shows how the app widget code interacts with the 

RemoteViewsFactory

, and how you can trigger updates:

One feature of app widgets that use collections is the ability to provide users with up-to-date content. For example, consider the Android 3.0 Gmail app widget, which provides users with a snapshot of their inbox. To make this possible, you need to be able to trigger your 

RemoteViewsFactory

 and collection view to fetch and display new data. You achieve this with the 

AppWidgetManager

 call 

notifyAppWidgetViewDataChanged()

. This call results in a callback to your 

RemoteViewsFactory

’s 

onDataSetChanged()

 method, which gives you the opportunity to fetch any new data. Note that you can perform processing-intensive operations synchronously within the 

onDataSetChanged()

 callback. You are guaranteed that this call will be completed before the metadata or view data is fetched from the 

RemoteViewsFactory

. In addition, you can perform processing-intensive operations within the 

getViewAt()

 method. If this call takes a long time, the loading view (specified by the 

RemoteViewsFactory

’s 

getLoadingView()

 method) will be displayed in the corresponding position of the collection view until it returns.