The Android Home screen available on most Android devices allows the user
to embed app widgets for quick
access to content. If you're building a Home replacement or a similar app,
you can also allow the user to embed app widgets by implementing an
AppWidgetHost
.
This is not something that most apps will ever need to do, but if you are
creating your own host, it's important to understand the contractual obligations
a host implicitly agrees to.
This document focuses on the responsibilities involved in implementing a custom
AppWidgetHost
. For an example of how to implement an
AppWidgetHost
, see the source code for the
Android Home screen
Launcher.
Here is an overview of key classes and concepts involved in implementing a custom
AppWidgetHost
:
- App Widget Host—
The
AppWidgetHost
provides the interaction with the AppWidget service for apps, like the home screen, that want to embed app widgets in their UI. AnAppWidgetHost
must have an ID that is unique within the host's own package. This ID remains persistent across all uses of the host. The ID is typically a hard-coded value that you assign in your application. - App Widget ID—
Each app widget instance is assigned a unique ID at the time of binding
(see
bindAppWidgetIdIfAllowed()
, discussed in more detail in Binding app widgets). The unique ID is obtained by the host usingallocateAppWidgetId()
. This ID is persistent across the lifetime of the widget, that is, until it is deleted from the host. Any host-specific state (such as the size and location of the widget) should be persisted by the hosting package and associated with the app widget ID. - App Widget Host View—
AppWidgetHostView
can be thought of as a frame that the widget is wrapped in whenever it needs to be displayed. An app widget is assigned to anAppWidgetHostView
every time the widget is inflated by the host. - Options Bundle—
The
AppWidgetHost
uses the options bundle to communicate information to theAppWidgetProvider
about how the widget is being displayed (for example, size range, and whether the widget is on a lockscreen or the home screen). This information allows theAppWidgetProvider
to tailor the widget's contents and appearance based on how and where it is displayed. You useupdateAppWidgetOptions()
andupdateAppWidgetSize()
to modify an app widget's bundle. Both of these methods trigger a callback to theAppWidgetProvider
.
Binding App Widgets
When a user adds an app widget to a host, a process called
binding occurs. Binding refers to associating
a particular app widget ID to a specific host and to a specific
AppWidgetProvider
. There are different
ways of achieving this, depending on what version of Android your
app is running on.
Binding app widgets on Android 4.0 and lower
On devices running Android version 4.0 and lower, users add app widgets
via a system activity that allows users to select a widget. This implicitly
does a permission check—that is, by adding the app widget, the user is
implicitly granting permission to your app to add app widgets to the host.
Here is an example that illustrates
this approach, taken from the original
Launcher. In this snippet, an event handler invokes
startActivityForResult()
with the request code REQUEST_PICK_APPWIDGET
in response to a
user action:
private static final int REQUEST_CREATE_APPWIDGET = 5; private static final int REQUEST_PICK_APPWIDGET = 9; ... public void onClick(DialogInterface dialog, int which) { switch (which) { ... case AddAdapter.ITEM_APPWIDGET: { ... int appWidgetId = Launcher.this.mAppWidgetHost.allocateAppWidgetId(); Intent pickIntent = new Intent(AppWidgetManager.ACTION_APPWIDGET_PICK); pickIntent.putExtra (AppWidgetManager.EXTRA_APPWIDGET_ID, appWidgetId); ... startActivityForResult(pickIntent, REQUEST_PICK_APPWIDGET); break; } ... }
When the system activity finishes, it returns a result with the user's chosen
app widget to your activity. In the following example, the activity responds
by calling addAppWidget()
to add the app widget:
public final class Launcher extends Activity implements View.OnClickListener, OnLongClickListener { ... @Override protected void onActivityResult(int requestCode, int resultCode, Intent data) { mWaitingForResult = false; if (resultCode == RESULT_OK && mAddItemCellInfo != null) { switch (requestCode) { ... case REQUEST_PICK_APPWIDGET: addAppWidget(data); break; case REQUEST_CREATE_APPWIDGET: completeAddAppWidget(data, mAddItemCellInfo, !mDesktopLocked); break; } } ... } }
The method addAppWidget()
checks to see if the app widget
needs to be configured before it's added:
void addAppWidget(Intent data) { int appWidgetId = data.getIntExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, -1); String customWidget = data.getStringExtra(EXTRA_CUSTOM_WIDGET); AppWidgetProviderInfo appWidget = mAppWidgetManager.getAppWidgetInfo(appWidgetId); if (appWidget.configure != null) { // Launch over to configure widget, if needed. Intent intent = new Intent(AppWidgetManager.ACTION_APPWIDGET_CONFIGURE); intent.setComponent(appWidget.configure); intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, appWidgetId); startActivityForResult(intent, REQUEST_CREATE_APPWIDGET); } else { // Otherwise, finish adding the widget. } }
For more discussion of configuration, see Creating an App Widget Configuration Activity.
Once the app widget is ready, the next step is to do the
actual work of adding it to the workspace. The
original Launcher uses a method called completeAddAppWidget()
to do this.
Binding app widgets on Android 4.1 and higher
Android 4.1 adds APIs for a more streamlined binding process.
These APIs also make it possible for a host to provide a custom UI for
binding. To use this improved process, your app must declare the
BIND_APPWIDGET
permission in its manifest:
<uses-permission android:name="android.permission.BIND_APPWIDGET" />
But this is just the first step. At runtime the user must
explicitly grant permission to your app to allow it to add app widgets
to the host. To test whether your app has permission to add the widget,
you use the
bindAppWidgetIdIfAllowed()
method.
If bindAppWidgetIdIfAllowed()
returns false
, your app must display a dialog prompting the
user to grant permission
("allow" or "always allow," to cover all future app widget additions).
This snippet gives an example of how to display the dialog:
Intent intent = new Intent(AppWidgetManager.ACTION_APPWIDGET_BIND); intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, appWidgetId); intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_PROVIDER, info.componentName); // This is the options bundle discussed above intent.putExtra(AppWidgetManager.EXTRA_APPWIDGET_OPTIONS, options); startActivityForResult(intent, REQUEST_BIND_APPWIDGET);
The host also has to check whether the user added an app widget that needs configuration. For more discussion of this topic, see Creating an App Widget Configuration Activity.
Host Responsibilities
What Version are You Targeting?
The approach you use in implementing your host should depend on what Android version you're targeting. Many of the features described in this section were introduced in 3.0 or later. For example:
- Android 3.0 (API Level 11) introduces auto-advance behavior for widgets.
- Android 3.1 (API Level 12) introduces the ability to resize widgets.
- Android 4.0 (API Level 15) introduces a change in padding policy that puts the responsibility on the host to manage padding.
- Android 4.1 (API Level 16) adds an API that allows the widget provider to get more detailed information about the environment in which its widget instances are being hosted.
- Android 4.2 (API Level 17) introduces the options bundle and the
bindAppWidgetIdIfAllowed()
method. It also introduces lockscreen widgets.
If you are targeting earlier devices, refer to the original Launcher as an example.
Widget developers can specify a number of configuration settings
for widgets using the
AppWidgetProviderInfo metadata.
These configuration options, discussed in more detail below, can be
retrieved by the host from the AppWidgetProviderInfo
object associated with a widget provider.
Regardless of the version of Android you are targeting, all hosts have the following responsibilities:
- When adding a widget, you must allocate the widget ID as described above.
You must also make sure that when a widget is removed from the host, you call
deleteAppWidgetId()
to deallocate the widget ID. - When adding a widget, be sure to launch its configuration activity if it exists, as described in Updating the App Widget from the Configuration Activity. This is a necessary step for many app widgets before they can be properly displayed.
- Every app widget specifies a minimum width and height in dps, as defined in the
AppWidgetProviderInfo
metadata (usingandroid:minWidth
andandroid:minHeight
). Make sure that the widget is laid out with at least this many dps. For example, many hosts align icons and widgets in a grid. In this scenario, by default the host should add the app widget using the minimum number of cells that satisfy theminWidth
andminHeight
constraints.
In addition to the requirements listed above, specific platform versions introduce features that place new responsibilities on the host. These are described in the following sections.
Android 3.0
Android 3.0 (API Level 11) introduces the ability for a widget to specify autoAdvanceViewId()
.
This view ID should point to an instance of an
Advanceable
, such as StackView
or AdapterViewFlipper
. This indicates that the host
should call advance()
on this
view at an interval deemed appropriate by the host (taking into account whether
it makes sense to advance the widget—for example, the host probably
wouldn't want to advance a widget if it were on another page, or
if the screen were turned off).
Android 3.1
Android 3.1 (API Level 12) introduces the ability to resize widgets.
A widget can specify that it is resizable using the
android:resizeMode
attribute in the AppWidgetProviderInfo
metadata, and indicate whether it supports horizontal and/or
vertical resizing. Introduced in Android 4.0 (API Level 14), the widget can also specify a
android:minResizeWidth
and/or android:minResizeHeight
.
It is the host’s responsibility to make it possible for the
widget to be resized horizontally and/or vertically, as specified
by the widget. A widget that specifies that it is resizable can be
resized arbitrarily large, but should not be resized smaller than
the values specified by android:minResizeWidth
and android:minResizeHeight
.
For a sample implementation, see
AppWidgetResizeFrame
in Launcher2
.
Android 4.0
Android 4.0 (API Level 15) introduces a change in padding policy that
puts the responsibility on the host to manage padding. As of 4.0, app
widgets no longer include their own padding. Instead, the system adds
padding for each widget, based the characteristics of the current screen.
This leads to a more uniform, consistent presentation of widgets in a grid.
To assist applications that host app widgets, the platform provides
the method
getDefaultPaddingForWidget()
.
Applications can call this method to get the system-defined padding
and account for it when computing the number of cells to allocate to the widget.
Android 4.1
Android 4.1 (API Level 16) adds an API that allows the widget provider to get more detailed information about the environment in which its widget instances are being hosted. Specifically, the host hints to the widget provider about the size at which the widget is being displayed. It is the host’s responsibility to provide this size information.
The host provides this information via
updateAppWidgetSize()
.
The size is specified as a minimum and maximum width/height in dps.
The reason that a range is specified (as opposed to a fixed size)
is because the width and height of a widget may change with orientation.
You don’t want the host to have to update all of its widgets on rotation,
as this could cause serious system slowdown. These values should be
updated once upon the widget being placed, any time the widget is resized,
and any time the launcher inflates the widget for the first time in a
given boot (as the values aren’t persisted across boot).
Android 4.2
Android 4.2 (API Level 17) adds the ability for the options bundle
to be specified at bind time. This is the ideal way to specify app
widget options, including size, as it gives the AppWidgetProvider
immediate access to the options
data on the first update. This can be achieved by using the method bindAppWidgetIdIfAllowed()
. For more discussion of this topic,
see Binding app widgets.
Android 4.2 also introduces lockscreen widgets. When hosting widgets
on the lockscreen, the host must specify this information within the app
widget options bundle (the AppWidgetProvider
can use this information to style
the widget appropriately). To designate a widget as a lockscreen widget, use updateAppWidgetOptions()
and include the field
OPTION_APPWIDGET_HOST_CATEGORY
with the value WIDGET_CATEGORY_KEYGUARD
.
This option defaults to
WIDGET_CATEGORY_HOME_SCREEN
,
so it is not explicitly required to set this for a home screen host.
Make sure that your host adds only app widgets that are appropriate
for your app—for example, if your host is a home screen, ensure
that the
android:widgetCategory
attribute in the
AppWidgetProviderInfo
metadata includes
the flag WIDGET_CATEGORY_HOME_SCREEN
.
Similarly, for the lockscreen, ensure that field includes the flag WIDGET_CATEGORY_KEYGUARD
. For more
discussion of this topic, see
Enabling App Widgets on the Lockscreen.