docs/html/guide/topics/appwidgets/host.jd

page.title=App Widget Host
page.tags=AppWidgetHost,home screen,launcher
@jd:body

<div id="qv-wrapper">
  <div id="qv">

    <h2>In this document</h2>
        <ol>
          <li><a href="#host-binding">Binding App Widgets</a>
          <ol>
            <li><a href="#binding-pre">Binding app widgets on Android 4.0 and lower</a></li>
            <li><a href="#binding-41">Binding app widgets on Android 4.1 and higher</a></li>
         </ol>
          </li>
          <li><a href="#host-state">Host Responsibilities</a>
          <ol>
            <li><a href="#30">Android 3.0</a></li>
            <li><a href="#31">Android 3.1</a></li>
            <li><a href="#40">Android 4.0</a></li>
            <li><a href="#41">Android 4.1</li>
            <li><a href="#42">Android 4.2</a></li>
         </ol>
         </li>
      </ol>
  </div>
</div>


<p>The Android Home screen available on most Android devices allows the user
to embed <a href="{@docRoot}guide/topics/appwidgets/index.html">app widgets</a> 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
{@link android.appwidget.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.</p>

<p>This document focuses on the responsibilities involved in implementing a custom
{@link android.appwidget.AppWidgetHost}. For an example of how to implement an
{@link android.appwidget.AppWidgetHost}, see the source code for the
Android Home screen
<a href="https://android.googlesource.com/platform/packages/apps/Launcher2/+/master/src/com/android/launcher2/Launcher.java">
Launcher</a>.


<p>Here is an overview of key classes and concepts involved in implementing a custom
{@link android.appwidget.AppWidgetHost}:</p>
<ul>
    <li><strong>App Widget Host</strong>&mdash;
    The {@link android.appwidget.AppWidgetHost} provides the interaction
with the AppWidget service for apps, like the home screen, that want to embed
app widgets in their UI. An {@link android.appwidget.AppWidgetHost} 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.</li>

    <li><strong>App Widget ID</strong>&mdash;
    Each app widget instance is assigned a unique ID at the time of binding
(see {@link android.appwidget.AppWidgetManager#bindAppWidgetIdIfAllowed bindAppWidgetIdIfAllowed()},
discussed in more detail in <a href="#binding">Binding app widgets</a>).
The unique ID is obtained by the host using {@link android.appwidget.AppWidgetHost#allocateAppWidgetId() allocateAppWidgetId()}. 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.
</li>

    <li><strong>App Widget Host View</strong>&mdash;
    {@link android.appwidget.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 an {@link android.appwidget.AppWidgetHostView} every time the
widget is inflated by the host. </li>
    <li><strong>Options Bundle</strong>&mdash;
The {@link android.appwidget.AppWidgetHost} uses the options bundle to communicate
information to the {@link android.appwidget.AppWidgetProvider} 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 the
{@link android.appwidget.AppWidgetProvider} to tailor the widget's contents
and appearance based on how and where it is  displayed.
You use
{@link android.appwidget.AppWidgetHostView#updateAppWidgetOptions(android.os.Bundle) updateAppWidgetOptions()}
and
{@link android.appwidget.AppWidgetHostView#updateAppWidgetSize updateAppWidgetSize()}

to modify an app widget's
bundle. Both of these methods trigger a callback to the
{@link android.appwidget.AppWidgetProvider}.</p></li>
</ul>

<h2 id="host-binding">Binding App Widgets</h2>

<p>When a user adds an app widget to a host, a process called
<em>binding</em> occurs. <em>Binding</em> refers to associating
a particular app widget ID to a specific host and to a specific
{@link android.appwidget.AppWidgetProvider}. There are different
ways of achieving this, depending on what version of Android your
app is running on.</p>

<h3 id="binding-pre">Binding app widgets on Android 4.0 and lower</h3>

<p>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&mdash;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
<a href="https://android.googlesource.com/platform/packages/apps/Launcher/+/master/src/com/android/launcher/Launcher.java">Launcher</a>. In this snippet, an event handler invokes
{@link android.app.Activity#startActivityForResult(android.content.Intent,int) startActivityForResult()}
with the request code {@code REQUEST_PICK_APPWIDGET} in response to a
user action:</p>

<pre>
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;
    }
    ...
}</pre>

<p>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 {@code addAppWidget()} to add the app widget:</p>

<pre>public final class Launcher extends Activity
        implements View.OnClickListener, OnLongClickListener {
    ...
    &#64;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;
                }
        }
        ...
    }
}</pre>

<p>The method {@code addAppWidget()} checks to see if the app widget
needs to be configured before it's added:</p>

<pre>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.
    }
}</pre>

<p>For more discussion of configuration,
see <a href="{@docRoot}guide/topics/appwidgets/index.html#Configuring">Creating an
App Widget Configuration Activity</a>.</p>

<p>Once the app widget is ready, the next step is to do the
actual work of adding it to the workspace. The
<a href="https://android.googlesource.com/platform/packages/apps/Launcher/+/master/src/com/android/launcher/Launcher.java">original Launcher</a> uses a method called {@code completeAddAppWidget()}
to do this.</p>

<h3 id="binding-41">Binding app widgets on Android 4.1 and higher</h3>

<p>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
{@link android.Manifest.permission#BIND_APPWIDGET} permission in its manifest:</p>

<pre>&lt;uses-permission android:name="android.permission.BIND_APPWIDGET" /&gt;
</pre>


<p>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
{@link android.appwidget.AppWidgetManager#bindAppWidgetIdIfAllowed bindAppWidgetIdIfAllowed()}
method.
If {@link android.appwidget.AppWidgetManager#bindAppWidgetIdIfAllowed bindAppWidgetIdIfAllowed()}
returns {@code 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:</p>

<pre>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);
</pre>

<p>The host also has to check whether the user added
an app widget that needs configuration. For more discussion of this topic,
see
<a href="{@docRoot}guide/topics/appwidgets/index.html#Configuring">Creating
an App Widget Configuration Activity</a>.</p>

<h2 id="host-state">Host Responsibilities</h2>

<div class="sidebox-wrapper">
<div class="sidebox">
  <h2>What Version are You Targeting?</h2>
  <p>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:</p>
<ul>
<li>Android 3.0 (API Level 11) introduces auto-advance behavior for widgets.</li>
<li>Android 3.1 (API Level 12) introduces the ability to resize widgets.</li>
<li>Android 4.0 (API Level 15) introduces a change in padding policy that
puts the responsibility on the
host to manage padding.</li>
<li>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.</li>
<li>Android 4.2 (API Level 17) introduces the options bundle and the
{@link android.appwidget.AppWidgetManager#bindAppWidgetIdIfAllowed(int,android.content.ComponentName,android.os.Bundle) bindAppWidgetIdIfAllowed()}
method. It also introduces lockscreen widgets.</li>
</ul>
<p>If you are targeting earlier devices, refer to the original
<a href="https://android.googlesource.com/platform/packages/apps/Launcher/+/master/src/com/android/launcher/Launcher.java">Launcher</a> as an example.
</div>
</div>

<p>Widget developers can specify a number of configuration settings
for widgets using the <a href="{@docRoot}guide/topics/appwidgets/index.html#MetaData">
AppWidgetProviderInfo metadata</a>.
These configuration options, discussed in more detail below, can be
retrieved by the host from the {@link android.appwidget.AppWidgetProviderInfo}
object associated with a widget provider.</p>

<p>Regardless of the version of Android you are targeting, all hosts
have the following responsibilities:</p>

<ul>
<li>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 {@link android.appwidget.AppWidgetHost#deleteAppWidgetId deleteAppWidgetId()}
to deallocate the widget ID.</li>

<li>When adding a widget, be sure to launch its configuration activity
if it exists, as described in
<a href="{@docRoot}guide/topics/appwidgets/index.html#UpdatingFromTheConfiguration">
Updating the App Widget
from the Configuration Activity</a>. This is a necessary step for many app widgets before
they can be properly displayed.</li>

<li>Every app widget specifies a minimum width and height in dps, as defined in the {@link android.appwidget.AppWidgetProviderInfo} metadata
(using {@link android.appwidget.AppWidgetProviderInfo#minWidth android:minWidth} and
{@link android.appwidget.AppWidgetProviderInfo#minHeight android: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 the {@code minWidth} and {@code minHeight} constraints.</li>

</ul>

<p>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.</p>

<h3 id="30">Android 3.0</h3>

<p>Android 3.0 (API Level 11) introduces the ability for a widget to specify {@link android.appwidget.AppWidgetProviderInfo#autoAdvanceViewId autoAdvanceViewId()}.
This view ID should point to an instance of an
{@link android.widget.Advanceable}, such as {@link android.widget.StackView}
or {@link android.widget.AdapterViewFlipper}. This indicates that the host
should call {@link android.widget.Advanceable#advance advance()} on this
view at an interval deemed appropriate by the host (taking into account whether
it makes sense to advance the widget&mdash;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).</p>

<h3 id="31">Android 3.1</h3>

<p>Android 3.1 (API Level 12) introduces the ability to resize widgets.
A widget can specify that it is resizable using the
{@link android.appwidget.AppWidgetProviderInfo#resizeMode android:resizeMode}
attribute in the {@link android.appwidget.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
{@link android.appwidget.AppWidgetProviderInfo#minResizeWidth android:minResizeWidth}
and/or {@link android.appwidget.AppWidgetProviderInfo#minResizeHeight android:minResizeHeight}.</p>

<p>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 {@link
android.appwidget.AppWidgetProviderInfo#minResizeWidth android:minResizeWidth}
and {@link
android.appwidget.AppWidgetProviderInfo#minResizeHeight android:minResizeHeight}.
For a sample implementation, see  <a href="https://android.googlesource.com/platform/packages/apps/Launcher2/+/master/src/com/android/launcher2/AppWidgetResizeFrame.java">
{@code AppWidgetResizeFrame}</a> in {@code Launcher2}.</p>


<h3 id="40">Android 4.0</h3>

<p>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
{@link android.appwidget.AppWidgetHostView#getDefaultPaddingForWidget 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.</p>

<h3 id="41">Android 4.1</h3>

<p>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.</p>

<p>The host provides this information via
{@link android.appwidget.AppWidgetHostView#updateAppWidgetSize 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).</p>


<h3 id="42">Android 4.2</h3>

<p>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 {@link
android.appwidget.AppWidgetProvider} immediate access to the options
data on the first update. This can be achieved by using the method {@link android.appwidget.AppWidgetManager#bindAppWidgetIdIfAllowed(int,android.content.ComponentName,android.os.Bundle) bindAppWidgetIdIfAllowed()}. For more discussion of this topic,
see <a href="#host-binding">Binding app widgets</a>.</p>

<p>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 {@link
android.appwidget.AppWidgetProvider} can use this information to style
the widget appropriately). To designate a widget as a lockscreen widget, use {@link android.appwidget.AppWidgetHostView#updateAppWidgetOptions updateAppWidgetOptions()}
and include the field
{@link android.appwidget.AppWidgetManager#OPTION_APPWIDGET_HOST_CATEGORY}
with the value {@link android.appwidget.AppWidgetProviderInfo#WIDGET_CATEGORY_KEYGUARD}.
This option defaults to
{@link android.appwidget.AppWidgetProviderInfo#WIDGET_CATEGORY_HOME_SCREEN},
so it is not explicitly required to set this for a home screen host.</p>

<p>Make sure that your host adds only app widgets that are appropriate
for your app&mdash;for example, if your host is a home screen, ensure
that the
{@link android.appwidget.AppWidgetProviderInfo#widgetCategory android:widgetCategory}
attribute in the
{@link android.appwidget.AppWidgetProviderInfo} metadata includes
the flag {@link android.appwidget.AppWidgetProviderInfo#WIDGET_CATEGORY_HOME_SCREEN}.
Similarly, for the lockscreen, ensure that field includes the flag  {@link android.appwidget.AppWidgetProviderInfo#WIDGET_CATEGORY_KEYGUARD}. For more
discussion of this topic, see
<a href="{@docRoot}guide/topics/appwidgets/index.html#lockscreen">
Enabling App Widgets on the Lockscreen</a>.</p>