Check if a widget is exists on homescreen using appWidgetId

Question

I am using AlarmManager to update my widgets. And I want to stop it if there is no widget on homescreen. But I am facing a problem with detecting if there is no widget on home screen.

As whenever I try to get the AppWidgetIds using this way:

AppWidgetManager appWidgetManager = AppWidgetManager.getInstance(context);

int[] appWidgetIDs = appWidgetManager
    .getAppWidgetIds(new ComponentName(context, Widget.class));

I get the a length of appWidgetIDs while actually there is no widget on homescreen. Why?

Therefore, I would like to know if there is a way to detect that a widget id is exists on homescreen.

Thank you upfront.

Answer 1

Congratulations, you've encountered phantom appwidgets. It appears to be documented on the Android issue tracker . They usually occur when the configuration activity for an appwidget is canceled, though it seems to be through improper implementation of the configuration activity; developers neglect to include the appwidget ID as an extra when setting the activity result to RESULT_CANCELED . (even Google's ApiDemos sample application neglects to do this!)

The proper implementation is like this:

public class AppWidgetConfigActivity extends Activity {

    private int appWidgetId;
    private Intent resultValue;

    protected void onCreate(bundle saved) {
        super.onCreate(saved);

        // get the appwidget id from the intent
        Intent intent = getIntent();
        appWidgetId = intent.getIntExtra(AppWidgetManager.EXTRA_APPWIDGET_ID,
                AppWidgetManager.INVALID_APPWIDGET_ID);

        // make the result intent and set the result to canceled
        resultValue = new Intent();
        resultValue.putExtra(AppWidgetManager.EXTRA_APPWIDGET_ID, appWidgetId);
        setResult(RESULT_CANCELED, resultValue);

        // if we weren't started properly, finish here
        if (appwidgetId == AppWidgetManager.INVALID_APPWIDGET_ID) {
            finish();
        }

        /* ... */
    }

    /* ... */

    private void finishConfigure() {
        /* finish configuring appwidget ... */
        setResult(RESULT_OK, resultValue);
    }
}

Thus far I know of no way to detect the presence of a phantom appwidget without doing your own bookkeeping. I suggest storing a SharedPreferences value indicating that the configuration activity was not canceled and then querying this value in your other code. You can also use this information to "delete" a phantom widget if you come across one. In your appwidget configuration activity:

private void finishConfigure() {
    /* finish configuring appwidget ... */
    setResult(RESULT_OK, resultValue);

    String key = String.format("appwidget%d_configured", appwidgetId);
    SharedPreferences prefs = getSharedPreferences("widget_prefs", 0);
    prefs.edit().putBoolean(key, true).commit;
}

Then you can check that you have at least one non-phantom appwidget like so:

AppWidgetManager appWidgetManager = AppWidgetManager.getInstance(context);
AppWidgetHost appWidgetHost = new AppWidgetHost(context, 1); // for removing phantoms
SharedPreferences prefs = getSharedPreferences("widget_prefs", 0);
boolean hasWidget = false;

int[] appWidgetIDs = appWidgetManager.getAppWidgetIds(new ComponentName(context, Widget.class));
for (int i = 0; i < appWidgetIDs.length; i++) {
    int id = appWidgetIDs[i];
    String key = String.format("appwidget%d_configured", id);
    if (prefs.getBoolean(key, false)) {
        hasWidget = true;
    } else {
        // delete the phantom appwidget
        appWidgetHost.deleteAppWidgetId(id);
    }
}

if (hasWidget) {
    // proceed
} else {
    // turn off alarms
}