Wednesday, September 5, 2012

Android SharedPreferences gotcha

I have another gotcha for you. Can you tell what's wrong with the following code?

public class MyFragment extends Fragment implements OnSharedPreferenceChangeListener {
  private TextView mInfo;
  private SharedPreferences mPreferences;

  public static final String INFO_SP_KEY = "info";
  
  @Override
  public View onCreateView(LayoutInflater inflater, ViewGroup container,
      Bundle savedInstanceState) {
    return inflater.inflate(R.layout.my_fragment, container, false);
  }

  @Override
  public void onViewCreated(View view, Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    mInfo = (TextView) view.findViewById(R.id.info);
  }
  
  @Override
  public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
    if (key.equals(INFO_SP_KEY)) {
      updateInfo();
    }
  }

  @Override
  public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);
    mPreferences = PreferenceManager.getDefaultSharedPreferences(getActivity());
  }

  @Override
  public void onPause() {
    super.onPause();
    mPreferences.unregisterOnSharedPreferenceChangeListener(this);
  }

  @Override
  public void onResume() {
    super.onResume();
    mPreferences.registerOnSharedPreferenceChangeListener(this);
    updateInfo();
  }

  protected void updateInfo() {
    mInfo.setText(getString(R.string.info_text, mPreferences.getInt(INFO_SP_KEY, 0)));
  }
}
At first glance everything looks fine and in most cases it will work fine as well. However, if you a) set android:minSdkVersion to 8 or lower and b) change the shared preference from another thread (IntentService, SyncAdapter, etc.), you'll get the following crash:

09-05 07:16:58.993: E/AndroidRuntime(403): android.view.ViewRoot$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
09-05 07:16:58.993: E/AndroidRuntime(403): at android.view.ViewRoot.checkThread(ViewRoot.java:2802)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.view.ViewRoot.requestLayout(ViewRoot.java:594)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.view.View.requestLayout(View.java:8125)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.view.View.requestLayout(View.java:8125)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.view.View.requestLayout(View.java:8125)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.view.View.requestLayout(View.java:8125)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.view.View.requestLayout(View.java:8125)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.view.View.requestLayout(View.java:8125)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.view.View.requestLayout(View.java:8125)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.widget.TextView.checkForRelayout(TextView.java:5378)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.widget.TextView.setText(TextView.java:2688)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.widget.TextView.setText(TextView.java:2556)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.widget.TextView.setText(TextView.java:2531)
09-05 07:16:58.993: E/AndroidRuntime(403): at com.porcupineprogrammer.sharedpreferencesgotcha.BaseFragment.updateButtonText(BaseFragment.java:65)
09-05 07:16:58.993: E/AndroidRuntime(403): at com.porcupineprogrammer.sharedpreferencesgotcha.WrongFragment.onSharedPreferenceChanged(WrongFragment.java:12)
09-05 07:16:58.993: E/AndroidRuntime(403): at android.app.ContextImpl$SharedPreferencesImpl$EditorImpl.commit(ContextImpl.java:2830)
09-05 07:16:58.993: E/AndroidRuntime(403): at com.porcupineprogrammer.sharedpreferencesgotcha.BaseFragment$1$1.run(BaseFragment.java:36)
09-05 07:16:58.993: E/AndroidRuntime(403): at java.lang.Thread.run(Thread.java:1096)

Fortunately the obvious crashlog is obvious and you can solve this issue in about 5 seconds, by wrapping the UI action in Activity.runOnUiThread() method. Morbidly curious may track the root cause of this issue in GrepCode. Tl;dr: before Gingerbread the listeners are notified in the same thread as the SharedPreferences.commit() caller, in later releases commit() ensures the notifications are performed in UI thread.

Code of the sample application that demonstrates this issue is available on my github.