Displaying ProgressDialog within a PreferenceActivity

I've got a quite interesting issue when I try to display a ProgressDialog (the simple, spinner type) within a onPreferenceChange listener.

public class SettingsActivity extends PreferenceActivity {
  private ProgressDialog dialog;

  public void onCreate(Bundle savedInstanceState) {
    ListPreference pref= (ListPreference) findPreference("myPreference");  
    pref.setOnPreferenceChangeListener(new OnPreferenceChangeListener() {

      @Override
      public boolean onPreferenceChange(Preference preference, Object newValue) 开发者_如何学JAVA{
        dialog = ProgressDialog.show(SettingsActivity.this, "", "Doing stuff...", true);
        SystemClock.sleep(2000);
     }
    return true;
  }
}

The ProgressDialog shows up, but not until the method (sleep in this case) has finished. What am I doing wrong?

You're sleeping on the main UI thread, which stops the operating system from handling your application's events. This will stop your app from redrawing and as you discovered, can prevent new windows from actually appearing.

Instead of sleeping, try this:

final ProgressDialog dialog = ProgressDialog.show(
    SettingsActivity.this, "", "Doing stuff...", true);
new Handler().postDelayed(new Runnable() {
    public void run() {
      dialog.hide();
    }
  }, 2000);

You can use AsyncTask to run the function in a separate thread (see http://developer.android.com/resources/articles/painless-threading.html)

This is probably a bit unnecessary if you just want to invoke a sleep method, but should work even for other methods that otherwise would block the UI thread.

You could do something like this:

private class BackgroundTask extends AsyncTask<Integer, Integer, void> {
    private ProgressDialog dialog;

    @Override
    protected void onPreExecute() {
        dialog = ProgressDialog.show(SettingsActivity.this, "", "Doing stuff...", true);
    }

    @Override
    protected void doInBackground(Integer... params) {
        sleep(params[0]);
    }

    @Override
    protected void onPostExecute(Boolean result) {
        dialog.dismiss();
    }
 }

And than call it using the code below:

new BackgroundTask().execute(2000);