从 OnOptionsItemSelected 更新片段中的 ListView

Question

I have a HomeActivity which extends Activity that contains Actionbar items. The HomeActivity has 1 fragment (StatusFragment which extends Fragment). In the Fragment there is a ListView which uses a custom ArrayAdapter and a method call to supply the data.

private ParseUser[] GetUsers(){
    final ParseQuery<ParseUser> query = ParseUser.getQuery();
    ParseUser[] usersArray;

    try {
        List<ParseUser> users = query.find();
        usersArray = users.toArray(new ParseUser[users.size()]);
    } catch (ParseException e) {
        usersArray = null;
        e.printStackTrace();
    }
    return usersArray;
}

I'm having trouble getting the ListView to update from the OnOptionsItemSelected callback.

case R.id.home_ab_refresh:
    StatusFragment pFrag = (StatusFragment) getFragmentManager().findFragmentByTag("mFragment");
    pFrag.users = pFrag.GetUsers();
    pFrag.mAdapter.notifyDataSetChanged();
    return true;

1) Is this an appropriate way to access the Fragment from the Actionbar items (HomeActivity)?

2) Is there a better way to design this code?

Thanks much!

Answer 1

Re 1) I probably wouldn't do a findFragmentByTag() every time, and instead just stick the fragment into a member variable of the activity during the activity's onCreate() .

The main issue with the code is something else:

   pFrag.users = pFrag.GetUsers();
   pFrag.mAdapter.notifyDataSetChanged();

Here you violate the object-oriented design principle of loose coupling . The HomeActivity is too intimately bound to the implementation details of the StatusFragment . What you should do instead is move that code into the fragment and expose it as a single, public method that is named for the intent (goal, purpose) of the action, not its implementation .

   // In HomeActivity
   pFrag.reloadData();

   // In the fragment
   public void reloadData() {
       this.users = pFrag.GetUsers();
       this.mAdapter.notifyDataSetChanged();
   }

This way, it's easier to reuse the status fragment elsewhere. More importantly, it's easier to evolve that fragment, you can now completely change the internals without having to change the host activity. This is cleaner from a design perspective.

Re 2) Aside from the issue I already mentioned, you should consider returning an empty array rather than null when an exception occurs. It's generally a better idea to return empty array/collection from finder methods, because people tend to immediately use the result for an iterator or an addAll() or something like that, without null-checking it first.

Answer 2

First of all, you dont make a nullpointer check, since you cant be certain that the FragmentManager will actually return a validFragment.

you can however just catch the onOptionsMenuSelected event in the fragment itself, which will result in a much more capsulated code.

Besides that, when do you refresh the ListView? Wouldnt it make sense to update the listview automatically once the new data has arrived?