使用notifyDataSetChanged（）时，Android API ListView不会更新;

Question

I'm sorry to write about what seems to be a common problem, but I've already been on this one task for five hours, and I simply give up.

I'm making a small android app, all it's supposed to do right now is keep a small SQLite database and be able to write/edit notes on it. Everything works fine with SQL, my problem is with the ListView. No matter what I do, it just refuses to update when I add a new note. The only way to view the new note is when I enter edit mode in another and come back out.

Here's the relevant part of the source:

public class MainActivity extends Activity {

ArrayAdapter<NoteItem> mAdapter;
public static NoteDataBase ndb;
ListView listView;
ArrayList a;

    @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ndb = new NoteDataBase(this);
    a = ndb.getAllNotes();

            //LISTVIEW DO NOT TOUCH
/***********/   mAdapter = new ArrayAdapter<NoteItem>(this, 
/***********/       android.R.layout.simple_list_item_1, a);
/***********/       listView = (ListView) findViewById(R.id.listView1);
/***********/   listView.setAdapter(mAdapter);
            //*****************

}

//New Note button
public void newNoteAction(View view){
    ndb.addNote(new NoteItem("Title", "Contents"));
    a = ndb.getAllNotes();
    mAdapter = new ArrayAdapter<NoteItem>
                                (this, android.R.layout.simple_list_item_1, a);
    mAdapter.notifyDataSetChanged();
    System.out.println(ndb.getAllNotes().size());
}

The print at the end is just to see if it's actually creating the new note.

I hope I gave enough info, tell me if you need anything else. I tried almost everything that I read already, but after almost five hours on one task, there's a point where I'm gonna go crazy :)

Thanks,

Shef

Answer 1

public void newNoteAction(View view){
    ndb.addNote(new NoteItem("Title", "Contents"));
    // don't do this
    // you are creating a new adapter and not binding it back to listview
    // a = ndb.getAllNotes(); 
    // mAdapter = new ArrayAdapter<NoteItem>(this, android.R.layout.simple_list_item_1, a);
    mAdapter.add(new NoteItem("Title", "Contents"));
    mAdapter.notifyDataSetChanged();
    System.out.println(ndb.getAllNotes().size());
}

Answer 2

Looks like the problem is that you're creating a new instance of ArrayAdapter and adding to it, which isn't associated with the ListView .

notifyDataSetChanged() will send events to whatever is listening - the ListView is listening to the old adapter.

Answer 3

It seems like you wanted to update the adapter but instead you're creating a new adapter. Here, if you just call listView.setAdapter(mAdapter) again, it would set this new adapter to the ListView and it would work.

But creating a new adapter every time isnt a good way to do it. Since you're using a database, try reading up on CursorAdapter .