Android：从第二个活动返回时，在第一个活动中刷新ListView

Question

I have a problem with list refresh when user closes one activity and returns to previous. I see that problem is very common but I can't solve it.

I overridden onResume method:

@Override
public void onResume() {
    super.onResume();
    populateList();
}

populateList() is a method where I populate listView with list of Strings:

arrayAdapter = new CustomArrayAdapter(this, R.layout.symbol_item,list);
listView.setAdapter(arrayAdapter);

The problem is that when second activity is closed, new items are just added again in the ListView so I have every item doubled. Like it's not refreshed.

If I put notifyDataSetChanged() in onResume() it throws me nullPointerException because when activity is started first time there is no adapter initialized when activity is first time started.

I'm not sure how to handle this.

public class testActivity extends Activity {


    private int id=1;
    private ListView listView;
    private CustomArrayAdapter arrayAdapter;
    private ArrayList<String> list = new ArrayList<String>();
    ArrayList<Item> objectList = new ArrayList<Item>();

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_test);

    }

    @Override
    public void onResume() {
        super.onResume();
        populateList();
    }

    private void populateList() {
        try {
            objectList = new GetAsyncTask(id).execute();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ExecutionException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (TimeoutException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        int size = objectList.size();
        String name;

        for (int i = 0; i < size; i++) {
            name = objectList.get(i).getName();
            list.add(name);
        }

        arrayAdapter = new CustomArrayAdapter(this, R.layout.symbol_item,
                list);
        listView.setAdapter(arrayAdapter);
    }
}

Answer 1

well right off the bat, you could easily knock this away with a simple conditional statement that performs the command only if the adapter isn't null :

    if (adapter != null) {
        adapter.notifyDataSetChanged();
    }

But this seems to me like, at a deeper level, your code could be re-factored somewhat to be more efficient, though not necessarily more functional.

---

do it like this:

private int id = 1;
private ListView listView;
private CustomArrayAdapter arrayAdapter;
private ArrayList<String> list = new ArrayList<String>();

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_test);
}

@Override
public void onResume() {
    super.onResume();
    populateList();
}

private void populateList() {

    ArrayList<Item> objectList;
    try {
        objectList = new GetAsyncTask(id).execute();
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (ExecutionException e) {
        e.printStackTrace();
    } catch (TimeoutException e) {
        e.printStackTrace();
    }

    list.clear();
    for (int i = 0; i <objectList.size(); i++) {
        String name = objectList.get(i).getName();
        list.add(name);
    }
    if (arrayAdapter == null) {
        arrayAdapter = new CustomArrayAdapter(this, R.layout.symbol_item, list);
        listView.setAdapter(arrayAdapter);
    } else {
        arrayAdapter.notifyDataSetChanged();            
    }
}

Answer 2

I catch the list_of_files from device and put it in a list in populateList(). Then I apply adapter. On return that list on device is changed so it again does the same and put it in a list. The problem is in ListView there are still old items and new items are just added at the end. So it's duplicated.

One basic approach is to call list.clear() before adding the new data. This should erase the old data and prevent duplicates. (But it's hard to provide an exact answer without seeing the code in question...)

---

Addition
You should add this code to your onPostExecute() method inside GetAsyncTask :

int size = objectList.size();
String name;

list.clear(); // or list = new ArrayList<String>();
for (int i = 0; i < size; i++) {
    name = objectList.get(i).getName();
    list.add(name);
}

arrayAdapter = new CustomArrayAdapter(this, R.layout.symbol_item, list);
listView.setAdapter(arrayAdapter);

Unless GetAsyncTask is nested in your Activity, you will need to move a few variables to GetAsyncTask use this. But this approach is better because it doesn't force the Activity to wait for results (which might cause an "Application Not Responding" error.)

Answer 3

You are only showing part of the app code, so I may be offbase here, but I'm guessing you are using a Cursor to return the list variable and use list to create an ArrayAdapter...

If you want the data to be 'fresh', why not use a CursorLoader with the LoaderManager? There are several benefits to the LoaderManager (like moving the query off of the UI thread), and the LoaderManager does a bit of the heavy lifting for you when it comes to monitoring the DB for changes.

There is a blog post that I've found quite helpful - http://www.androiddesignpatterns.com/2012/07/understanding-loadermanager.html

This is all assuming you are pulling the data from a database...

Answer 4

The problem is that you are calling the function populateList() without clearing the list first. That way populate list simply just adds to the previous content.

This would be a right way :

public void onResume() {
super.onResume();
list = new ArrayList<String>();
populateList();  }

I am also not sure as to why you have set the adapter inside the function populateList()

You should try this.

    private ArrayList<String> list = new ArrayList<String>();

    private ArrayList<String> populateList() {
            try {
                objectList = new GetAsyncTask(id).execute();
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (ExecutionException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (TimeoutException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            int size = objectList.size();
            String name;

            for (int i = 0; i < size; i++) {
                name = objectList.get(i).getName();
                list.add(name);
            }
 return list;
        }
     arrayAdapter = new CustomArrayAdapter(this, R.layout.symbol_item,
                    populateList());
            listView.setAdapter(arrayAdapter);

That way by having the adapter out you can simply have this in the onResume() function

public void onResume() {
    super.onResume();
    list = new ArrayList<String>();
    }