Android：从第二个活动返回时,在第一个活动中刷新ListView

当用户关闭一个活动并返回到上一个活动时,我遇到列表刷新问题.我发现这个问题很常见,但我无法解决.

我重写了onResume方法：

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

populateList()是一个方法,我用字符串列表填充listView：

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

问题是,当第二个活动关闭时,新项目将再次添加到ListView中,因此我将每个项目加倍.喜欢它没有刷新.

如果我在onResume()中放置notifyDataSetChanged(),它会抛出nullPointerException,因为当第一次启动活动时,第一次启动活动时没有初始化适配器.

我不知道如何处理这个问题.

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);
    }
}

好吧,你可以轻松地用一个简单的条件语句敲掉它,只有当适配器不为空时才执行命令：

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

但在我看来,在更深层次上,您的代码可能会在某种程度上重新考虑,以提高效率,但不一定更具功能性.

像这样做：

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();            
    }
}