android - 如何从 BroadcastReceiver 更新 ListView？

Question

只更新 a 完全没问题TextView，但是当我尝试更新 a ListViewfromonReceive时，我一直NullPointerException从getView方法 ( BaseAdapter) 获取。我都尝试过 handler.post，runOnUiThread但问题仍然存在。我错过了什么？

public class MainActivity extends Activity {

    private SimpleBroadcastReceiver receiver;
    private List<User> users;
    private UserItemAdapter adapter;
    private ListView listview;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_test_service);
        setup();
        setupControls();
        setupViews();
    }

    private void setupViews() {
        users = new ArrayList<User>();
        User chan = new User();
        chan.setUsername("Username");
        users.add(chan);
        adapter = new UserItemAdapter(this, users);
        listview = (ListView) findViewById(R.id.listview);
        listview.setAdapter(adapter);
    }

    private void setupControls() {
        Button b = (Button) findViewById(R.id.button);
        b.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, SimpleIntentService.class);
                Toast.makeText(MainActivity.this, "Start Service", Toast.LENGTH_SHORT).show();
                startService(intent);
            }
        });
    }

    private void setup() {
        IntentFilter filter = new IntentFilter(SimpleBroadcastReceiver.ACTION_RESPONSE);
        filter.addCategory(Intent.CATEGORY_DEFAULT);
        receiver = new SimpleBroadcastReceiver(new Handler());
        registerReceiver(receiver, filter);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return true;
    }

    private void updateUi(final List<User> data) {
        this.runOnUiThread(new Runnable() {
            public void run() {
                for (User u : data) {
                    users.add(u);
                }
                adapter.notifyDataSetChanged();
            }
        });

    }

    public class SimpleBroadcastReceiver extends BroadcastReceiver {
        public static final String ACTION_RESPONSE = "com.csun.intent.action.MESSAGE_PROCESSED";
        private Handler handler;

        public SimpleBroadcastReceiver(Handler handler) {
            this.handler = handler;
        }

        @Override
        public void onReceive(Context context, Intent intent) {
            TextView t = (TextView) findViewById(com.csun.tutorplus.R.id.textview);
            t.setText("Hello");
            final ArrayList<User> data = (ArrayList<User>) intent.getSerializableExtra("users");
            updateUi(data);
        }
    }

}

用户项适配器

public class UserItemAdapter extends BaseAdapter {
    private static final String TAG = UserItemAdapter.class.getSimpleName();
    private List<User> users;
    private Context context;
    private LayoutInflater inflater;

    static class ViewHolder {
        ImageView photo;
        TextView username;
    }

    public UserItemAdapter(Context context, List<User> users) {
        this.context = context;
        this.users = users;
        this.inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    public int getCount() {
        return users.size();
    }

    public Object getItem(int position) {
        return users.get(position);
    }

    public long getItemId(int position) {
        return 0;
    }

    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder;
        if (convertView == null) {
            convertView = inflater.inflate(R.layout.item_user, null);
            holder = new ViewHolder();
            holder.photo = (ImageView) convertView.findViewById(R.id.item_user_XML_imageview_userphoto);
            holder.username = (TextView) convertView.findViewById(R.id.item_user_XML_textview_username);
        } else {
            holder = (ViewHolder) convertView.getTag();
        }
        holder.username.setText(users.get(position).getUsername());
        return convertView;
    }

}

错误日志

02-16 02:18:27.994: E/AndroidRuntime(26343): FATAL EXCEPTION: main
02-16 02:18:27.994: E/AndroidRuntime(26343): java.lang.NullPointerException
02-16 02:18:27.994: E/AndroidRuntime(26343):    at com.csun.tutorplus.UserItemAdapter.getView(UserItemAdapter.java:54)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.AbsListView.obtainView(AbsListView.java:1409)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.ListView.makeAndAddView(ListView.java:1793)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.ListView.fillSpecific(ListView.java:1338)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.ListView.layoutChildren(ListView.java:1624)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.AbsListView.onLayout(AbsListView.java:1260)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.view.View.layout(View.java:7277)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1254)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1130)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.LinearLayout.onLayout(LinearLayout.java:1047)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.view.View.layout(View.java:7277)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.FrameLayout.onLayout(FrameLayout.java:338)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.view.View.layout(View.java:7277)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1254)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1130)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.LinearLayout.onLayout(LinearLayout.java:1047)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.view.View.layout(View.java:7277)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.widget.FrameLayout.onLayout(FrameLayout.java:338)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.view.View.layout(View.java:7277)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.view.ViewRoot.performTraversals(ViewRoot.java:1203)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.view.ViewRoot.handleMessage(ViewRoot.java:1957)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.os.Handler.dispatchMessage(Handler.java:99)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.os.Looper.loop(Looper.java:150)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at android.app.ActivityThread.main(ActivityThread.java:4369)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at java.lang.reflect.Method.invokeNative(Native Method)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at java.lang.reflect.Method.invoke(Method.java:507)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:846)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:604)
02-16 02:18:27.994: E/AndroidRuntime(26343):    at dalvik.system.NativeStart.main(Native Method)