我的应用程序能够从 GCM 接收消息并将消息保存到手机上的 SQLlite 数据库。这些消息在具有列表视图的活动中是可见的。
在下面的代码中，onPause() 函数刷新了 listView。这不是一个好的实现，因为它仅在更新时未显示活动时才有效。如果活动在更新时显示，则列表是静态的并且不会更新。
问题：
显示活动时如何更新列表视图？或者有没有办法使用后台服务来更新适配器，这样每当显示活动时，它总是显示最新的数据。
这种功能目前在 android 上是不可能的，我需要实现一些其他的东西，比如“pull-to-refresh”？
在 OnResume() 中刷新 listview 会使应用程序崩溃，并显示空指针异常。
活动：
public class NotesView extends Activity implements OnItemClickListener {
ListView listView;
NoteAdapter objAdapter;
NotificationsDatabase db = new NotificationsDatabase(this);
List<Notes> listAlerts;
String note;
String time;
TextView noteView;
TextView timeView;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.note);
listView = (ListView) findViewById(R.id.notelist);
listView.setOnItemClickListener(this);
noteView = (TextView) findViewById(R.id.noteDisplay);
timeView = (TextView) findViewById(R.id.notetimeStampDisplay);
new MyTask().execute();
}
// My AsyncTask start...
class MyTask extends AsyncTask<Void, Void, List<Notes>> {
ProgressDialog pDialog;
@Override
protected void onPreExecute() {
super.onPreExecute();
pDialog = new ProgressDialog(NotesView.this);
pDialog.setMessage("Loading...");
pDialog.setCancelable(false);
pDialog.show();
if (isCancelled()) {
this.cancel(true);
}
}
@Override
protected List<Notes> doInBackground(Void... params) {
db.open();
listAlerts = db.getData();
if (isCancelled()) {
this.cancel(true);
}
return null;
}
protected void onPostExecute(List<Notes> alerts) {
if (null != pDialog && pDialog.isShowing()) {
pDialog.dismiss();
}
db.close();
setAdapterToListview();
}
}// end myTask
public void setAdapterToListview() {
objAdapter = new NoteAdapter(NotesView.this, R.layout.row_notes, listAlerts);
objAdapter.sortByNoteDesc();
objAdapter.notifyDataSetChanged();
listView.setAdapter(objAdapter);
}
@Override
public boolean onKeyDown(int keyCode, KeyEvent event) {
if (keyCode == KeyEvent.KEYCODE_BACK && event.getRepeatCount() == 0) {
Intent intent = new Intent(
NotesView.this.getApplicationContext(),
TabBarExample.class);
intent.putExtra("goToTab", "Alerts");
startActivity(intent);
return true;
}
return super.onKeyDown(keyCode, event);
}
public void onItemClick(AdapterView<?> parent, View viewDel, int position,
long id) {
for (int i = 0; i < 1; i++) {
Notes item = listAlerts.get(position);
int ids = item.getId();
note = item.getNote();
time = item.getTimeStamp();
}
System.out.println(note + " " + time);
//
}
@Override
protected void onResume() {
super.onResume();
}
@Override
protected void onPause() {
super.onPause();
setContentView(R.layout.note);
listView = (ListView) findViewById(R.id.notelist);
listView.setAdapter(null);
listView.setOnItemClickListener(this);
noteView = (TextView) findViewById(R.id.noteDisplay);
timeView = (TextView) findViewById(R.id.notetimeStampDisplay);
new MyTask().execute();
}
@Override
protected void onDestroy() {
}
}
来自 GCMIntentService 的代码片段
@Override
protected void onMessage(Context context, Intent intent) {
Log.i(TAG, "Received message");
//String message = getString(R.string.gcm_message);
System.out.println("onMessage: ");
Bundle extras = intent.getExtras();
String message = extras.getString("message");
String event_id_from_server = extras.getString("server_id");
// displayMessage(context, message);
generateNotification(context, message);
saveMsg(message);
System.out.println("server id is " + event_id_from_server);
if (event_id_from_server != null) {
updateLocalDatabase(event_id_from_server);
}
}
public void saveMsg(String msg) {
boolean worked = true;
try {
NotificationsDatabase entry = new NotificationsDatabase(GCMIntentService.this);
entry.open();
java.util.Date date = new java.util.Date();
Timestamp x = new Timestamp(date.getTime());
String timeStamp = x.toLocaleString();
entry.createEntry(msg, timeStamp);
entry.close();
//update adapter service
} catch (Exception e) {
worked = false;
String error = e.toString();
System.out.println(error);
} finally {
if (worked) {
}
}
}