当我的应用启动时，它会用片段替换布局，需要AsyncTask才能正确加载。当我从导航抽屉加载这个片段时，一切正常，但是它会在应用启动时加载，AsyncTask不会执行。我该如何解决这个问题？
MainActivity：
public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
Fragment fragment = null;
Class fragmentClass = null;
fragmentClass = NewsFragment.class;
try {
fragment = (Fragment) fragmentClass.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.replace(R.id.frameLayoutForFragments, fragment);
fragmentTransaction.commit();
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawer.setDrawerListener(toggle);
toggle.syncState();
NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
navigationView.setNavigationItemSelectedListener(this);
navigationView.getMenu().getItem(0).setChecked(true);
}
@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else {
super.onBackPressed();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
//Wybory elementów w navigationdrawer
Fragment fragment = null;
Class fragmentClass = null;
int id = item.getItemId();
if (id == R.id.nav_news) {
fragmentClass = NewsFragment.class;
} else if (id == R.id.nav_map) {
fragmentClass = MapFragment.class;
} else if (id == R.id.nav_buildings) {
fragmentClass = BuildingsFragment.class;
} else if (id == R.id.nav_manage) {
fragmentClass = MapFragment.class;
} else if (id == R.id.nav_info) {
fragmentClass = AppInfoFragment.class;
} else if (id == R.id.nav_bugreport) {
fragmentClass = ContactFragment.class;
}
try {
fragment = (Fragment) fragmentClass.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction().replace(R.id.frameLayoutForFragments, fragment).commit();
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}
启动时加载的片段：
public class NewsFragment extends Fragment {
public static ArrayList <ParsedWebData> list = new ArrayList<ParsedWebData>();
public NewsFragment() {
// Required empty public constructor
}
public static NewsFragment newInstance(Context context) {
NewsFragment fragment = new NewsFragment();
return fragment;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//Start AsyncTask w momencie ładowania fragmentu
AsyncXMLParser parser = new AsyncXMLParser();
parser.execute();
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_news, container, false);
ListView listView = (ListView) view.findViewById(R.id.listViewNews);
CustomListViewAdapter customListViewAdapter = new CustomListViewAdapter(getContext(), R.id.listViewNews, list);
listView.setAdapter(customListViewAdapter);
listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> a, View v, int position, long id) {
ParsedWebData singleData = list.get(position);
String url = singleData.getUrl();
Intent browserIntent = new Intent(Intent.ACTION_VIEW, Uri.parse(url));
startActivity(browserIntent);
}
});
return view;
}
}
的AsyncTask：
public class AsyncXMLParser extends AsyncTask <Void, Integer, ArrayList<ParsedWebData>> {
@Override
protected void onPreExecute() {
}
@Override
protected ArrayList<ParsedWebData> doInBackground(Void... params) {
ArrayList<ParsedWebData> list = new ArrayList<ParsedWebData>();
try {
XmlPullParserFactory factory = XmlPullParserFactory.newInstance();
factory.setNamespaceAware(false);
XmlPullParser xpp = factory.newPullParser();
// We will get the XML from an input stream
InputStream input = new URL("linkhere").openStream();
xpp.setInput(input, "UTF_8");
int eventType = xpp.getEventType();
String text = null;
ParsedWebData data = new ParsedWebData();
while (eventType != XmlPullParser.END_DOCUMENT) {
String tagname = xpp.getName();
switch (eventType) {
case XmlPullParser.START_TAG:
if (tagname.equalsIgnoreCase("item")) {
data = new ParsedWebData();
}
break;
case XmlPullParser.TEXT:
text = xpp.getText();
break;
case XmlPullParser.END_TAG:
if (tagname.equalsIgnoreCase("item")) {
// add employee object to list
list.add(data);
} else if (tagname.equalsIgnoreCase("title")) {
data.title = text;
} else if (tagname.equalsIgnoreCase("link")) {
data.url = text;
} else if (tagname.equalsIgnoreCase("description")) {
text = Jsoup.parse(text).text();
data.description = text;
}
break;
default:
break;
}
eventType = xpp.next();
}
} catch (XmlPullParserException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
NewsFragment.list = list;
return list;
}
@Override
protected void onPostExecute(ArrayList <ParsedWebData> result) {
}
}
我试图自己找到原因，并尝试在MainActivity中手动执行AsyncTask，但它没有帮助。任何想法有什么不对？
答案 0 :(得分：2)
在OnCreateView运行之后，AsyncTask（线程）结束了，这是错误的，所以你看不到你的数据。
您可以做的是将代码行NewsFragment.list = list;从doInBackground()移至onPostExecute()，然后调用adapter.notifyDataSetChanged()。但问题是，在您的设计中，您无法访问AsyncTask中的listView适配器
另外，将列表作为Fragment中的静态变量，以便您可以从AsyncTask访问它是非常糟糕的编程设计。
您应该删除静态列表变量，并重新设计AsyncTask，将listView传递给构造函数（您必须将它从onCreate移动到onCreateView），并将listView分配给AsyncTask中的成员。然后在onPostExecute中设置适配器（不在onCreateView中）
public class AsyncXMLParser extends AsyncTask <Void, Integer, ArrayList<ParsedWebData>> {
CustomListViewAdapter _adapter;
Public AsyncXMLParser(CustomListViewAdapter adapter) {
_adapter = adapter;
}
...
@Override
protected void onPostExecute(ArrayList <ParsedWebData> result) {
CustomListViewAdapter customListViewAdapter = new CustomListViewAdapter(getContext(), R.id.listViewNews, result);
listView.setAdapter(customListViewAdapter);
// Or, depending on design
_adapter.notifyDataSetChanged();
}
}
这样，列表不是静态变量（可以从片段中删除代码），只有在AsyncTask完成后才会填充listView。如果您以后需要访问实际的列表数据，可以从适配器获取它。
答案 1 :(得分：0)
我找不到Asynctask为什么不被执行的原因但是根据你想要做多少背景工作，你可能不想使用 的AsyncTask。 Asynctask应仅用于执行花费少量时间（最多几秒）的操作，因为它的生命周期与ui组件的生命周期相关联。这里的问题很可能就是你要杀死asynctask因为它连接到了ui。
我建议您使用intentservice。它们非常易于使用，并且可以在后台运行。由于在这种情况下你似乎无法找到Asynctask的问题，你可能会通过使用intentservices来节省时间。 https://developer.android.com/reference/android/app/IntentService.html