这是我的代码。在它的onPostExecute（）方法获得结果为null但我从doInBackground（）方法返回值。 doInBackground（）正在调用方法getWeatherDataFromJson（）。我在LOG中返回arrary resultStrs（参见代码）但在onPostExecute（）中结果为null。
public class ForecastFragment extends Fragment {
public ForecastFragment(){
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setHasOptionsMenu(true);
}
@Override
public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
// Inflate the menu; this adds items to the action bar if it is present.
inflater.inflate(R.menu.forecastfragment, menu);
}
@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();
if (id == R.id.action_refresh) {
FetchWeatherTask weatherTask = new FetchWeatherTask();
weatherTask.execute("600001");
return true;
}
return super.onOptionsItemSelected(item);
}
private ArrayAdapter<String> mForecastAdapter;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
FetchWeatherTask weatherTask = new FetchWeatherTask();
weatherTask.execute("600001");
// Will contain the raw JSON response as a string.
String[] forecastArray = {
"Today - Sunny - 88/64",
"Tomorrow - Foggy - 78/64",
"Wed - Cloudy - 72/64",
"Thurs - Asteroids - 75/55",
"Fri - Heavy Rain - 67/44",
"Sat - Sunny - 88/76",
"Today - Light Rain - 81/69",
};
List<String> weekForecast = new ArrayList<String>(
Arrays.asList(forecastArray)
);
mForecastAdapter =
new ArrayAdapter<String>(
// the current context
getActivity(),
//id of list item layout
R.layout.list_item_forecast,
// id of text view to populate
R.id.list_item_forecast_textview,
// array data
weekForecast);
ListView listView = (ListView) rootView.findViewById(R.id.listview_forecast);
listView.setAdapter(mForecastAdapter);
return rootView;
}
public class FetchWeatherTask extends AsyncTask<String, Void, String[]> {
private final String LOG_TAG = FetchWeatherTask.class.getSimpleName();
/* The date/time conversion code is going to be moved outside the asynctask later,
* so for convenience we're breaking it out into its own method now.
*/
private String getReadableDateString(long time){
// Because the API returns a unix timestamp (measured in seconds),
// it must be converted to milliseconds in order to be converted to valid date.
SimpleDateFormat shortenedDateFormat = new SimpleDateFormat("EEE MMM dd");
return shortenedDateFormat.format(time);
}
/**
* Prepare the weather high/lows for presentation.
*/
private String formatHighLows(double high, double low) {
// For presentation, assume the user doesn't care about tenths of a degree.
long roundedHigh = Math.round(high);
long roundedLow = Math.round(low);
String highLowStr = roundedHigh + "/" + roundedLow;
return highLowStr;
}
/**
* Take the String representing the complete forecast in JSON Format and
* pull out the data we need to construct the Strings needed for the wireframes.
*
* Fortunately parsing is easy: constructor takes the JSON string and converts it
* into an Object hierarchy for us.
*/
private String[] getWeatherDataFromJson(String forecastJsonStr, int numDays)
throws JSONException {
// These are the names of the JSON objects that need to be extracted.
final String OWM_LIST = "list";
final String OWM_WEATHER = "weather";
final String OWM_TEMPERATURE = "temp";
final String OWM_MAX = "max";
final String OWM_MIN = "min";
final String OWM_DESCRIPTION = "main";
JSONObject forecastJson = new JSONObject(forecastJsonStr);
JSONArray weatherArray = forecastJson.getJSONArray(OWM_LIST);
// OWM returns daily forecasts based upon the local time of the city that is being
// asked for, which means that we need to know the GMT offset to translate this data
// properly.
// Since this data is also sent in-order and the first day is always the
// current day, we're going to take advantage of that to get a nice
// normalized UTC date for all of our weather.
Time dayTime = new Time();
dayTime.setToNow();
// we start at the day returned by local time. Otherwise this is a mess.
int julianStartDay = Time.getJulianDay(System.currentTimeMillis(), dayTime.gmtoff);
// now we work exclusively in UTC
dayTime = new Time();
String[] resultStrs = new String[numDays];
for(int i = 0; i < weatherArray.length(); i++) {
// For now, using the format "Day, description, hi/low"
String day;
String description;
String highAndLow;
// Get the JSON object representing the day
JSONObject dayForecast = weatherArray.getJSONObject(i);
// The date/time is returned as a long. We need to convert that
// into something human-readable, since most people won't read "1400356800" as
// "this saturday".
long dateTime;
// Cheating to convert this to UTC time, which is what we want anyhow
dateTime = dayTime.setJulianDay(julianStartDay+i);
day = getReadableDateString(dateTime);
// description is in a child array called "weather", which is 1 element long.
JSONObject weatherObject = dayForecast.getJSONArray(OWM_WEATHER).getJSONObject(0);
description = weatherObject.getString(OWM_DESCRIPTION);
// Temperatures are in a child object called "temp". Try not to name variables
// "temp" when working with temperature. It confuses everybody.
JSONObject temperatureObject = dayForecast.getJSONObject(OWM_TEMPERATURE);
double high = temperatureObject.getDouble(OWM_MAX);
double low = temperatureObject.getDouble(OWM_MIN);
highAndLow = formatHighLows(high, low);
resultStrs[i] = day + " - " + description + " - " + highAndLow;
}
for (String s : resultStrs) {
Log.v(LOG_TAG, "Forecast entry: " + s);
}
return resultStrs;
}
@Override
protected String[] doInBackground(String... params) {
// These two need to be declared outside the try/catch
// so that they can be closed in the finally block.
HttpURLConnection urlConnection = null;
BufferedReader reader = null;
// Will contain the raw JSON response as a string.
String forecastJsonStr = null;
String format = "json";
String units = "metric";
int numDays = 7;
try {
// Construct the URL for the OpenWeatherMap query
// Possible parameters are avaiable at OWM's forecast API page, at
// http://openweathermap.org/API#forecast
final String BASE_URL = "http://api.openweathermap.org/data/2.5/forecast/daily?";
final String QUERY_PARAMS = "q";
final String FORMAT_PARAMS = "mode";
final String UNIT_PARAMS = "units";
final String DAYS_PARAMS = "cnt";
Uri builtUri = Uri.parse(BASE_URL).buildUpon()
.appendQueryParameter(QUERY_PARAMS, params[0])
.appendQueryParameter(FORMAT_PARAMS, format)
.appendQueryParameter(UNIT_PARAMS, units)
.appendQueryParameter(DAYS_PARAMS, Integer.toString(numDays))
.build();
URL url = new URL(builtUri.toString());
Log.v(LOG_TAG, "Built Url:" +builtUri.toString());
// Create the request to OpenWeatherMap, and open the connection
urlConnection = (HttpURLConnection) url.openConnection();
urlConnection.setRequestMethod("GET");
urlConnection.connect();
// Read the input stream into a String
InputStream inputStream = urlConnection.getInputStream();
StringBuffer buffer = new StringBuffer();
if (inputStream == null) {
// Nothing to do.
//forecastJsonStr = null;
return null;
}
reader = new BufferedReader(new InputStreamReader(inputStream));
String line;
while ((line = reader.readLine()) != null) {
// Since it's JSON, adding a newline isn't necessary (it won't affect parsing)
// But it does make debugging a *lot* easier if you print out the completed
// buffer for debugging.
buffer.append(line + "\n");
}
if (buffer.length() == 0) {
// Stream was empty. No point in parsing.
//forecastJsonStr = null;
return null;
}
forecastJsonStr = buffer.toString();
try {
return getWeatherDataFromJson(forecastJsonStr, numDays);
} catch (JSONException e) {
Log.e(LOG_TAG,e.getMessage());
e.printStackTrace();
}
} catch (IOException e) {
Log.e(LOG_TAG , "Error", e);
// If the code didn't successfully get the weather data, there's no point in attemping
// to parse it.
return null;
//forecastJsonStr = null;
//return forecastJsonStr;
} finally{
if (urlConnection != null) {
urlConnection.disconnect();
return null;
}
if (reader != null) {
try {
reader.close();
} catch (final IOException e) {
Log.e("PlaceholderFragment", "Error closing stream", e);
}
}
}
return null;
}
@Override
protected void onPostExecute(String[] result) {
if (result != null) {
Log.v(LOG_TAG,"dfgihuffhuffuhgfufdhuidfhudfuhli");
mForecastAdapter.clear();
for (String dayForecastStr : result) {
mForecastAdapter.add(dayForecastStr);
}
}else {
Log.v(LOG_TAG, "errprdvdfdffd");
}
}
}
}
答案 0 :(得分：0)
我认为，您之前使用的返回方式是返回getWeatherDataFromJson（forecastJsonStr，numDays）;＆＃39;很好的方法。但这没有发生，因为我认为你的finally块被执行而你的urlconnection对象不是null。所以它执行了“如果＆＃39; condition并返回null。并且最后一个return语句永远不会被调用。
作为次要选项，＆＃39; numDays＆＃39;是固定的，如果你可以发送一个＆quot; forecastJSONString＆＃39;作为onPost（String resStr）中的param。所以你可以在onPost（）方法中执行下一个操作，它会更简单。