java - AsyncTask 不调用 onPostExecute(Result result)

Question

• 我在这里尝试使用 AsyncTask 从 OWM API 显示 7 天的天气预报。
• doInBackground(String...param) 方法也工作正常。我检查了 LOGCAT。
• 异步完成执行后。我试图在菜单中的刷新按钮上刷新 ListView。但似乎 onPostExecute() 确实很关心。

预测 fragment .java

package com.example.sunshine;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Date;
import java.util.List;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import android.net.Uri;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ListView;

/**
 * A placeholder fragment containing a simple view.
 */

public class ForecastFragment extends Fragment {
  private ArrayAdapter<String> mForeCastAdapter;
  public String[] forecastArray;

  public ForecastFragment() {

  }

  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setHasOptionsMenu(true);
  }

  public void onCreateOptionsMenu(Menu menu, MenuInflater inflater) {
    inflater.inflate(R.menu.forecastfragment, menu);
  }

  public boolean onOptionsItemSelected(MenuItem item) {
    int id = item.getItemId();
    if (id == R.id.action_refresh) {
      new FetchWeatherTask().execute("44700");
      return true;
    }
    return super.onOptionsItemSelected(item);

  }

  protected void onPostExecute(String[] result) {

    if (result != null) {
      Log.v("msg", result[0]);
      mForeCastAdapter.clear();
      // forecastArray = result;

      for (String dayForecastStr : result) {
        mForeCastAdapter.add(dayForecastStr);
        // mForeCastAdapter.notifyDataSetChanged();
      }
    }
    mForeCastAdapter.notifyDataSetChanged();

  }

  @Override
  public View onCreateView(LayoutInflater inflater, ViewGroup container,
      Bundle savedInstanceState)
  {
    String[] list = { "1", "2", "3" };
    List<String> weekForeCast = new ArrayList<String>(
        Arrays.asList(list));

    mForeCastAdapter = new ArrayAdapter<String>(getActivity(),
        R.layout.list_item_forecast, R.id.list_item_forecast_textview,
        weekForeCast);

    View rootView = inflater.inflate(R.layout.fragment_main, container,
        false);

    ListView weekList = (ListView) rootView
        .findViewById(R.id.list_item_forecast);

    weekList.setAdapter(mForeCastAdapter);

    return rootView;
  }

}

class FetchWeatherTask extends AsyncTask<String, Void, String[]> {

  private final String LOG_TAG = FetchWeatherTask.class.getSimpleName();
  protected ArrayAdapter<String> mForeCastAdapter;

  /*
   * 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.

    Date date = new Date(time * 1000);
    SimpleDateFormat format = new SimpleDateFormat("E, MMM d");
    return format.format(date).toString();
  }

  /**
   * 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_DATETIME = "dt";
    final String OWM_DESCRIPTION = "main";

    JSONObject forecastJson = new JSONObject(forecastJsonStr);
    JSONArray weatherArray = forecastJson.getJSONArray(OWM_LIST);

    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 = dayForecast.getLong(OWM_DATETIME);
      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 Array : " + 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
      // URL url = new
      // URL("http://api.openweathermap.org/data/2.5/forecast/daily?" +
      // "q=94043&mode=json&units=metric&cnt=7");

  final String FORECAST_BASE_URL= ="http://api.openweathermap.org/        
                                      data/2.5/forecast/daily?";

      final String QUERY_PARAM = "q";
      final String MODE_PARAM = "mode";
      final String UNIT_PARAM = "units";
      final String DAYS_PARAM = "cnt";

      Uri urlBuild = Uri
          .parse(FORECAST_BASE_URL)
          .buildUpon()
          .appendQueryParameter(QUERY_PARAM, params[0])
          .appendQueryParameter(MODE_PARAM, format)
          .appendQueryParameter(UNIT_PARAM, units)
          .appendQueryParameter(DAYS_PARAM, Integer.toString(numDays))
          .build();

      URL url = new URL(urlBuild.toString());

      //Log.v(LOG_TAG, "Build URL " + urlBuild.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.
        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.
        return null;
      }

      forecastJsonStr = buffer.toString();
      // Log.v(LOG_TAG, "Forecast JSON String" + forecastJsonStr);
    }

    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;
    }
    finally {
      if (urlConnection != null) {
        urlConnection.disconnect();
      }

      if (reader != null) {
        try {
          reader.close();
        } catch (final IOException e) {
          Log.e(LOG_TAG, "Error closing stream", e);
        }
      }
    }

    try {
      return getWeatherDataFromJson(forecastJsonStr, numDays);

    }catch(JSONException e){
      Log.e(LOG_TAG,e.getMessage(), e);
      e.printStackTrace();
    }

    return null;
  }
}

Answer 1

尝试在 AsyncTask 类而不是 Fragment 类中使用方法 onPostExecute。 AsyncTask 类如下所示:

private class MyTask extends AsyncTask<String, Void, String[]> {

  @Override
  protected String[] doInBackground(String... params) {
  }

  @Override
  protected void onPostExecute(String result[]) {
  }

  @Override
  protected void onPreExecute() {
  }

  @Override
  protected void onProgressUpdate(Void... values) {
  }
}

对于您的情况，您需要将 onPostExecute 方法放在您的 FetchWeatherTask 类中:

class FetchWeatherTask extends AsyncTask<String, Void, String[]> {
    ....

    @Override
    protected void onPostExecute(String result[]) {
      if (result != null) {
        Log.v("msg", result[0]);
        mForeCastAdapter.clear();
        //forecastArray = result;

        for (String dayForecastStr : result) {
            mForeCastAdapter.add(dayForecastStr);
            //mForeCastAdapter.notifyDataSetChanged();
        }
      }

      mForeCastAdapter.notifyDataSetChanged();
    }
    ....
}