從AsyncTask返回對象時出現問題

Question

我有一個擴展AsyncTask的類（RestClient.java）：package org.stocktwits.helper;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONException;
import org.json.JSONObject;

import android.os.AsyncTask;
import android.util.Log;

public class RestClient extends AsyncTask<String, Void, JSONObject>{
    public JSONObject jsonObj = null;
    private static String convertStreamToString(InputStream is) {
        /*
         * To convert the InputStream to String we use the BufferedReader.readLine()
         * method. We iterate until the BufferedReader return null which means
         * there's no more data to read. Each line will appended to a StringBuilder
         * and returned as String.
         */
        BufferedReader reader = new BufferedReader(new InputStreamReader(is));
        StringBuilder sb = new StringBuilder();

        String line = null;
        try {
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        return sb.toString();
    }


    /* This is a test function which will connects to a given
     * rest service and prints it's response to Android Log with
     * labels "Praeda".
     */
    public static JSONObject connect(String url)
    {

        HttpClient httpclient = new DefaultHttpClient();


        // Prepare a request object
        HttpGet httpget = new HttpGet(url); 

        // Execute the request
        HttpResponse response;
        try {
            response = httpclient.execute(httpget);
            // Examine the response status
            Log.i("Praeda",response.getStatusLine().toString());

            // Get hold of the response entity
            HttpEntity entity = response.getEntity();

            if (entity != null) {

                // A Simple JSON Response Read
                InputStream instream = entity.getContent();
                String result= convertStreamToString(instream);

                // A Simple JSONObject Creation
                JSONObject json=new JSONObject(result);

                // Closing the input stream will trigger connection release
                instream.close();

                return json;
            }


        } catch (ClientProtocolException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (JSONException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return null;
    }

    @Override
    protected JSONObject doInBackground(String... urls) {
        return connect(urls[0]);
    }

    @Override
    protected void onPostExecute(JSONObject json ) {
        this.jsonObj = json;
    }

    public void setJSONObject(JSONObject jsonFromUI){
        this.jsonObj = jsonFromUI;
    }

    public JSONObject getJSONObject(){
        return this.jsonObj;
    }

}

我試圖在我的Main類（Main.java）上執行AsyncTask：

    RestClient rc = new RestClient();
    JSONObject json = new JSONObject();
    rc.setJSONObject(json);
    rc.execute(buildQuery());
    json = rc.getJSONObject();

//do some stuff with the json object
try { JSONObject query = json.getJSONObject("query");
//...
}

json為null，因為它在onPostExecute（）之前調用。 如何獲取JSON？

更新：我需要在onPostExecute（）中運行以下try塊：

try {

            JSONObject query = json.getJSONObject("query");
            JSONObject results = query.getJSONObject("results");

            if (query.getString("count").equals("1")) { // YQL JSON doesn't
                // return an array for
                // single quotes
                JSONObject quote = results.getJSONObject("quote");

                Quote myQuote = new Quote();
                myQuote.setName(quote.getString("Name"));
                myQuote.setSymbol(quote.getString("Symbol"));
                myQuote.setLastTradePriceOnly(quote
                        .getString("LastTradePriceOnly"));
                myQuote.setChange(quote.getString("Change"));
                myQuote.setOpen(quote.getString("Open"));
                myQuote.setMarketCapitalization(quote
                        .getString("MarketCapitalization"));
                myQuote.setDaysHigh(quote.getString("DaysHigh"));
                myQuote.setYearHigh(quote.getString("YearHigh"));
                myQuote.setDaysLow(quote.getString("DaysLow"));
                myQuote.setYearLow(quote.getString("YearLow"));
                myQuote.setVolume(quote.getString("Volume"));
                myQuote.setAverageDailyVolume(quote
                        .getString("AverageDailyVolume"));
                myQuote.setPeRatio(quote.getString("PERatio"));
                myQuote.setDividendYield(quote.getString("DividendYield"));
                myQuote.setPercentChange(quote.getString("PercentChange"));

                quotesAdapter.add(myQuote);}

Answer 1

嘿，您可以使用偵聽器解決此問題。 我已經稍微更改了代碼以使用此代碼。

package com.insidetip.uob.data;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONException;
import org.json.JSONObject;

import android.app.ProgressDialog;
import android.content.Context;
import android.os.AsyncTask;
import android.util.Log;

public class JSONClient extends AsyncTask<String, Void, JSONObject>{
    ProgressDialog progressDialog ;
    GetJSONListener getJSONListener;
    Context curContext;
    public JSONClient(Context context, GetJSONListener listener){
        this.getJSONListener = listener;
        curContext = context;
    }
    private static String convertStreamToString(InputStream is) {
        /*
         * To convert the InputStream to String we use the BufferedReader.readLine()
         * method. We iterate until the BufferedReader return null which means
         * there's no more data to read. Each line will appended to a StringBuilder
         * and returned as String.
         */
        BufferedReader reader = new BufferedReader(new InputStreamReader(is));
        StringBuilder sb = new StringBuilder();

        String line = null;
        try {
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        return sb.toString();
    }


    public static JSONObject connect(String url)
    {
        HttpClient httpclient = new DefaultHttpClient();

        // Prepare a request object
        HttpGet httpget = new HttpGet(url); 

        // Execute the request
        HttpResponse response;
        try {
            response = httpclient.execute(httpget);
            // Examine the response status
            Log.i("Praeda",response.getStatusLine().toString());

            // Get hold of the response entity
            HttpEntity entity = response.getEntity();

            if (entity != null) {

                // A Simple JSON Response Read
                InputStream instream = entity.getContent();
                String result= convertStreamToString(instream);

                // A Simple JSONObject Creation
                JSONObject json=new JSONObject(result);

                // Closing the input stream will trigger connection release
                instream.close();

                return json;
            }


        } catch (ClientProtocolException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (JSONException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return null;
    }
    @Override
    public void onPreExecute() {
        progressDialog = new ProgressDialog(curContext);
        progressDialog.setMessage("Loading..Please wait..");
        progressDialog.setCancelable(false);
        progressDialog.setIndeterminate(true);
        progressDialog.show();

    }

    @Override
    protected JSONObject doInBackground(String... urls) {
        return connect(urls[0]);
    }

    @Override
    protected void onPostExecute(JSONObject json ) {
        getJSONListener.onRemoteCallComplete(json);
        progressDialog.dismiss();
    }
}

像這樣在調用類中使用。

    JSONClient client = new JSONClient(context, listener);
    client.execute(URL);

不要忘記實現監聽器

public interface GetJSONListener {
    public void onRemoteCallComplete(JSONObject jsonFromNet);
}

Answer 2

我被doInBackground的結果誤​​認為可以在onPostExecute中使用

doInBackground（Params ...），在PreExecute（）完成執行后立即在后台線程上調用。 此步驟用於執行可能需要很長時間的后台計算。 異步任務的參數將傳遞到此步驟。 計算結果必須通過此步驟返回，並將傳遞回最后一步 。 此步驟還可以使用publishProgress（Progress ...）發布一個或多個進度單位。 這些值在onProgressUpdate（Progress ...）步驟中發布在UI線程上。

@Override

protected void onPostExecute(JSONObject json ) {
// DO stuff here ( it's UI thread )
 mJsonFromTheActivity = json;
}

Answer 3

execute（）始終返回AsyncTask本身。 從doInBackground（）返回的對象將在onPostExecute（）中交給您。

Answer 4

如果您將asynctask作為活動的嵌套內部類，則可以將其中一個活動變量設置為asynctask的結果