从AsyncTask类返回数据

• Z时代
• 2024-01-10
• 分类：问答

如何从AsyncTask中获取数据？我的MainActivity正在调用触发AsyncTask的DataCall.getJSON函数，但是我不确定如何将数据返回到原始Activity。

String state_data =  DataCall.getJSON(spinnerURL,spinnerContentType);

public class DataCall extends Activity {
    private static final String TAG = "MyApp";
    private class DownloadWebPageTask extends AsyncTask<String, Void, String> {
        protected String doInBackground(String... urls) {
            String response = "";
            for (String url : urls) {
                DefaultHttpClient client = new DefaultHttpClient();
                HttpGet httpGet = new HttpGet(url);
                try {
                    HttpResponse execute = client.execute(httpGet);
                    InputStream content = execute.getEntity().getContent();
                    BufferedReader buffer = new BufferedReader(
                            new InputStreamReader(content));
                    String s = "";
                    while ((s = buffer.readLine()) != null) {
                        response += s;
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
            return response;
        }
        protected void onPostExecute(String result) {
            //THIS IS WHERE I NEED TO RETURN MY DATA TO THE MAIN ACTIVITY. (I am guessing)
        }
        }
    public void getJSON(String myUrlString, String contentType) {
        DownloadWebPageTask task = new DownloadWebPageTask();
        task.execute(new String[] { "http://www.mywebsite.com/" + myUrlString });
    }
}

回答：

对我来说，关键是创建一个名为URLWithParams的类或其他类，因为AsyncTask将只允许发送一种类型的输入，并且我需要URL和HTTP请求的参数。

public class URLWithParams {
    public String url;
    public List<NameValuePair> nameValuePairs;
    public URLWithParams()
    {
        nameValuePairs = new ArrayList<NameValuePair>();
    }
}

然后将其发送到JSONClient：

public class JSONClient extends AsyncTask<URLWithParams, Void, String> {
    private final static String TAG = "JSONClient";
    ProgressDialog progressDialog ;
    GetJSONListener getJSONListener;
    public JSONClient(GetJSONListener listener){
        this.getJSONListener = listener;
    }
    @Override
    protected String doInBackground(URLWithParams... urls) {
        return connect(urls[0].url, urls[0].nameValuePairs);
    }
    public static String connect(String url, List<NameValuePair> pairs)
    {
        HttpClient httpclient = new DefaultHttpClient();
        if(url == null)
        {
            Log.d(TAG, "want to connect, but url is null");
        }
        else
        {
            Log.d(TAG, "starting connect with url " + url);
        }
        if(pairs == null)
        {
            Log.d(TAG, "want to connect, though pairs is null");
        }
        else
        {
            Log.d(TAG, "starting connect with this many pairs: " + pairs.size());
            for(NameValuePair dog : pairs)
            {
                Log.d(TAG, "example: " + dog.toString());
            }
        }
        // Execute the request
        HttpResponse response;
        try {
            // Prepare a request object
            HttpPost httpPost = new HttpPost(url); 
            httpPost.setEntity(new UrlEncodedFormEntity(pairs));
            response = httpclient.execute(httpPost);
            // Examine the response status
            Log.i(TAG,response.getStatusLine().toString());
            BufferedReader reader = new BufferedReader(new InputStreamReader(response.getEntity().getContent(), "UTF-8"));
            String json = reader.readLine();
            return json;
        } catch (ClientProtocolException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return null;
    }
    @Override
    protected void onPostExecute(String json ) {
        getJSONListener.onRemoteCallComplete(json);
    }
    public interface GetJSONListener {
        public void onRemoteCallComplete(String jsonFromNet);
    }
}

然后从我的主班这样叫

public class BookCatalog implements GetJSONListener {
    private final String TAG = this.getClass().getSimpleName();
    private String catalog_url = "URL";
    private void getCatalogFromServer() {
        URLWithParams mURLWithParams = new URLWithParams();
        mURLWithParams.url = catalog_url;
        try {
            JSONClient asyncPoster = new JSONClient(this);
            asyncPoster.execute(mURLWithParams);
        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    @Override
    public void onRemoteCallComplete(String jsonBookCatalogList) {
        Log.d(TAG, "received json catalog:");
        Log.d(TAG, jsonBookCatalogList);
    JSONObject bookCatalogResult;
    try {
        bookCatalogResult = (JSONObject) new JSONTokener(jsonBookCatalogList).nextValue();
        JSONArray books = bookCatalogResult.getJSONArray("books");
        if(books != null) {
            ArrayList<String> newBookOrdering = new ArrayList<String>();
            int num_books = books.length();
            BookCatalogEntry temp;
            DebugLog.d(TAG, "apparently we found " + Integer.toString(num_books) + " books.");
            for(int book_id = 0; book_id < num_books; book_id++) {
                JSONObject book = books.getJSONObject(book_id);
                String title = book.getString("title");
                int version = book.getInt("price");
            }
        }
    } catch (JSONException e) {
        e.printStackTrace();
    }
    }
}

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