如何正确使用 doinBackground() 方法来检索 RSS 项目

Question

现在我正在创建一个 RSS 阅读器，主要只是试图让项目的标题和描述显示在 ListView 上。我之前在没有 RSS 数据的情况下对其进行了测试，并确认该应用程序正确列出了我创建的项目。但是，在尝试对来自 RSS 的数据进行相同操作后，我遇到了检索实际 RSS 数据以及如何使用 doinBackground 方法的问题。

在阅读了 Google 关于 doinBackground 的文档后，我了解到它的类 (Async) 允许执行后台操作并将其结果显示在 UI 线程中。但是，我通常在提取 RSS 数据以及 doinBackground() 如何适合我的代码时遇到问题。有关如何正确检索数据和有效使用 doinbackground() 的任何想法？

我遇到问题的代码类别是 Headlines 和 RSSManager。代码如下：

标题片段

import android.os.Bundle;
import android.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import java.net.MalformedURLException;
import java.net.URL;

public class Headlines extends Fragment {
EditText editText;
Button gobutton;
ListView listView;

public Headlines() {
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_headlines, container, false);
    editText = (EditText)v.findViewById(R.id.urlText);
    gobutton = (Button)v.findViewById(R.id.goButton);
    listView = (ListView)v.findViewById(R.id.listView);
    RSSFeedManager rfm = new RSSFeedManager();
    News [] news = new News[100]; // i shouldnt have to set the size of the array here since I did it in getFeed() in RSSFeedManager.java
    try {
        news = rfm.getFeed(String.valueOf(new URL("http://rss.cnn.com/rss/cnn_world.rss")));
    } catch (MalformedURLException e) {
        e.printStackTrace();
    }
    RssAdapter adapter = new RssAdapter(this.getActivity(),news);
    listView.setAdapter(adapter);
    /*gobutton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

        }
    });*/
    return v;
}

}

RSSFeedManager

import android.os.AsyncTask;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.NodeList;

public class RSSFeedManager extends AsyncTask<String,Void,String> {
public URL rssURL;
News[] articles;

public News[] getFeed(String url) {
    try {
        String strURL = url;
        rssURL = new URL(url);
        DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
        Document doc = builder.parse(rssURL.openStream());

        //using Nodelist to get items within the rss, then creating
        //a News array the same size of the amount of items within the rss
        //then setting up a temporary "News" item which will be the temp object
        //used for storing multiple objects that contain title and description
        //of each item
        NodeList items = doc.getElementsByTagName("item");
        News[] articles = new News[items.getLength()];
        News news = null;

        //traverse through items and place the contents of each item within an RssItem object
        //then add to it to the News Array
        for (int i = 0; i < items.getLength(); i++) {
            Element item = (Element) items.item(i);
            news.setTitle(getValue(item, "title"));
            news.setDescription(getValue(item, "description"));
            articles[i] = news;
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return articles;
}

public String getValue(Element parent, String nodeName) {
    return parent.getElementsByTagName(nodeName).item(0).getFirstChild().getNodeValue();
}

@Override
protected String doInBackground(String... url) {
    String rssURL = url[0];
    URL urlTemp;
    try {
        //pulling the url from the params and converting it to type URL and then establishing a connection
        urlTemp = new URL(rssURL);
        HttpURLConnection urlConnection = (HttpURLConnection) urlTemp.openConnection();
        urlConnection.connect();
        /*
        *im thinking i need to call the getFeed() method
        *after establishing the httpurlconnection however
        *I also thought I may just need to move the getFeed()
        *code within doinBackground. Lost at this point due to the
        * return types of getFeed and doinBackground
        */
    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    return null;
}

}

Answer 1

在挑选出每一行代码之后，我发现了我的 RSSFeedManager 类、doInBackground() 方法和 Headlines 类的一些问题。

从 RSSFeedManager 开始，这里有一些问题。我将实例化的News [] articles 实例化为类变量，然后在getFeeds() 方法中重新定义它。显然这会导致一些问题，并将articles 返回为空。我还删除了strURL 和rssURL，因为我做错了。不需要将 URL 传递给 getFeeds()，而是需要从 URL 传递 XML。我还稍微修改了代码以使用 News 类的构造函数。

这是 RSSFeedManager 的固定代码：

public class RSSFeedManager{

News[] articles;

public News[] getFeed(String html) {
    try {
        DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
        Document doc = builder.parse(new ByteArrayInputStream(html.getBytes()));

        NodeList items = doc.getElementsByTagName("item");
        articles = new News[items.getLength()];

        for (int i = 0; i < items.getLength(); i++) {
            Element item = (Element) items.item(i);
            News news = new News(getValue(item, "title"),getValue(item, "description").substring(0,100),"");
            articles[i] = news;
        }
    } catch (Exception e) {
        //e.printStackTrace();
        Log.d("EXCEPTION PARSING",e.toString());
    }
    return articles;
}

public String getValue(Element parent, String nodeName) {
    return parent.getElementsByTagName(nodeName).item(0).getFirstChild().getNodeValue();
}
}

正如我之前提到的，我意识到 RSSFeedManager 应该从 URL 接收 XML，而不是从 URL 本身接收 XML，并且 doInBackground() 应该发生在一个新类中。本质上，带有 Downloader 类的 doInBackground() 方法（扩展 AsyncTask 正在接收输入的 url，然后从该 RSS url 收集 XML。

这里是下载器类

public class Downloader extends AsyncTask<String, Void, String> {

@Override
protected String doInBackground(String... urls) {
    String result ="";
    try{
        URL url = new URL(urls[0]);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        InputStream in = connection.getInputStream();
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));
        String line = "";
        while((line=reader.readLine())!= null){
            result= result + line;
        }
        connection.disconnect();

    } catch (Exception e) {
        Log.e("Error fetching", e.toString());
    }

    return result;
}
}

因此，在完成这两个课程之后，我知道我需要在 Headlines 课程中解决一些问题。我最初蛮力地强制代码进行测试，以确保来自 RSS 提要的文章正确显示，并且他们确实如此，然后着手正确实现它。为了提醒人们，该程序的目的是显示来自用户指定的 RSS 提要的文章，方法是在 editText 中输入一个 url，这是一个 EditText 对象。然后用户按下goButton，类型为Button，然后列出来自editText的所有文章。我通过为goButton 创建一个onClickListener 并创建Downloader 和RssFeedManager 类的对象来实现这一点，以便调用它们的方法来建立连接、检索URL 的XML，然后对其进行解析。

这是 Headline 类代码的 sn-p，我可以将所有内容联系在一起。

 public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_headlines, container, false);
    editText = (EditText)v.findViewById(R.id.urlText);
    gobutton = (Button)v.findViewById(R.id.goButton);
    listView = (ListView)v.findViewById(R.id.listView);
    parent = this.getActivity();
    gobutton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            RSSFeedManager rfm = new RSSFeedManager();
            String html = "";
            try {
                Downloader d = new Downloader();
                d.execute(editText.getText().toString());
                html = d.get();
                Log.d("HTML CAME BACK", html);
                news = rfm.getFeed(html);
                RssAdapter adapter = new RssAdapter(parent, news);
                listView.setAdapter(adapter);
            } catch (InterruptedException e) {
                Log.e("ERROR!!!!!", e.toString());
            } catch (ExecutionException e) {
                Log.e("ERROR!!!!!!!", e.toString());
                Toast.makeText(parent, e.toString(), Toast.LENGTH_LONG).show();
            } catch (Exception e) {
                Log.e("WEIRD!", e.toString());
                Toast.makeText(parent, e.toString(), Toast.LENGTH_LONG).show();
            }


        }
    });

所有这些更改都有助于解决问题，让我能够进一步完成这个程序，并了解更多关于 android 开发的信息。