Question

现在我正在创建一个RSS阅读器，主要是试图让项目的标题和描述显示在ListView上。我之前没有使用RSS数据测试它，并确认应用程序正确列出了我创建的项目。但是，在尝试使用RSS中的数据后，我遇到了检索实际RSS数据以及如何使用doinBackground方法的问题。

在阅读了有关doinBackground的Google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，我在提取RSS数据方面遇到了一些问题，而且doinBackground（）如何适合我的代码。关于如何正确检索数据并有效使用doinbackground（）的任何想法？

我遇到问题的代码类是Headlines和RSSManager。这是代码：

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

在挑选出每行代码之后，我想出了我的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null。我还删除了strURL和rssURL因为我正在以错误的方式解决这个问题。没有必要将URL传递给getFeeds()，而是需要从URL传递XML。我还尝试了一些代码来使用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而不是URL本身接收XML，并且新类是doInBackground()应该发生的地方。本质上，带有Downloader类的doInBackground()方法（扩展AsyncTask）接收输入的URL，然后从该RSS url收集XML。

这是Downloader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;
}
}

所以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。我最初粗暴地强迫代码进行测试，以确保RSS源中的文章正确显示，然后他们做了，然后开始正确实现它。为了提醒人们，该程序的目的是通过将URL记录到editText EditText对象中来显示用户指定的RSS源中的文章。然后，用户按下类型为goButton的{{1}}，然后列出来自Button的所有网址文章。我通过为editText创建onClickListener并创建goButton和Downloader类的对象来完成此操作，以便调用其方法来建立连接，检索URL的XML，然后解析它。

以下是Headline课程代码的片段，我可以将所有内容组合在一起。

RssFeedManager

所有这些变化都有助于解决问题并使我能够进一步完成此计划，并了解有关Android开发的更多信息。

如何正确使用doinBackground（）方法来检索RSS项目

1 个答案: