我一直在关注这本书深入浅出Android的，而我被困在第3章。

这一点的应用程序是一个非常基本的布局; 3文本视图和1的图像来看，应该​​从美国航空航天局的RSS日常图像读取后更新。

我已经完成了一章，但现在运行应用程序时只显示一个空白屏幕。

任何帮助AP preciated。这是code：

 公共类MainActivity延伸活动{

 @覆盖
    公共无效的onCreate（包savedInstanceState）{
        super.onCreate（savedInstanceState）;
        的setContentView（R.layout.activity_main）;
        IotdHandler处理器=新IotdHandler（）;
        handler.processFeed（）;
        resetDisplay（handler.getTitle（），handler.getDate（），handler.getImage（），handler.getDescription（））;
    }

    公共类IotdHandler扩展的DefaultHandler {
        私人字符串URL =htt​​p://www.nasa.gov/rss/dyn/image_of_the_day.rss;
        私人布尔INURL = FALSE;
        私人INTITLE布尔值= FALSE;
        私人布尔inDescription = FALSE;
        私人布尔inItem = FALSE;
        私人布尔inDate = FALSE;
        私人位图图像= NULL;
        私人字符串标题= NULL;
        私人StringBuffer的说明=新的StringBuffer（）;
        私人字符串日期= NULL;


        公共无效processFeed（）{
            尝试 {
            的SAXParserFactory厂=
            SAXParserFactory.newInstance（）;
            的SAXParser解析器= factory.newSAXParser（）;
            XMLReader的读卡器= parser.getXMLReader（）;
            reader.setContentHandler（本）;
            的InputStream的InputStream =新的网址（URL）.openStream（）;
            reader.parse（新的InputSource（InputStream中））;
            }赶上（例外五）{}
        }

            私人位图getBitmap（字符串URL）{
                尝试 {
                HttpURLConnection的连接=（HttpURLConnection类）新的网址（URL）.openConnection（）;
                connection.setDoInput（真正的）;
                connection.connect（）;
                输入的InputStream = connection.getInputStream（）;
                位图bilde = BitmapFactory.de codeStream（输入）;
                input.close（）;
                返回bilde;
                }赶上（IOException异常IOE）{返回null; }
                }

            公共无效的startElement（URL字符串，字符串的localName，字符串QNAME，属性的属性）抛出的SAXException {
                    如果（localName.endsWith（JPG））{INURL = TRUE; }
                    其他{INURL = FALSE; }

                    如果（localName.startsWith（项目））{inItem = TRUE; }
                    否则，如果（inItem）{

                        如果（localName.equals（标题））{= INTITLE真实; }
                        其他INTITLE {= FALSE; }

                        如果（localName.equals（说明））{inDescription = TRUE; }
                        其他{inDescription = FALSE; }

                        如果（localName.equals（pubdate的））{inDate = TRUE; }
                        其他{inDate = FALSE; }
                        }
                    }


            公共无效字符（字符CH []，诠释开始，诠释长度）{字符串的字符=新的String（CH）.substring（启动，启动+长度）;
                如果（inurl这样和放大器;＆安培; URL == NULL）{图像= getBitmap（字）; }
                如果（与INTITLE功放;＆安培;标题== NULL）{标题=字符; }
                如果（inDescription）{description.append（字）; }
                如果（inDate＆安培;＆放大器;日期== NULL）{日期=字符; }
         }

        公共位图的getImage（）{返回图像; }
        公共字符串的getTitle（）{返回称号; }
        公共StringBuffer的getDescription（）{返回描述; }
        公共字符串GETDATE（）{返回日期; }
}

    私人无效resetDisplay（标题字符串，字符串日期，位图图像，StringBuffer的描述）{

        TextView的titleview的=（的TextView）findViewById（R.id.imageTitle）;
        titleView.setText（职称）;

        TextView的dateView =（TextView中）findViewById（R.id.imageDate）;
        dateView.setText（日期）;

        ImageView的ImageView的=（ImageView的）findViewById（R.id.imageDisplay）;
        imageView.setImageBitmap（图像）;

        TextView的descriptionView =（TextView中）findViewById（R.id.imageDescription）;
        descriptionView.setText（介绍）;
    }

@覆盖
公共布尔onCreateOptionsMenu（功能菜单）{
   //充气菜单;这增加了项目操作栏，如果它是present。
   。getMenuInflater（）膨胀（R.menu.main，菜单）;
   返回true;
}
}
 

解决方案

您应该使用的AsyncTask 作为一个内部类。

  @覆盖
公共无效的onCreate（包savedInstanceState）{
    super.onCreate（savedInstanceState）;
    的setContentView（R.layout.activity_main）;
    IotdHandler处理器=新IotdHandler（）;
    新MyTask的（）执行（）。
}
 

，然后解析doInBackground（文件），并调用resetDisplay在onPostExecute（）。

 公共类MyTask的扩展AsyncTask的＆LT;虚空，虚空，虚空＆GT; {

@覆盖
保护无效doInBackground（虚空...... PARAMS）{
    handler.processFeed（）;
    返回null;
}

@覆盖
保护无效onPostExecute（无效的结果）{
    resetDisplay（handler.getTitle（），handler.getDate（），handler.getImage（），handler.getDescription（））;
    super.onPostExecute（结果）;
}
}
 

有关更多信息如何传递参数，返回结果等。 AsyncTask的文档

I've been following the book "Head First Android", and I'm stuck in Chapter 3.

This little app is a really basic layout; 3 text views and 1 image view, that should update after reading from NASA RSS daily image.

I've completed the chapter but now when running the app only shows a Blank screen.

Any help appreciated. This is the code:

public class MainActivity extends Activity {

 @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        IotdHandler handler = new IotdHandler ();
        handler.processFeed();
        resetDisplay (handler.getTitle(), handler.getDate(), handler.getImage(), handler.getDescription());
    }

    public class IotdHandler extends DefaultHandler {
        private String url = "http://www.nasa.gov/rss/dyn/image_of_the_day.rss";
        private boolean inUrl = false;
        private boolean inTitle = false;
        private boolean inDescription = false;
        private boolean inItem = false;
        private boolean inDate = false;
        private Bitmap image = null;
        private String title = null;
        private StringBuffer description = new StringBuffer();
        private String date = null;


        public void processFeed() {
            try {
            SAXParserFactory factory =
            SAXParserFactory.newInstance();
            SAXParser parser = factory.newSAXParser();
            XMLReader reader = parser.getXMLReader();
            reader.setContentHandler(this);
            InputStream inputStream = new URL(url).openStream();
            reader.parse(new InputSource(inputStream));
            } catch (Exception e) {  }
        }

            private Bitmap getBitmap(String url) {
                try {
                HttpURLConnection connection = (HttpURLConnection)new URL(url).openConnection();
                connection.setDoInput(true);
                connection.connect();
                InputStream input = connection.getInputStream();
                Bitmap bilde = BitmapFactory.decodeStream(input);
                input.close();
                return bilde;
                } catch (IOException ioe) { return null; }
                }

            public void startElement(String url, String localName, String qName, Attributes attributes) throws SAXException {
                    if (localName.endsWith(".jpg")) { inUrl = true; }
                    else { inUrl = false; }

                    if (localName.startsWith("item")) { inItem = true; }
                    else if (inItem) {

                        if (localName.equals("title")) { inTitle = true; }
                        else { inTitle = false; }

                        if (localName.equals("description")) { inDescription = true; }
                        else { inDescription = false; }

                        if (localName.equals("pubDate")) { inDate = true; }
                        else { inDate = false; }
                        }
                    }


            public void characters(char ch[], int start, int length) { String chars = new String(ch).substring(start, start + length);
                if (inUrl && url == null) { image = getBitmap(chars); }
                if (inTitle && title == null) { title = chars; }
                if (inDescription) { description.append(chars); }
                if (inDate && date == null) { date = chars; }
         }

        public Bitmap getImage() { return image; }
        public String getTitle() { return title; }
        public StringBuffer getDescription() { return description; }
        public String getDate() { return date; }
}

    private void resetDisplay (String title, String date, Bitmap image, StringBuffer description) {

        TextView titleView = (TextView) findViewById (R.id.imageTitle);
        titleView.setText(title);

        TextView dateView = (TextView) findViewById(R.id.imageDate);
        dateView.setText(date);

        ImageView imageView = (ImageView) findViewById (R.id.imageDisplay);
        imageView.setImageBitmap(image);

        TextView descriptionView = (TextView) findViewById (R.id.imageDescription);
        descriptionView.setText(description);
    }

@Override
public boolean onCreateOptionsMenu(Menu menu) {
   // Inflate the menu; this adds items to the action bar if it is present.
   getMenuInflater().inflate(R.menu.main, menu);
   return true;
}
}

You should use AsyncTask as an inner class.

 @Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    IotdHandler handler = new IotdHandler ();
    new MyTask().execute();
}

and then parse document in doInBackground() and call resetDisplay in onPostExecute().

public class MyTask extends AsyncTask<Void, Void, Void>{

@Override
protected Void doInBackground(Void... params) {
    handler.processFeed();
    return null;
}

@Override
protected void onPostExecute(Void result) {
    resetDisplay (handler.getTitle(), handler.getDate(), handler.getImage(), handler.getDescription());
    super.onPostExecute(result);
}
}

For more info how to pass parameter,return result etc.. AsyncTask Document