Android 使用Pull方法解析XML文件的方法

Pull解析方法給應用程序完全的控制文檔該怎麼樣被解析。Android中對Pull方法提供了支持的API，主要是
復制代碼 代碼如下:
org.xmlpull.v1.XmlPullParser;
org.xmlpull.v1.XmlPullParserFactory;

二個類，其中主要使用的是XmlPullParser，XmlPullParserFactory是一個工廠，用於構建XmlPullParser對象。
應用程序通過調用XmlPullParser.next()等方法來產生Event，然後再處理Event。可以看到它與Push方法的不同，Push方法是由Parser自己主動產生Event，回調給應用程序。而Pull方法是主動的調用Parser的方法才能產生事件。
假如XML中的語句是這樣的："<author country="United States">James Elliott</author>"，author是TAG，country是ATTRIBUTE，"James Elliott"是TEXT。
要想解析文檔先要構建一個XmlPullParser對象
復制代碼 代碼如下:
final XmlPullParserFactory factory = XmlPullParserFactory.newInstance();
factory.setNamespaceAware(true);
final XmlPullParser parser = factory.newPullParser();

Pull解析是一個遍歷文檔的過程，每次調用next()，nextTag(), nextToken()和nextText()都會向前推進文檔，並使Parser停留在某些事件上面，但是不能倒退。
然後把文檔設置給Parser
復制代碼 代碼如下:
parser.setInput(new StringReader("<author country=\"United States\">James Elliott</author>");

這時，文檔剛被初始化，所以它應該位於文檔的開始，事件應該是START_DOCUMENT，可以通過XmlPullParser.getEventType()來獲取。然後調用next()會產生
START_TAG，這個事件告訴應用程序一個標簽已經開始了，調用getName()會返回"author";再next()會產生
TEXT事件，調用getText()會返回"James Elliott"，再next()，會產生
END_TAG，這個告訴你一個標簽已經處理完了，再next()，會產生
END_DOCUMENT，它告訴你整個文檔已經處理完成了。
除了next()外，nextToken()也可以使用，只不過它會返回更加詳細的事件，比如 COMMENT, CDSECT, DOCDECL, ENTITY等等非常詳細的信息。如果程序得到比較底層的信息，可以用nextToken()來驅動並處理詳細的事件。需要注意一點的是TEXT事件是有可能返回空白的White Spaces比如換行符或空格等。
另外有二個非常實用的方法nextTag()和nextText()
nextTag()--首先它會忽略White Spaces，如果可以確定下一個是START_TAG或END_TAG，就可以調用nextTag()直接跳過去。通常它有二個用處：當START_TAG時，如果能確定這個TAG含有子TAG，那麼就可以調用nextTag()產生子標簽的START_TAG事件；當END_TAG時，如果確定不是文檔結尾，就可以調用nextTag()產生下一個標簽的START_TAG。在這二種情況下如果用next()會有TEXT事件，但返回的是換行符或空白符。
nextText()--它只能在START_TAG時調用。當下一個元素是TEXT時，TEXT的內容會返回；當下一個元素是END_TAG時，也就是說這個標簽的內容為空，那麼空字串返回；這個方法返回後，Parser會停在END_TAG上。比如：
復制代碼 代碼如下:
<author>James Elliott</author>
<author></author>
<author/>

當START_TAG時，調用nextText()，依次返回：
"James Elliott"
""(empty)
""(empty)
這個方法在處理沒有子標簽的標簽時很有用。比如：
復制代碼 代碼如下:
<title>What Is Hibernate</title>
<author>James Elliott</author>
<category>Web</category>

就可以用以下代碼來處理：
復制代碼 代碼如下:
        while (eventType != XmlPullParser.END_TAG) {
            switch (eventType) {
            case XmlPullParser.START_TAG:
                tag = parser.getName();
                final String content = parser.nextText();
                Log.e(TAG, tag + ": [" + content + "]");
                eventType = parser.nextTag();
                break;
            default:
                break;
            }
        }

這就要比用next()來處理方便多了，可讀性也大大的加強了。
最後附上一個解析XML的實例Android程序
復制代碼 代碼如下:
import java.io.IOException;
import java.io.InputStream;
import org.xmlpull.v1.XmlPullParser;
import org.xmlpull.v1.XmlPullParserException;
import org.xmlpull.v1.XmlPullParserFactory;
import android.util.Log;
public class RssPullParser extends RssParser {
    private final String TAG = FeedSettings.GLOBAL_TAG;

    private InputStream mInputStream;

    public RssPullParser(InputStream is) {
        mInputStream = is;
    }

    public void parse() throws ReaderBaseException, XmlPullParserException, IOException {
        if (mInputStream == null) {
            throw new ReaderBaseException("no input source, did you initialize this class correctly?");
        }
        final XmlPullParserFactory factory = XmlPullParserFactory.newInstance();
        factory.setNamespaceAware(true);
        final XmlPullParser parser = factory.newPullParser();

        parser.setInput(mInputStream);
        int eventType = parser.getEventType();
        if (eventType != XmlPullParser.START_DOCUMENT) {
            throw new ReaderBaseException("Not starting with 'start_document'");
        }
        eventType = parseRss(parser);
        if (eventType != XmlPullParser.END_DOCUMENT) {
            throw new ReaderBaseException("not ending with 'end_document', do you finish parsing?");
        }
        if (mInputStream != null) {
            mInputStream.close();
        } else {
            Log.e(TAG, "inputstream is null, XmlPullParser closed it??");
        }
    }

    /**
     * Parsing the Xml document. Current type must be Start_Document.
     * After calling this, Parser is positioned at END_DOCUMENT.
     * @param parser
     * @return event end_document
     * @throws XmlPullParserException
     * @throws ReaderBaseException
     * @throws IOException
     */
    private int parseRss(XmlPullParser parser) throws XmlPullParserException, ReaderBaseException, IOException {
        int eventType = parser.getEventType();
        if (eventType != XmlPullParser.START_DOCUMENT) {
            throw new ReaderBaseException("not starting with 'start_document', is this a new document?");
        }
        Log.e(TAG, "starting document, are you aware of that!");
        eventType = parser.next();
        while (eventType != XmlPullParser.END_DOCUMENT) {
            switch (eventType) {
            case XmlPullParser.START_TAG: {
                Log.e(TAG, "start tag: '" + parser.getName() + "'");
                final String tagName = parser.getName();
                if (tagName.equals(RssFeed.TAG_RSS)) {
                    Log.e(TAG, "starting an RSS feed <<");
                    final int attrSize = parser.getAttributeCount();
                    for (int i = 0; i < attrSize; i++) {
                        Log.e(TAG, "attr '" + parser.getAttributeName(i) + "=" + parser.getAttributeValue(i) + "'");
                    }
                } else if (tagName.equals(RssFeed.TAG_CHANNEL)) {
                    Log.e(TAG, "\tstarting an Channel <<");
                    parseChannel(parser);
                }
                break;
            }
            case XmlPullParser.END_TAG: {
                Log.e(TAG, "end tag: '" + parser.getName() + "'");
                final String tagName = parser.getName();
                if (tagName.equals(RssFeed.TAG_RSS)) {
                    Log.e(TAG, ">> edning an RSS feed");
                } else if (tagName.equals(RssFeed.TAG_CHANNEL)) {
                    Log.e(TAG, "\t>> ending an Channel");    
                }
                break;
            }
            default:
                break;
            }
            eventType = parser.next();
        }
        Log.e(TAG, "end of document, it is over");
        return parser.getEventType();
    }

    /**
     * Parse a channel. MUST be start tag of an channel, otherwise exception thrown.
     * Param XmlPullParser
     * After calling this function, parser is positioned at END_TAG of Channel.
     * return end tag of a channel
     * @throws XmlPullParserException
     * @throws ReaderBaseException
     * @throws IOException
     */
    private int parseChannel(XmlPullParser parser) throws XmlPullParserException, ReaderBaseException, IOException {
        int eventType = parser.getEventType();
        String tagName = parser.getName();
        if (eventType != XmlPullParser.START_TAG || !RssFeed.TAG_CHANNEL.equals(tagName)) {
            throw new ReaderBaseException("not start with 'start tag', is this a start of a channel?");
        }
        Log.e(TAG, "\tstarting " + tagName);
        eventType = parser.nextTag();
        while (eventType != XmlPullParser.END_TAG) {
            switch (eventType) {
            case XmlPullParser.START_TAG: {
                final String tag = parser.getName();
                if (tag.equals(RssFeed.TAG_IMAGE)) {
                    parseImage(parser);
                } else if (tag.equals(RssFeed.TAG_ITEM)) {
                    parseItem(parser);
                } else {
                    final String content = parser.nextText();
                    Log.e(TAG, tag + ": [" + content + "]");
                }
                // now it SHOULD be at END_TAG, ensure it
                if (parser.getEventType() != XmlPullParser.END_TAG) {
                    throw new ReaderBaseException("not ending with 'end tag', did you finish parsing sub item?");
                }
                eventType = parser.nextTag();
                break;
            }
            default:
                break;
            }
        }
        Log.e(TAG, "\tending " + parser.getName());
        return parser.getEventType();
    }

    /**
     * Parse image in a channel.
     * Precondition: position must be at START_TAG and tag MUST be 'image'
     * Postcondition: position is END_TAG of '/image'
     * @throws IOException
     * @throws XmlPullParserException
     * @throws ReaderBaseException
     */
    private int parseImage(XmlPullParser parser) throws XmlPullParserException, IOException, ReaderBaseException {
        int eventType = parser.getEventType();
        String tag = parser.getName();
        if (eventType != XmlPullParser.START_TAG || !RssFeed.TAG_IMAGE.equals(tag)) {
            throw new ReaderBaseException("not start with 'start tag', is this a start of an image?");
        }
        Log.e(TAG, "\t\tstarting image " + tag);
        eventType = parser.nextTag();
        while (eventType != XmlPullParser.END_TAG) {
            switch (eventType) {
            case XmlPullParser.START_TAG:
                tag = parser.getName();
                Log.e(TAG, tag + ": [" + parser.nextText() + "]");
                // now it SHOULD be at END_TAG, ensure it
                if (parser.getEventType() != XmlPullParser.END_TAG) {
                    throw new ReaderBaseException("not ending with 'end tag', did you finish parsing sub item?");
                }
                eventType = parser.nextTag();
                break;
            default:
                break;
            }
        }
        Log.e(TAG, "\t\tending image " + parser.getName());
        return parser.getEventType();
    }

    /**
     * Parse an item in a channel.
     * Precondition: position must be at START_TAG and tag MUST be 'item'
     * Postcondition: position is END_TAG of '/item'
     * @throws IOException
     * @throws XmlPullParserException
     * @throws ReaderBaseException
     */
    private int parseItem(XmlPullParser parser) throws XmlPullParserException, IOException, ReaderBaseException {
        int eventType = parser.getEventType();
        String tag = parser.getName();
        if (eventType != XmlPullParser.START_TAG || !RssFeed.TAG_ITEM.equals(tag)) {
            throw new ReaderBaseException("not start with 'start tag', is this a start of an item?");
        }
        Log.e(TAG, "\t\tstarting " + tag);
        eventType = parser.nextTag();
        while (eventType != XmlPullParser.END_TAG) {
            switch (eventType) {
            case XmlPullParser.START_TAG:
                tag = parser.getName();
                final String content = parser.nextText();
                Log.e(TAG, tag + ": [" + content + "]");
                // now it SHOULD be at END_TAG, ensure it
                if (parser.getEventType() != XmlPullParser.END_TAG) {
                    throw new ReaderBaseException("not ending with 'end tag', did you finish parsing sub item?");
                }
                eventType = parser.nextTag();
                break;
            default:
                break;
            }
        }
        Log.e(TAG, "\t\tending " + parser.getName());
        return parser.getEventType();
    }
}