ListView異步加載網絡數據

很多情況，我們需要從網絡加載數據，這個時候我們需要異步的方法去加載，並且實現數據緩存到本地的功能。

在主界面定義定義一個listview控件

main.xml代碼

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <ListView
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:id="@+id/listView"/>

</LinearLayout>

在定義listview條目顯示的界面

listview_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal" >

    <ImageView
        android:layout_width="120dp"
        android:layout_height="120dp"
        android:id="@+id/imageView"
        />

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textSize="18sp"
        android:textColor="#FFFFFF"
        android:id="@+id/textView"
        />
</LinearLayout>

自定義適配器類，繼承BaseAdapter

public class ContactAdapter extends BaseAdapter {
    private List<Contact> data;
    private int listviewItem;
    private File cache;
    LayoutInflater layoutInflater;

    public ContactAdapter(Context context, List<Contact> data, int listviewItem, File cache) {
        this.data = data;
        this.listviewItem = listviewItem;
        this.cache = cache;
        layoutInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }
    /**
     * 得到數據的總數
     */
    public int getCount() {
        return data.size();
    }
    /**
     * 根據數據索引得到集合所對應的數據
     */
    public Object getItem(int position) {
        return data.get(position);
    }

    public long getItemId(int position) {
        return position;
    }
<span style="white-space:pre">  </span>//listview顯示每一個條目都會調用此方法

<span style="white-space:pre">  </span>//convertView: ListView考慮到性能，會對第一屏的顯示的條目進行緩存

public View getView(int position, View convertView, ViewGroup parent) {
    ImageView imageView = null;
    TextView textView = null;

    if(convertView == null){
        convertView = layoutInflater.inflate(listviewItem, null);
        imageView = (ImageView) convertView.findViewById(R.id.imageView);
        textView = (TextView) convertView.findViewById(R.id.textView);
        convertView.setTag(new DataWrapper(imageView, textView));
    }else{
        DataWrapper dataWrapper = (DataWrapper) convertView.getTag();
        imageView = dataWrapper.imageView;
        textView = dataWrapper.textView;
    }
    Contact contact = data.get(position);
    textView.setText(contact.name);
    asyncImageLoad(imageView, contact.image);
    return convertView;
}
   private void asyncImageLoad(ImageView imageView, String path) {
    AsyncImageTask asyncImageTask = new AsyncImageTask(imageView);
    asyncImageTask.execute(path);

}

   private final class AsyncImageTask extends AsyncTask<String, Integer, Uri>{
    private ImageView imageView;
    public AsyncImageTask(ImageView imageView) {
        this.imageView = imageView;
    }
    protected Uri doInBackground(String... params) {//子線程中執行的
        try {

<span style="white-space:pre">              </span>//得到圖片的uri地址
                return ContactService.getImage(params[0], cache);
            } catch (Exception e) {
                e.printStackTrace();
            }
            return null;
        }
        protected void onPostExecute(Uri result) {//運行在主線程
            if(result!=null && imageView!= null)
                imageView.setImageURI(result);
        }
    }

<span style="white-space:pre">  </span>//數據包裹類，如果顯示一個條目就去查找控件id的話，這樣會很耗性能
    private final class DataWrapper{
        public ImageView imageView;
        public TextView textView;
        public DataWrapper(ImageView imageView, TextView textView) {
            this.imageView = imageView;
            this.textView = textView;
        }
    }
}

工具類

public class ContactService {

    /**
     * 獲取聯繫人
     * @return
     */
    public static List<Contact> getContacts() throws Exception{
        String path = "http://192.168.1.100:8080/web/list.xml";
        HttpURLConnection conn = (HttpURLConnection) new URL(path).openConnection();
        conn.setConnectTimeout(5000);
        conn.setRequestMethod("GET");
        if(conn.getResponseCode() == 200){
            return parseXML(conn.getInputStream());
        }
        return null;
    }
    /**
     * 解析xml文件，並獲取所有聯繫人信息
     * @param xml
     * @return
     * @throws Exception
     */
    private static List<Contact> parseXML(InputStream xml) throws Exception{
        List<Contact> contacts = new ArrayList<Contact>();
        Contact contact = null;
        XmlPullParser pullParser = Xml.newPullParser();
        pullParser.setInput(xml, "UTF-8");
        int event = pullParser.getEventType();
        while(event != XmlPullParser.END_DOCUMENT){
            switch (event) {
            case XmlPullParser.START_TAG:
                if("contact".equals(pullParser.getName())){
                    contact = new Contact();
                    contact.id = new Integer(pullParser.getAttributeValue(0));
                }else if("name".equals(pullParser.getName())){
                    contact.name = pullParser.nextText();
                }else if("image".equals(pullParser.getName())){
                    contact.image = pullParser.getAttributeValue(0);
                }
                break;

            case XmlPullParser.END_TAG:
                if("contact".equals(pullParser.getName())){
                    contacts.add(contact);
                    contact = null;
                }
                break;
            }
            event = pullParser.next();
        }
        return contacts;
    }
    /**
     * 獲取網絡圖片,如果圖片存在於緩存中，就返回該圖片，否則從網絡中加載該圖片並緩存起來
     * @param path 圖片路徑
     * @return
     */
    public static Uri getImage(String path, File cacheDir) throws Exception{// path -> MD5 ->32字符串.jpg
        File localFile = new File(cacheDir, MD5.getMD5(path)+ path.substring(path.lastIndexOf(".")));
        if(localFile.exists()){
            return Uri.fromFile(localFile);
        }else{
            HttpURLConnection conn = (HttpURLConnection) new URL(path).openConnection();
            conn.setConnectTimeout(5000);
            conn.setRequestMethod("GET");
            if(conn.getResponseCode() == 200){
                FileOutputStream outStream = new FileOutputStream(localFile);
                InputStream inputStream = conn.getInputStream();
                byte[] buffer = new byte[1024];
                int len = 0;
                while( (len = inputStream.read(buffer)) != -1){
                    outStream.write(buffer, 0, len);
                }
                inputStream.close();
                outStream.close();
                return Uri.fromFile(localFile);
            }
        }
        return null;
    }

}

主界面Activity的代碼

public class MainActivity extends Activity {
    ListView listView;
    File cache;

    Handler handler = new Handler(){
        public void handleMessage(Message msg) {
             listView.setAdapter(new ContactAdapter(MainActivity.this, (List<Contact>)msg.obj,
                     R.layout.listview_item, cache));
        }
    };

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        listView = (ListView) this.findViewById(R.id.listView);

        cache = new File(Environment.getExternalStorageDirectory(), "cache");
        if(!cache.exists()) cache.mkdirs();

        new Thread(new Runnable() {
            public void run() {
                try {
                    List<Contact> data = ContactService.getContacts();
                    handler.sendMessage(handler.obtainMessage(22, data));
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }

    @Override
    protected void onDestroy() {
        for(File file : cache.listFiles()){
            file.delete();
        }
        cache.delete();
        super.onDestroy();
    }

}

注意：由於應用訪問到網絡和sdcard，所以在功能清單文件中需要加入權限

<uses-permission android:name="android.permission.INTERNET"/>
 <!-- 在SDCard中創建與刪除文件權限 -->
<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>
<!-- 往SDCard寫入數據權限 -->
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>