ListView异步加载网络数据

很多情况，我们需要从网络加载数据，这个时候我们需要异步的方法去加载，并且实现数据缓存到本地的功能。

在主界面定义定义一个listview控件

main.xml代码

<?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>

在定义listview条目显示的界面

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>

自定义适配器类，继承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);
	}
	/**
	 * 得到数据的总数
	 */
	public int getCount() {
		return data.size();
	}
	/**
	 * 根据数据索引得到集合所对应的数据
	 */
	public Object getItem(int position) {
		return data.get(position);
	}
	
	public long getItemId(int position) {
		return position;
	}
	//listview显示每一个条目都会调用此方法

	//convertView: ListView考虑到性能，会对第一屏的显示的条目进行缓存

	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) {//子线程中执行的
			try {

				//得到图片的uri地址
				return ContactService.getImage(params[0], cache);
			} catch (Exception e) {
				e.printStackTrace();
			}
			return null;
		}
		protected void onPostExecute(Uri result) {//运行在主线程
			if(result!=null && imageView!= null)
				imageView.setImageURI(result);
		}	
    }

	//数据包裹类，如果显示一个条目就去查找控件id的话，这样会很耗性能
	private final class DataWrapper{
		public ImageView imageView;
		public TextView textView;
		public DataWrapper(ImageView imageView, TextView textView) {
			this.imageView = imageView;
			this.textView = textView;
		}
	}
}

工具类

public class ContactService {

	/**
	 * 获取联系人
	 * @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，并获取所有联系人信息
	 * @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;
	}
	/**
	 * 获取网络图片,如果图片存在于缓存中，就返回该图片，否则从网络中加载该图片并缓存起来
	 * @param path 图片路径
	 * @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的代码

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：由于应用访问到网络和sdcard，所以在功能清单文件中需要加入权限

<uses-permission android:name="android.permission.INTERNET"/>
 <!-- 在SDCard中创建与删除文件权限 -->
<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>
<!-- 往SDCard写入数据权限 -->
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

本文主要是观看传智播客视频后自己做的笔记，具体的代码可以去传智播客的网站下载，这里就不提供了。