ListView的异步加载（笔记，多线程和AsyncTask)
异步加载最常用的两种方式： 多线程，线程池 AsyncTask
实例操作：
从一个网站上获取Json数据，然后将数据在ListView上显示。
1.创建item_layout布局 ， 修改主界面布局
item_layout.xml
activity_main.xml
接下来在主页面实例化listView ,创建内部类newsAsyncTask继承AsyncTask, 创建类NewsBean, 读取json数据，将其解析到list中去， 创建一个Adapter，讲list中的数据，装载到listView中去，知识点在注释中
NewsBean.java
package com.example.newsimooc;
public class NewsBean {
public String newsIconUrl;//图片
public String newsTitle;
public String newsContent;
}
NewsAdapter.java
package com.example.newsimooc;
import java.util.ArrayList;
import java.util.List;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;
public class NewsAdapter extends BaseAdapter {
private List mList;
private LayoutInflater mInflater;
public NewsAdapter(Context context, List data){
mList = data;
mInflater = LayoutInflater.from(context);
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return mList.size();
}
@Override
public Object getItem(int position) {
// TODO Auto-generated method stub
return mList.get(position);
}
@Override
public long getItemId(int position) {
// TODO Auto-generated method stub
return position;
}
@Override
public View getView(int position, View converView, ViewGroup arg2) {
// TODO Auto-generated method stub
ViewHolder viewHolder = null;
if(converView == null){
viewHolder = new ViewHolder();
converView = mInflater.inflate(R.layout.item_layout, null);
viewHolder.ivIcon =(ImageView) converView.findViewById(R.id.iv_icon);
viewHolder.tvTitle = (TextView)converView.findViewById(R.id.tv_title);
viewHolder.tvContent = (TextView)converView.findViewById(R.id.tv_content);
converView.setTag(viewHolder);
}else{
viewHolder = (ViewHolder) converView.getTag();
}
viewHolder.ivIcon.setImageResource(R.drawable.ic_launcher);
viewHolder.tvTitle.setText(mList.get(position).newsTitle);
viewHolder.tvContent.setText(mList.get(position).newsContent);
return converView;
}
class ViewHolder{
public TextView tvTitle, tvContent;
public ImageView ivIcon;
}
}
MainActivity.java
package com.example.newsimooc;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import android.os.AsyncTask;
import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.view.Menu;
import android.widget.ListView;
public class MainActivity extends Activity {
private ListView mListView;
private static String URL = "http://www.imooc.com/api/teacher?type=4&num=34";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mListView = (ListView) findViewById(R.id.lv_main);
new NewsAsyncTask().execute(URL);
}
//获取Jason数据并解析到newsbean中,将url对应的Jsong格式数据转化为我们封装的NewsBean对象
private List getJsonData(String url){
List newsBeansList = new ArrayList();
try {
String jsonString = readStream(new URL(url).openStream());
//new URL(url).openStream()此句与url.openConnection().getInputStream()相同
//可根据URL直接联网获取网络数据，简单粗暴，返回值类型为InputStream
JSONObject jsonObject;
NewsBean newsBean;
try {
jsonObject = new JSONObject(jsonString);
//获取数据集中的每个对象，把每个对象都封装到一个NewsBean中
JSONArray jsonArray = jsonObject.getJSONArray("data");//data是数据集的名字，查看捕获到的JSONSTRING字符串的内容就可以看到
for(int i = 0; i >{
//返回的是NewsBean的集合
@Override
protected List doInBackground(String... params) {
// TODO Auto-generated method stub
return getJsonData(params[0]); //传入的参数只有一个，就是URL那个网址
}
//将数据设置到listView上
@Override
protected void onPostExecute(List newsBeans) { //doInBackground返回的是一个List
// TODO Auto-generated method stub
super.onPostExecute(newsBeans);
NewsAdapter adapter = new NewsAdapter(MainActivity.this, newsBeans);
mListView.setAdapter(adapter);
}
}
}
至此，已经实现了抓取数据，并在listView上显示，如图：
注意到这里图片是默认的，并没有访问网络，接下来就是采用异步处理加载网络图片。
此时的源代码：还未实现图片的异步加载
然后增加缓存机制，已经下载的图片就不用再下载了。
如何提高用户体验：使用缓存
LRU算法：Least Recently Used 近期最少使用算法 Android提供了LruCache类来实现这个缓存算法
ImageLoader.java 用来异步加载图片
package com.example.newsimooc;
import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import android.R.integer;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Handler;
import android.os.Message;
import android.support.v4.util.LruCache;
import android.webkit.URLUtil;
import android.widget.ImageView;
//专门用来加载图片
public class ImageLoader {
private ImageView mImageView;
private String mUrl;
//创建Cache
private LruCachemCaches;//第一个是K，关键字，第二个是V，值，需要缓存的对象
//实际上是一个map
public ImageLoader(){
//获取最大可用内存
int maxMemory = (int) Runtime.getRuntime().maxMemory();
int cacheSize = maxMemory / 4;
mCaches = new LruCache(cacheSize){
//必须重写
@Override
protected int sizeOf(String key, Bitmap value) {
//在每次存入缓存的时候调用
return value.getByteCount();
}
};
}
//增加到缓存
public void addBitmapToCache(String url, Bitmap bitmap){
if(getBitmapFromCache(url)== null ){
mCaches.put(url, bitmap);
}
}
//从缓存中获取数据
public Bitmap getBitmapFromCache(String url){
return mCaches.get(url);
}
private Handler mhandler = new Handler(){ //多线程方法中使用的
public void handleMessage(android.os.Message msg) {
super.handleMessage(msg);
if (mImageView.getTag().equals(mUrl)) {
mImageView.setImageBitmap((Bitmap) msg.obj);
}
};
};
//使用多线程的方式显示图片
public void showImageByThread(ImageView imageView, final String url){
mImageView = imageView;
mUrl = url;
new Thread(){
@Override
public void run() {
// TODO Auto-generated method stub
super.run();
Bitmap bitmap = getBitmapFromURL(url);
//此时bitmap不能直接装载在imageView上，要以消息的形式发送给主线程
//由handler装载图片
Message message =Message.obtain();
message.obj = bitmap;
mhandler.sendMessage(message);
}
}.start();
}
//将url解析为bitmap
public Bitmap getBitmapFromURL(String urlString){
Bitmap bitmap;
InputStream is = null;
try {
URL url = new URL(urlString);
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
is = new BufferedInputStream(connection.getInputStream());
bitmap = BitmapFactory.decodeStream(is);
connection.disconnect();
return bitmap;
}
catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}finally{
try {
is.close();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
return null;
}
public void showImageByAsyncTask(ImageView imageView, String url){
//从缓存中取出对应的图片
Bitmap bitmap = getBitmapFromCache(url); //从内存中找到图片
if(bitmap == null){//如果没有，必须去网络下载
new NewsAsyncTask(imageView, url).execute(url);
}else {
imageView.setImageBitmap(bitmap);
}
}
private class NewsAsyncTask extends AsyncTask{
private ImageView mImageView;
private String mUrl;
public NewsAsyncTask(ImageView imageView ,String urlString){
mImageView = imageView;
mUrl = urlString;
}
@Override
protected Bitmap doInBackground(String... params) {
// TODO Auto-generated method stub
String url = params[0];
//从网络获取图片
Bitmap bitmap = getBitmapFromURL(params[0]);
if(bitmap != null){
//下载到了图片 就保存到Cache中去
addBitmapToCache(url, bitmap);
}
return bitmap;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
// TODO Auto-generated method stub
super.onPostExecute(bitmap);
if(mImageView.getTag().equals(mUrl))
mImageView.setImageBitmap(bitmap);
}
}
}
package com.example.newsimooc;
import java.util.ArrayList;
import java.util.List;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AbsListView;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;
public class NewsAdapter extends BaseAdapter{
private List mList;
private LayoutInflater mInflater;
private ImageLoader mImageLoader;
public NewsAdapter(Context context, List data){
mList = data;
mInflater = LayoutInflater.from(context);
mImageLoader = new ImageLoader();//保证了只有一个LRUCache
}
@Override
public int getCount() {
// TODO Auto-generated method stub
return mList.size();
}
@Override
public Object getItem(int position) {
// TODO Auto-generated method stub
return mList.get(position);
}
@Override
public long getItemId(int position) {
// TODO Auto-generated method stub
return position;
}
@Override
public View getView(int position, View converView, ViewGroup arg2) {
// TODO Auto-generated method stub
ViewHolder viewHolder = null;
if(converView == null){
viewHolder = new ViewHolder();
converView = mInflater.inflate(R.layout.item_layout, null);
viewHolder.ivIcon =(ImageView) converView.findViewById(R.id.iv_icon);
viewHolder.tvTitle = (TextView)converView.findViewById(R.id.tv_title);
viewHolder.tvContent = (TextView)converView.findViewById(R.id.tv_content);
converView.setTag(viewHolder);
}else{
viewHolder = (ViewHolder) converView.getTag();
}
viewHolder.ivIcon.setImageResource(R.drawable.ic_launcher);
String urlString = mList.get(position).newsIconUrl;
viewHolder.ivIcon.setTag(urlString);//身份和URL绑定，是为了解决加载新图片时由于
//缓存机制，先加载旧图片再加载新图片
//new ImageLoader().showImageByThread(viewHolder.ivIcon,
//urlString);//使用多线程的方式加载图片
mImageLoader.showImageByAsyncTask(viewHolder.ivIcon, urlString);
viewHolder.tvTitle.setText(mList.get(position).newsTitle);
viewHolder.tvContent.setText(mList.get(position).newsContent);
return converView;
}
class ViewHolder{
public TextView tvTitle, tvContent;
public ImageView ivIcon;
}
}
MainActivity.java
package com.example.newsimooc;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import android.os.AsyncTask;
import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.view.Menu;
import android.widget.ListView;
public class MainActivity extends Activity {
private ListView mListView;
private static String URL = "http://www.imooc.com/api/teacher?type=4&num=34";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mListView = (ListView) findViewById(R.id.lv_main);
new NewsAsyncTask().execute(URL);
}
//获取Jason数据并解析到newsbean中,将url对应的Jsong格式数据转化为我们封装的NewsBean对象
private List getJsonData(String url){
List newsBeansList = new ArrayList();
try {
String jsonString = readStream(new URL(url).openStream());
//new URL(url).openStream()此句与url.openConnection().getInputStream()相同
//可根据URL直接联网获取网络数据，简单粗暴，返回值类型为InputStream
JSONObject jsonObject;
NewsBean newsBean;
try {
jsonObject = new JSONObject(jsonString);
//获取数据集中的每个对象，把每个对象都封装到一个NewsBean中
JSONArray jsonArray = jsonObject.getJSONArray("data");//data是数据集的名字，查看捕获到的JSONSTRING字符串的内容就可以看到
for(int i = 0; i >{
//返回的是NewsBean的集合
@Override
protected List doInBackground(String... params) {
// TODO Auto-generated method stub
return getJsonData(params[0]); //传入的参数只有一个，就是URL那个网址
}
//将数据设置到listView上
@Override
protected void onPostExecute(List newsBeans) { //doInBackground返回的是一个List
// TODO Auto-generated method stub
super.onPostExecute(newsBeans);
NewsAdapter adapter = new NewsAdapter(MainActivity.this, newsBeans);
mListView.setAdapter(adapter);
}
}
}
源代码下载： listView异步加载（多线程和AsyncTask)以及缓存的使用
如何提高效率：
进一步优化：
ListView滑动停止后才加载可见项
ListView滑动时，取消所有加载项也就是滑动时不进行UI绘制
代码有些地方不太懂，这里只贴出源代码：