ListView的异步载入（笔记，多线程和AsyncTask)

异步载入最经常使用的两种方式： 多线程，线程池     AsyncTask

实例操作：

从一个站点上获取Json数据。然后将数据在ListView上显示。

1.创建item_layout布局 ， 改动主界面布局

item_layout.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="wrap_content"
    android:padding="4dp"
    android:orientation="horizontal" >
    
    <ImageView 
        android:id="@+id/iv_icon"
        android:layout_width="64dp"
        android:layout_height="64dp"
        android:src="@drawable/ic_launcher"
        />
    
    <LinearLayout 
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:gravity="center"
        android:paddingLeft="4dp">
        
        <TextView 
            android:id="@+id/tv_title"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textSize="15sp"
            android:text="Title"
            android:maxLines="1"
            />
        <TextView 
            android:id="@+id/tv_content"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:textSize="10sp"
            android:maxLines="3"
            android:text="Content"/>
        
    </LinearLayout>
    

</LinearLayout>

activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >

   <ListView 
       android:id="@+id/lv_main"
       android:layout_width="match_parent"
       android:layout_height="match_parent">
   </ListView>

</RelativeLayout>

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，知识点在凝视中

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<NewsBean> mList;
	private LayoutInflater mInflater;
	
	public NewsAdapter(Context context, List<NewsBean> 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相应的Jsong格式数据转化为我们封装的NewsBean对象
    private List<NewsBean> getJsonData(String url){
    	List<NewsBean> newsBeansList = new ArrayList<NewsBean>();
    	try {
			String jsonString = readStream(new URL(url).openStream());
			//new URL(url).openStream()此句与url.openConnection().getInputStream()同样
			//可依据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);
				
				//获取数据集中的每一个对象。把每一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字符串的内容就能够看到
				for(int i = 0; i <jsonArray.length(); i++){
					jsonObject = jsonArray.getJSONObject(i);
					newsBean = new NewsBean();
					newsBean.newsIconUrl = jsonObject.getString("picSmall");
					newsBean.newsTitle = jsonObject.getString("name");
					newsBean.newsContent = jsonObject.getString("description");
					newsBeansList.add(newsBean);
					
				}
			} catch (JSONException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
    	return newsBeansList;
    	
    }
    
    private String  readStream(InputStream is){ //通过is解析网页返回的数据
    	InputStreamReader isr;
    	String result = "";
    	try {
    		String line = "";
			isr = new InputStreamReader(is, "utf-8");//字节流转化为字符流
			BufferedReader br = new BufferedReader(isr);
			try {
				while((line = br.readLine()) != null){
					result += line;
					
				}
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		} catch (UnsupportedEncodingException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
    	return result;//网页数据组成的字符串
    }
    
    
    //实现网络的异步訪问
    class NewsAsyncTask extends AsyncTask<String, Void, List<NewsBean>>{
    	//返回的是NewsBean的集合

		@Override
		protected List<NewsBean> doInBackground(String... params) {
			// TODO Auto-generated method stub
			return getJsonData(params[0]); //传入的參数仅仅有一个。就是URL那个网址
		}
		
		//将数据设置到listView上
		@Override
		protected void onPostExecute(List<NewsBean> 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，并没有訪问网络。接下来就是採用异步处理载入网络图片。

此时的源码：还未实现图片的异步载入

然后添加缓存机制，已经下载的图片就不用再下载了。

怎样提高用户体验：使用缓存

LRU算法：Least Recently Used 最近最少使用算法 Android提供了LruCache类来实现这个缓存算法

ImageLoader.java  用来异步载入图片

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;


//专门用来载入图片
public class ImageLoader {
	
	private ImageView mImageView;
	private String mUrl;
	//创建Cache
	private LruCache<String, Bitmap>mCaches;//第一个是K。keyword。第二个是V。值，须要缓存的对象
	//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<String, Bitmap>(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();
			}
		};
	}
	
	//添加到缓存
	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){
		//从缓存中取出相应的图片
		Bitmap bitmap = getBitmapFromCache(url); //从内存中找到图片
		if(bitmap == null){//假设没有，必须去网络下载
			new NewsAsyncTask(imageView, url).execute(url);
		}else {
			imageView.setImageBitmap(bitmap);
		}
	}
	
	private class NewsAsyncTask extends AsyncTask<String, Void, Bitmap>{

		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);
			
		}
	}

}
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.AbsListView;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;

public class NewsAdapter extends BaseAdapter{

	
	private List<NewsBean> mList;
	private LayoutInflater mInflater;
	private ImageLoader mImageLoader;
	
	public NewsAdapter(Context context, List<NewsBean> data){
		mList = data;
		mInflater = LayoutInflater.from(context);
		mImageLoader = new ImageLoader();//保证了仅仅有一个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。是为了解决载入新图片时因为
		//缓存机制，先载入旧图片再载入新图片
		//new ImageLoader().showImageByThread(viewHolder.ivIcon,
				//urlString);//使用多线程的方式载入图片
		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相应的Jsong格式数据转化为我们封装的NewsBean对象
    private List<NewsBean> getJsonData(String url){
    	List<NewsBean> newsBeansList = new ArrayList<NewsBean>();
    	try {
			String jsonString = readStream(new URL(url).openStream());
			//new URL(url).openStream()此句与url.openConnection().getInputStream()同样
			//可依据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);
				
				//获取数据集中的每一个对象，把每一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字符串的内容就能够看到
				for(int i = 0; i <jsonArray.length(); i++){
					jsonObject = jsonArray.getJSONObject(i);
					newsBean = new NewsBean();
					newsBean.newsIconUrl = jsonObject.getString("picSmall");
					newsBean.newsTitle = jsonObject.getString("name");
					newsBean.newsContent = jsonObject.getString("description");
					newsBeansList.add(newsBean);
					
				}
			} catch (JSONException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
    	return newsBeansList;
    	
    }
    
    private String  readStream(InputStream is){ //通过is解析网页返回的数据
    	InputStreamReader isr;
    	String result = "";
    	try {
    		String line = "";
			isr = new InputStreamReader(is, "utf-8");//字节流转化为字符流
			BufferedReader br = new BufferedReader(isr);
			try {
				while((line = br.readLine()) != null){
					result += line;
					
				}
			} catch (IOException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		} catch (UnsupportedEncodingException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
    	return result;//网页数据组成的字符串
    }
    
    
    //实现网络的异步訪问
    class NewsAsyncTask extends AsyncTask<String, Void, List<NewsBean>>{
    	//返回的是NewsBean的集合

		@Override
		protected List<NewsBean> doInBackground(String... params) {
			// TODO Auto-generated method stub
			return getJsonData(params[0]); //传入的參数仅仅有一个，就是URL那个网址
		}
		
		//将数据设置到listView上
		@Override
		protected void onPostExecute(List<NewsBean> 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);
		}
    	
    }
    
   
    
}

源码下载：listView异步载入（多线程和AsyncTask)以及缓存的使用

怎样提高效率：

进一步优化：

ListView滑动停止后才载入可见项

ListView滑动时，取消全部载入项

也就是滑动时不进行UI绘制

代码有些地方不太懂。这里仅仅贴出源码：




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原文地址：https://www.cnblogs.com/wzjhoutai/p/7205683.html