ListView异步加载图片

ListView异步加载图片是非常实用的方法，凡是是要通过网络获取图片资源一般使用这种方法比较好，用户体验好，不用让用户等待下去，下面就说实现方法，先贴上主方法的代码： 

package cn.wangmeng.test;

 

import java.io.IOException;

import java.io.InputStream;

import java.lang.ref.SoftReference;

import java.net.MalformedURLException;

import java.net.URL;

import java.util.HashMap;

 

import android.graphics.drawable.Drawable;

import android.os.Handler;

import android.os.Message;

 

public class AsyncImageLoader {

 

private HashMap<String, SoftReference<Drawable>> imageCache;

 

public AsyncImageLoader() {

imageCache = new HashMap<String, SoftReference<Drawable>>();

}

 

public Drawable loadDrawable(final String imageUrl, final ImageCallback imageCallback) {

if (imageCache.containsKey(imageUrl)) {

SoftReference<Drawable> softReference = imageCache.get(imageUrl);

Drawable drawable = softReference.get();

if (drawable != null) {

return drawable;

}

}

final Handler handler = new Handler() {

public void handleMessage(Message message) {

imageCallback.imageLoaded((Drawable) message.obj, imageUrl);

}

};

new Thread() {

@Override

public void run() {

Drawable drawable = loadImageFromUrl(imageUrl);

imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));

Message message = handler.obtainMessage(0, drawable);

handler.sendMessage(message);

}

}.start();

return null;

}

 

public static Drawable loadImageFromUrl(String url) {

URL m;

InputStream i = null;

try {

m = new URL(url);

i = (InputStream) m.getContent();

} catch (MalformedURLException e1) {

e1.printStackTrace();

} catch (IOException e) {

e.printStackTrace();

}

Drawable d = Drawable.createFromStream(i, "src");

return d;

}

 

public interface ImageCallback {

public void imageLoaded(Drawable imageDrawable, String imageUrl);

}

 

}

以上代码是实现异步获取图片的主方法，SoftReference是软引用，是为了更好的为了系统回收变量，重复的URL直接返回已有的资源，实现回调函数，让数据成功后，更新到UI线程。 

几个辅助类文件：

package cn.wangmeng.test;

 

public class ImageAndText {

private String imageUrl;

private String text;

 

public ImageAndText(String imageUrl, String text) {

this.imageUrl = imageUrl;

this.text = text;

}

public String getImageUrl() {

return imageUrl;

}

public String getText() {

return text;

}

}

package cn.wangmeng.test;

 

import android.view.View;

import android.widget.ImageView;

import android.widget.TextView;

 

public class ViewCache {

 

private View baseView;

private TextView textView;

private ImageView imageView;

 

public ViewCache(View baseView) {

this.baseView = baseView;

}

 

public TextView getTextView() {

if (textView == null) {

textView = (TextView) baseView.findViewById(R.id.text);

}

return textView;

}

 

public ImageView getImageView() {

if (imageView == null) {

imageView = (ImageView) baseView.findViewById(R.id.image);

}

return imageView;

}

 

}

ViewCache是辅助获取adapter的子元素布局 

package cn.wangmeng.test;

 

import java.util.List;

 

import cn.wangmeng.test.AsyncImageLoader.ImageCallback;

 

import android.app.Activity;

import android.graphics.drawable.Drawable;

import android.view.LayoutInflater;

import android.view.View;

import android.view.ViewGroup;

import android.widget.ArrayAdapter;

import android.widget.ImageView;

import android.widget.ListView;

import android.widget.TextView;

 

public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> {

 

private ListView listView;

private AsyncImageLoader asyncImageLoader;

 

public ImageAndTextListAdapter(Activity activity, List<ImageAndText> imageAndTexts, ListView listView) {

super(activity, 0, imageAndTexts);

this.listView = listView;

asyncImageLoader = new AsyncImageLoader();

}

 

public View getView(int position, View convertView, ViewGroup parent) {

Activity activity = (Activity) getContext();

 

// Inflate the views from XML

View rowView = convertView;

ViewCache viewCache;

if (rowView == null) {

LayoutInflater inflater = activity.getLayoutInflater();

rowView = inflater.inflate(R.layout.image_and_text_row, null);

viewCache = new ViewCache(rowView);

rowView.setTag(viewCache);

} else {

viewCache = (ViewCache) rowView.getTag();

}

ImageAndText imageAndText = getItem(position);

 

// Load the image and set it on the ImageView

String imageUrl = imageAndText.getImageUrl();

ImageView imageView = viewCache.getImageView();

imageView.setTag(imageUrl);

Drawable cachedImage = asyncImageLoader.loadDrawable(imageUrl, new ImageCallback() {

public void imageLoaded(Drawable imageDrawable, String imageUrl) {

ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);

if (imageViewByTag != null) {

imageViewByTag.setImageDrawable(imageDrawable);

}

}

});

if (cachedImage == null) {

imageView.setImageResource(R.drawable.default_image);

}else{

imageView.setImageDrawable(cachedImage);

}

// Set the text on the TextView

TextView textView = viewCache.getTextView();

textView.setText(imageAndText.getText());

 

return rowView;

}

 

}

ImageAndTextListAdapter是实现ListView的Adapter，里面有个技巧就是imageView.setTag(imageUrl)，setTag是存储数据的，这样是为了保证在回调函数时，listview去更新自己对应item,大家仔细阅读就知道了。 

最后贴出布局文件：

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:orientation="horizontal"

android:layout_width="fill_parent"

android:layout_height="wrap_content">

 

<ImageView android:id="@+id/image"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

/>

 

<TextView android:id="@+id/text"

android:layout_width="wrap_content"

android:layout_height="wrap_content"/>

 

</LinearLayout>