学步园

r写这篇文章并不是教大家怎么样用listview异步加载图片，因为这样的文章在网上已经有很多了，比如这位仁兄写的就很好：

http://www.iteye.com/topic/685986

我也是因为看了这篇文章而受到了启发。

先说说这篇文章的优点把，开启线程异步加载图片，然后刷新UI显示图片，而且通过弱引用缓存网络加载的图片，节省了再次连接网络的开销。

这样做无疑是非常可取的方法，但是加载图片时仍然会感觉到轻微的卡屏现象，特别是listview里的item在进行快速滑动的时候。

我找了一下原因，可能是在listview快速滑动屏幕的时候划过的item太多 而且每次调用getView方法后就会异步的在过去某个时间内用handler刷新一下UI，

如果在同一时间调用handler刷新UI次数多了就会造成这样的卡屏现象。

后来又一想，其实我们完全没有必要在listview正在滑动的时候去后台加载图片（不管这是图片是在缓存里还是在网络上），这样无疑造成了很大的资源浪费。

我们只需要在listview滑动停止之后再去加载listview里面显示的几个item里面的图片就好了。

根据以上想法，我做了一些设计改造：

1.在adapter 的 getview方法里面启动加载图片的thread，如果listview在滑动则wait

2.监听listview滑动停止事件，获得listview显示的item的最上面和最下面的序号，并唤醒所有加载图片的thread，判断加载图片的序号是否是在范围内，如果是则继续加载，如果不是则结束thread

Java代码

1. @Override 
2. public View getView(int position, View convertView, ViewGroup parent) { 
3.     if(convertView == null){ 
4.         convertView = mInflater.inflate(R.layout.book_item_adapter,
   null); 
5.     } 
6.     BookModel model = mModels.get(position); 
7.     convertView.setTag(position); 
8.     ImageView iv = (ImageView) convertView.findViewById(R.id.sItemIcon); 
9.     TextView sItemTitle =  (TextView) convertView.findViewById(R.id.sItemTitle); 
10.     TextView sItemInfo =  (TextView) convertView.findViewById(R.id.sItemInfo); 
11.     sItemTitle.setText(model.book_name); 
12.     sItemInfo.setText(model.out_book_url); 
13.     iv.setBackgroundResource(R.drawable.rc_item_bg); 
14.     syncImageLoader.loadImage(position,model.out_book_pic,imageLoadListener); 
15.     return  convertView; 
16. } 
17.  
18. SyncImageLoader.OnImageLoadListener imageLoadListener =
    new SyncImageLoader.OnImageLoadListener(){ 
19.  
20.     @Override 
21.     public void onImageLoad(Integer t, Drawable drawable) { 
22.         //BookModel model = (BookModel) getItem(t); 
23.         View view = mListView.findViewWithTag(t); 
24.         if(view != null){ 
25.             ImageView iv = (ImageView) view.findViewById(R.id.sItemIcon); 
26.             iv.setBackgroundDrawable(drawable); 
27.         } 
28.     } 
29.     @Override 
30.     public void onError(Integer t) { 
31.         BookModel model = (BookModel) getItem(t); 
32.         View view = mListView.findViewWithTag(model); 
33.         if(view != null){ 
34.             ImageView iv = (ImageView) view.findViewById(R.id.sItemIcon); 
35.             iv.setBackgroundResource(R.drawable.rc_item_bg); 
36.         } 
37.     } 
38.      
39. }; 
40.  
41. public void loadImage(){ 
42.     int start = mListView.getFirstVisiblePosition(); 
43.     int end =mListView.getLastVisiblePosition(); 
44.     if(end >= getCount()){ 
45.         end = getCount() -1; 
    
46.     } 
47.     syncImageLoader.setLoadLimit(start, end); 
48.     syncImageLoader.unlock(); 
49. } 
50.  
51. AbsListView.OnScrollListener onScrollListener = new AbsListView.OnScrollListener() { 
52.      
53.     @Override 
54.     public void onScrollStateChanged(AbsListView view,int scrollState) { 
    
55.         switch (scrollState) { 
    
56.             case AbsListView.OnScrollListener.SCROLL_STATE_FLING: 
57.                 DebugUtil.debug("SCROLL_STATE_FLING"); 
58.                 syncImageLoader.lock(); 
59.                 break; 
60.             case AbsListView.OnScrollListener.SCROLL_STATE_IDLE: 
61.                 DebugUtil.debug("SCROLL_STATE_IDLE"); 
62.                 loadImage(); 
63.                 //loadImage(); 
    
64.                 break; 
65.             case AbsListView.OnScrollListener.SCROLL_STATE_TOUCH_SCROLL: 
66.                 syncImageLoader.lock(); 
67.                 break; 
68.  
69.             default: 
70.                 break; 
71.         } 
72.          
73.     } 
74.      
75.     @Override 
76.     public void onScroll(AbsListView view,int firstVisibleItem, 
    
77.             int visibleItemCount,
    int totalItemCount) { 
78.         // TODO Auto-generated method stub 
79.          
80.     } 
81. }; 

@Override
public View getView(int position, View convertView, ViewGroup parent) {
	if(convertView == null){
		convertView = mInflater.inflate(R.layout.book_item_adapter, null);
	}
	BookModel model = mModels.get(position);
	convertView.setTag(position);
	ImageView iv = (ImageView) convertView.findViewById(R.id.sItemIcon);
	TextView sItemTitle =  (TextView) convertView.findViewById(R.id.sItemTitle);
	TextView sItemInfo =  (TextView) convertView.findViewById(R.id.sItemInfo);
	sItemTitle.setText(model.book_name);
	sItemInfo.setText(model.out_book_url);
	iv.setBackgroundResource(R.drawable.rc_item_bg);
	syncImageLoader.loadImage(position,model.out_book_pic,imageLoadListener);
	return  convertView;
}

SyncImageLoader.OnImageLoadListener imageLoadListener = new SyncImageLoader.OnImageLoadListener(){

	@Override
	public void onImageLoad(Integer t, Drawable drawable) {
		//BookModel model = (BookModel) getItem(t);
		View view = mListView.findViewWithTag(t);
		if(view != null){
			ImageView iv = (ImageView) view.findViewById(R.id.sItemIcon);
			iv.setBackgroundDrawable(drawable);
		}
	}
	@Override
	public void onError(Integer t) {
		BookModel model = (BookModel) getItem(t);
		View view = mListView.findViewWithTag(model);
		if(view != null){
			ImageView iv = (ImageView) view.findViewById(R.id.sItemIcon);
			iv.setBackgroundResource(R.drawable.rc_item_bg);
		}
	}
	
};

public void loadImage(){
	int start = mListView.getFirstVisiblePosition();
	int end =mListView.getLastVisiblePosition();
	if(end >= getCount()){
		end = getCount() -1;
	}
	syncImageLoader.setLoadLimit(start, end);
	syncImageLoader.unlock();
}

AbsListView.OnScrollListener onScrollListener = new AbsListView.OnScrollListener() {
	
	@Override
	public void onScrollStateChanged(AbsListView view, int scrollState) {
		switch (scrollState) {
			case AbsListView.OnScrollListener.SCROLL_STATE_FLING:
				DebugUtil.debug("SCROLL_STATE_FLING");
				syncImageLoader.lock();
				break;
			case AbsListView.OnScrollListener.SCROLL_STATE_IDLE:
				DebugUtil.debug("SCROLL_STATE_IDLE");
				loadImage();
				//loadImage();
				break;
			case AbsListView.OnScrollListener.SCROLL_STATE_TOUCH_SCROLL:
				syncImageLoader.lock();
				break;

			default:
				break;
		}
		
	}
	
	@Override
	public void onScroll(AbsListView view, int firstVisibleItem,
			int visibleItemCount, int totalItemCount) {
		// TODO Auto-generated method stub
		
	}
};

package cindy.android.test.synclistview;

Syncimageloader代码

1. import java.io.DataInputStream; 
2. import java.io.File; 
3. import java.io.FileInputStream; 
4. import java.io.FileOutputStream; 
5. import java.io.IOException; 
6. import java.io.InputStream; 
7. import java.lang.ref.SoftReference; 
8. import java.net.URL; 
9. import java.util.HashMap; 
10.  
11. import android.graphics.drawable.Drawable; 
12. import android.os.Environment; 
13. import android.os.Handler; 
14.  
15. public class SyncImageLoader { 
16.  
17.     private Object lock = new Object(); 
18.      
19.     private boolean mAllowLoad = true; 
20.      
21.     private boolean firstLoad = true; 
22.      
23.     private int mStartLoadLimit = 0; 
24.      
25.     private int mStopLoadLimit = 0; 
    
26.      
27.     final Handler handler = new Handler(); 
28.      
29.     private HashMap<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();    
30.      
31.     public interface OnImageLoadListener { 
32.         public void onImageLoad(Integer t, Drawable drawable); 
33.         public void onError(Integer t); 
34.     } 
35.      
36.     public void setLoadLimit(int startLoadLimit,int stopLoadLimit){ 
37.         if(startLoadLimit > stopLoadLimit){ 
38.             return; 
39.         } 
40.         mStartLoadLimit = startLoadLimit; 
41.         mStopLoadLimit = stopLoadLimit; 
42.     } 
43.      
44.     public void restore(){ 
45.         mAllowLoad = true; 
46.         firstLoad = true; 
47.     } 
48.          
49.     public void lock(){ 
50.         mAllowLoad = false; 
51.         firstLoad = false; 
52.     } 
53.      
54.     public void unlock(){ 
55.         mAllowLoad = true; 
56.         synchronized (lock) { 
57.             lock.notifyAll(); 
58.         } 
59.     } 
60.  
61.     public void loadImage(Integer t, String imageUrl, 
62.             OnImageLoadListener listener) { 
63.         final OnImageLoadListener mListener = listener; 
64.         final String mImageUrl = imageUrl; 
65.         final Integer mt = t; 
66.          
67.         new Thread(new Runnable() { 
68.  
69.             @Override 
70.             public void run() { 
71.                 if(!mAllowLoad){ 
72.                     DebugUtil.debug("prepare to load"); 
73.                     synchronized (lock) { 
74.                         try { 
75.                             lock.wait(); 
76.                         } catch (InterruptedException e) { 
77.                             // TODO Auto-generated catch block 
78.                             e.printStackTrace(); 
79.                         } 
80.                     } 
81.                 } 
82.                  
83.                 if(mAllowLoad && firstLoad){ 
84.                     loadImage(mImageUrl, mt, mListener); 
85.                 } 
86.                  
87.                 if(mAllowLoad && mt <= mStopLoadLimit && mt >= mStartLoadLimit){ 
88.                     loadImage(mImageUrl, mt, mListener); 
89.                 } 
90.             } 
91.  
92.         }).start(); 
93.     } 
94.      
95.     private void loadImage(final String mImageUrl,final Integer mt,final OnImageLoadListener mListener){ 
96.          
97.         if (imageCache.containsKey(mImageUrl)) {   
98.             SoftReference<Drawable> softReference = imageCache.get(mImageUrl);   
99.             final Drawable d = softReference.get();   
100.             if (d != null) {   
101.                 handler.post(new Runnable() { 
102.                     @Override 
103.                     public void run() { 
104.                         if(mAllowLoad){ 
105.                             mListener.onImageLoad(mt, d); 
106.                         } 
107.                     } 
108.                 }); 
109.                 return;   
110.             }   
111.         }   
112.         try { 
113.             final Drawable d = loadImageFromUrl(mImageUrl); 
114.             if(d != null){ 
115.                 imageCache.put(mImageUrl, new SoftReference<Drawable>(d)); 
116.             } 
117.             handler.post(new Runnable() { 
118.                 @Override 
119.                 public void run() { 
120.                     if(mAllowLoad){ 
121.                         mListener.onImageLoad(mt, d); 
122.                     } 
123.                 } 
124.             }); 
125.         } catch (IOException e) { 
126.             handler.post(new Runnable() { 
127.                 @Override 
128.                 public void run() { 
129.                     mListener.onError(mt); 
130.                 } 
131.             }); 
132.             e.printStackTrace(); 
133.         } 
134.     } 
135.  
136.     public static Drawable loadImageFromUrl(String url) throws IOException { 
137.         DebugUtil.debug(url); 
138.         if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){ 
139.             File f = new File(Environment.getExternalStorageDirectory()+"/TestSyncListView/"+MD5.getMD5(url)); 
140.             if(f.exists()){ 
141.                 FileInputStream fis = new FileInputStream(f); 
142.                 Drawable d = Drawable.createFromStream(fis,
     "src"); 
143.                 return d; 
144.             } 
145.             URL m = new URL(url); 
146.             InputStream i = (InputStream) m.getContent(); 
147.             DataInputStream in = new DataInputStream(i); 
148.             FileOutputStream out = new FileOutputStream(f); 
149.             byte[] buffer = new byte[1024]; 
150.             int   byteread=0; 
     
151.             while ((byteread = in.read(buffer)) != -1) { 
152.                 out.write(buffer, 0, byteread); 
153.             } 
154.             in.close(); 
155.             out.close(); 
156.             Drawable d = Drawable.createFromStream(i,
     "src"); 
157.             return loadImageFromUrl(url); 
158.         }else{ 
159.             URL m = new URL(url); 
160.             InputStream i = (InputStream) m.getContent(); 
161.             Drawable d = Drawable.createFromStream(i,
     "src"); 
162.             return d; 
163.         } 
164.          
165.     } 
166. } 

import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.lang.ref.SoftReference;
import java.net.URL;
import java.util.HashMap;

import android.graphics.drawable.Drawable;
import android.os.Environment;
import android.os.Handler;

public class SyncImageLoader {

	private Object lock = new Object();
	
	private boolean mAllowLoad = true;
	
	private boolean firstLoad = true;
	
	private int mStartLoadLimit = 0;
	
	private int mStopLoadLimit = 0;
	
	final Handler handler = new Handler();
	
	private HashMap<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();   
	
	public interface OnImageLoadListener {
		public void onImageLoad(Integer t, Drawable drawable);
		public void onError(Integer t);
	}
	
	public void setLoadLimit(int startLoadLimit,int stopLoadLimit){
		if(startLoadLimit > stopLoadLimit){
			return;
		}
		mStartLoadLimit = startLoadLimit;
		mStopLoadLimit = stopLoadLimit;
	}
	
	public void restore(){
		mAllowLoad = true;
		firstLoad = true;
	}
		
	public void lock(){
		mAllowLoad = false;
		firstLoad = false;
	}
	
	public void unlock(){
		mAllowLoad = true;
		synchronized (lock) {
			lock.notifyAll();
		}
	}

	public void loadImage(Integer t, String imageUrl,
			OnImageLoadListener listener) {
		final OnImageLoadListener mListener = listener;
		final String mImageUrl = imageUrl;
		final Integer mt = t;
		
		new Thread(new Runnable() {

			@Override
			public void run() {
				if(!mAllowLoad){
					DebugUtil.debug("prepare to load");
					synchronized (lock) {
						try {
							lock.wait();
						} catch (InterruptedException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
						}
					}
				}
				
				if(mAllowLoad && firstLoad){
					loadImage(mImageUrl, mt, mListener);
				}
				
				if(mAllowLoad && mt <= mStopLoadLimit && mt >= mStartLoadLimit){
					loadImage(mImageUrl, mt, mListener);
				}
			}

		}).start();
	}
	
	private void loadImage(final String mImageUrl,final Integer mt,final OnImageLoadListener mListener){
		
		if (imageCache.containsKey(mImageUrl)) {  
            SoftReference<Drawable> softReference = imageCache.get(mImageUrl);  
            final Drawable d = softReference.get();  
            if (d != null) {  
            	handler.post(new Runnable() {
    				@Override
    				public void run() {
    					if(mAllowLoad){
    						mListener.onImageLoad(mt, d);
    					}
    				}
    			});
                return;  
            }  
        }  
		try {
			final Drawable d = loadImageFromUrl(mImageUrl);
			if(d != null){
                imageCache.put(mImageUrl, new SoftReference<Drawable>(d));
			}
			handler.post(new Runnable() {
				@Override
				public void run() {
					if(mAllowLoad){
						mListener.onImageLoad(mt, d);
					}
				}
			});
		} catch (IOException e) {
			handler.post(new Runnable() {
				@Override
				public void run() {
					mListener.onError(mt);
				}
			});
			e.printStackTrace();
		}
	}

	public static Drawable loadImageFromUrl(String url) throws IOException {
		DebugUtil.debug(url);
		if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){
			File f = new File(Environment.getExternalStorageDirectory()+"/TestSyncListView/"+MD5.getMD5(url));
			if(f.exists()){
				FileInputStream fis = new FileInputStream(f);
				Drawable d = Drawable.createFromStream(fis, "src");
				return d;
			}
			URL m = new URL(url);
			InputStream i = (InputStream) m.getContent();
			DataInputStream in = new DataInputStream(i);
			FileOutputStream out = new FileOutputStream(f);
			byte[] buffer = new byte[1024];
			int   byteread=0;
			while ((byteread = in.read(buffer)) != -1) {
				out.write(buffer, 0, byteread);
			}
			in.close();
			out.close();
			Drawable d = Drawable.createFromStream(i, "src");
			return loadImageFromUrl(url);
		}else{
			URL m = new URL(url);
			InputStream i = (InputStream) m.getContent();
			Drawable d = Drawable.createFromStream(i, "src");
			return d;
		}
		
	}
}

为了让大家更好的理解，我添加了源代码例子，还特地美化了一下UI

除了本身已有的弱引用缓存图片，我还添加了本地SD卡缓存图片（这两种缓存方法各有好处，如果图片经常变化建议内存缓存图片，如果是不经常修改的图片建议SD卡缓存）