优化 Android ListView 异步加载图片

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


部分代码如下：
Java代码  收藏代码
  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  收藏代码
  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）

欢迎大家拍砖讨论


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转http://note.sdo.com/u/jesse_luo/n/sCPcz~jQFr6pnM2k0001nH
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原文地址：https://www.cnblogs.com/jiezzy/p/2557422.html