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://cindy-lee.iteye.com/blog/1300818

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：

@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

    }
};

@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
		
	}
};

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;
        }

    }
}

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）

用真机测试了一下，感觉无比流畅（测试机是U8500非常垃圾的一种低端机）

欢迎大家拍砖讨论