本文实例讲述了Android ListView异步加载图片方法。分享给大家供大家参考，具体如下：
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 } };
package cindy.android.test.synclistview; 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; } } }
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）
希望本文所述对大家Android程序设计有所帮助。
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