Android ListView异步加载图片方法详解

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）

更多关于Android相关内容感兴趣的读者可查看本站专题：《Android开发入门与进阶教程》、《Android多媒体操作技巧汇总(音频,视频,录音等)》、《Android基本组件用法总结》、《Android视图View技巧总结》、《Android布局layout技巧总结》及《Android控件用法总结》

希望本文所述对大家Android程序设计有所帮助。