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Android ListView异步加载图片方法详解

2020-01-02 07:03:21
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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:

@Overridepublic 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。

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