如果不做任何处理,直接用网络加载图片在网速快的情况下可能没什么不好的感觉,但是如果使用移动流量或是网络不好的时候,问题就来了,要么用户会抱怨流量使用太多,要么抱怨图片加载太慢,如论从哪个角度出发,都不是好的体验!要提高用户体验,我们就要使用缓存。Android中数据缓存的方式有很多,相关介绍的文章也比较多,比如和http://www.jb51.net/article/38162.htm等。今天学习是是Lru缓存。

LRU缓存是把最近最少使用的数据移除,让给最新读取的数据。而往往最常读取的,也是读取次数最多的,所以,利用LRU缓存,我们能够提高应用的效率及用户体验度。Andorid本身提供了LruCache类来实现这个缓存算法 。

在ImageLoader中利用LruCache缓存:

public class ImageLoader {
private LruCache<String, Bitmap> mCaches;// 创建LruCache对象
private ImageView mImageView;
private ListView listView;
private Set<ImageAsyncTask> mTask;


@SuppressLint("NewApi")
public ImageLoader(ListView listView) {
this.listView = listView;
mTask = new HashSet<ImageAsyncTask>();
int maxMemory = (int) Runtime.getRuntime().maxMemory();// 获取最大可用内存
int cacheSize = maxMemory / 8;// 设置缓存数据的最大占用内存量为最大值1/8
mCaches = new LruCache<String, Bitmap>(cacheSize) {
@Override
protected int sizeOf(String key, Bitmap value) {
return value.getByteCount();// 每次存入缓存的时候调用,返回bitmap的大小
}
};
}


@SuppressLint("NewApi")
/**
* 增加缓存数据,增加前判断数据是否存在
* @description:
* @author ldm
* @date 2015-8-11 下午7:51:04
*/
public void setLruCaches(String url, Bitmap bitmap) {
if (getLruCaches(url) == null) {// 如果缓存中不存在url对应的Bitmap,则把bitmap加入mCaches
mCaches.put(url, bitmap);
}
}


/**
* 从缓存中获取数据
* @description:
* @author ldm
* @date 2015-8-11 下午7:51:22
*/
@SuppressLint("NewApi")
public Bitmap getLruCaches(String url) {
return mCaches.get(url);// 通过url获取缓存中对应的bitmap


}


/**
*从url中获取到Bitmap
* @description:
* @author ldm
* @date 2015-8-11 下午1:55:12
*/
public Bitmap getBitmapByUrl(String urlStr) {
Bitmap bitmap = null;
InputStream is = null;
try {
URL url = new URL(urlStr);
HttpURLConnection con = (HttpURLConnection) url.openConnection();
is = new BufferedInputStream(con.getInputStream());
bitmap = BitmapFactory.decodeStream(is);
con.disconnect();
return bitmap;
}
catch (Exception e) {
e.printStackTrace();
}
finally {
try {
is.close();
}
catch (IOException e) {
e.printStackTrace();
}
}
return null;
}


public void loadImgByAsyncTask(ImageView img, String url) {
mImageView = img;
// 从缓存中取出图片
Bitmap bitmap = getLruCaches(url);
if (bitmap == null) {// 如果能在中无图片,则就从网络下载
mImageView.setImageResource(R.drawable.ic_launcher);//设置默认图片
new ImageAsyncTask(url).execute(url);
}
else {// 缓存中有图片,则直接显示出来
mImageView.setImageBitmap(bitmap);
}
}


private class ImageAsyncTask extends AsyncTask<String, Void, Bitmap> {
private ImageView imageView;
private String mUrl;


public ImageAsyncTask(String mUrl) {
this.mUrl = mUrl;
}


@Override
protected Bitmap doInBackground(String... params) {
Bitmap bitmap = getBitmapByUrl(params[0]);// 获取图片
if (bitmap != null) {
setLruCaches(params[0], bitmap);
}
return getBitmapByUrl(params[0]);
}


@Override
protected void onPostExecute(Bitmap result) {
ImageView img = (ImageView) listView.findViewWithTag(mUrl);
if (img != null && result != null) {
imageView.setImageBitmap(result);
}
mTask.remove(this);
}
}


public void setImageView(int start, int end) {
for (int i = start; i < end; i++) {
String url = DataAdapter.URLS[i];
Bitmap bitmap = getLruCaches(url);
if (bitmap == null) {// 如果能在中无图片,则就从网络下载
ImageAsyncTask task = new ImageAsyncTask(url);
task.execute(url);
mTask.add(task);
}
else {// 缓存中有图片,则直接显示出来
ImageView img = (ImageView) listView.findViewWithTag(url);
img.setImageBitmap(bitmap);
}
}
}
public void stopAllTask(){
if(mTask.size()>0){
for (ImageAsyncTask task : mTask) {
task.cancel(false);
}
}
}
 }

   对应ListView的数据适配器DataAdapter:

public class DataAdapter extends BaseAdapter implements OnScrollListener {
private Context mContext;
private List<DataBean> list;
private ImageLoader mImageLoader;
private int mSart;
private int mEnd;
public static String[] URLS;
private ListView listView;
private boolean isFirst;//是否是第一次进入 
public DataAdapter(Context mContext, List<DataBean> list, ListView listView) {
this.listView = listView;
this.mContext = mContext;
this.list = list;
mImageLoader = new ImageLoader(listView);
URLS = new String[list.size()];
for (int i = 0; i < list.size(); i++) {
URLS[i] = list.get(i).getImgUrl();
}
isFirst=true;
listView.setOnScrollListener(this);
}

@Override
public int getCount() {
// TODO Auto-generated method stub
return list.size();
}

@Override
public Object getItem(int arg0) {
// TODO Auto-generated method stub
return list.get(arg0);
}

@Override
public long getItemId(int arg0) {
// TODO Auto-generated method stub
return arg0;
}

@Override
public View getView(int arg0, View view, ViewGroup arg2) {
ViewHolder holder = null;
if (view == null) {
holder = new ViewHolder();
view = LayoutInflater.from(mContext).inflate(R.layout.item_layout, null);
holder.iv = (ImageView) view.findViewById(R.id.item_iv);
holder.titleTv = (TextView) view.findViewById(R.id.item_title);
holder.contentTv = (TextView) view.findViewById(R.id.item_content);
view.setTag(holder);
}
else {
holder = (ViewHolder) view.getTag();
}
holder.titleTv.setText(list.get(arg0).getTitle());
holder.contentTv.setText(list.get(arg0).getContent());
holder.iv.setTag(list.get(arg0).getImgUrl());// 为ImageView设置tag
// new ImageLoader().loaderImageThread(holder.iv, list.get(arg0).getImgUrl());//用线程加载图片
mImageLoader.loadImgByAsyncTask(holder.iv, list.get(arg0).getImgUrl());
return view;
}

/***
* ListView在流动过程中调用 
*/
@Override
public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
mSart = firstVisibleItem;// 可见第一个item
mEnd = firstVisibleItem + visibleItemCount;// 可见的最后一个item
if(isFirst&&visibleItemCount>0){//第一次加载数据时数据处理
mImageLoader.setImageView(mSart, mEnd);
isFirst=false;
}
}

/***
* ListView在流动状态变化时调用 
*/
@Override
public void onScrollStateChanged(AbsListView view, int scrollState) {
if (scrollState == SCROLL_STATE_IDLE) {// 流动停止,此时加载可见项数据
mImageLoader.setImageView(mSart, mEnd);
}
else {// 停止加载数据
mImageLoader.stopAllTask();
}
}
class ViewHolder {
TextView titleTv;
TextView contentTv;
ImageView iv;
}
 }