下面是我使用的 LazyLoader 的示例。注意我正在为位图使用 SoftReferences，现在使用LruCache.
这将从 web / sdcard / memory 异步加载图像，并从占位符图像创建淡入效果。
public class ImageLoader {
private static MemoryCacheNew memoryCache=new MemoryCacheNew();
private static FileCache fileCache;
private static BitmapFactory.Options bitmapOptions;
private static int mInSampleSize;
public ImageLoader(Context context, int inSampleSize){
fileCache=new FileCache(context);
context = null;
bitmapOptions = new BitmapFactory.Options();
bitmapOptions.inSampleSize = mInSampleSize = inSampleSize;
bitmapOptions.inPreferredConfig = Config.RGB_565;
bitmapOptions.inInputShareable = true;
bitmapOptions.inDither = false;
}
final static int PLACEHOLDER_IMAGE = R.drawable.store_placeholder;
public void DisplayImage(String url, ImageView imageView, boolean checkTags){
try{
new AsyncPhotoTask(imageView, url, checkTags).execute();
}catch(Exception e){
e.printStackTrace();
}
}
public void DisplayImage(String url, ImageView imageView)
{
DisplayImage(url, imageView, true);
}
private static Bitmap getBitmap(String url)
{
File f=fileCache.getFile(url);
if(f!= null){
//from SD cache
Bitmap b = decodeFile(f);
if(b!=null)
return b;
}
//from web
try {
Bitmap bitmap=null;
URL imageUrl;
imageUrl = new URL(url);
HttpURLConnection conn = (HttpURLConnection)imageUrl.openConnection();
conn.setConnectTimeout(30000);
conn.setReadTimeout(30000);
InputStream is=conn.getInputStream();
OutputStream os = new FileOutputStream(f);
Utils.CopyStream(is, os);
is.close();
os.close();
bitmap = decodeFile(f);
return bitmap;
} catch (Exception ex){
ex.printStackTrace();
return null;
}
}
//decodes image and scales it to reduce memory consumption
private static Bitmap decodeFile(File f){
try {
return BitmapFactory.decodeStream(new FileInputStream(f), null, bitmapOptions);
} catch (FileNotFoundException e) {
} catch (OutOfMemoryError err){
System.gc();
}
return null;
}
private static class AsyncPhotoLoad extends AsyncTask<Void, Void, TransitionDrawable>{
private Bitmap bmp;
private ImageView imageView;
private String url;
private boolean checkTags;
public AsyncPhotoLoad(ImageView imageView, String url, boolean checkTags
){
this.imageView = imageView;
this.url = url;
this.checkTags = checkTags;
}
@Override
protected TransitionDrawable doInBackground(Void... arg0) {
//check that this is the correct imageview
TransitionDrawable transition = null;
try{
if(checkTags){
String tag = (String)imageView.getTag();
if(!tag.equals(url))
return null;
}
bmp = getBitmap(url);
if(bmp != null){
memoryCache.put(url, bmp, mInSampleSize);
Drawable oldDrawable = imageView.getDrawable();
if(!(oldDrawable instanceof TransitionDrawable)){
Drawable layers[] = new Drawable[2];
layers[0] = imageView.getDrawable();
layers[1] = new BitmapDrawable(bmp);
transition = new TransitionDrawable(layers);
}
}
}catch(Exception e){
e.printStackTrace();
}
return transition;
}
@Override
protected void onPostExecute(TransitionDrawable result) {
if(result != null){
try{
if(checkTags){
String tag = (String)imageView.getTag();
if(!tag.equals(url)){
return;
}
}
imageView.setImageDrawable(result);
result.startTransition(300);
} catch(Exception e){
e.printStackTrace();
}
} else {
if(checkTags){
try{
String tag = (String)imageView.getTag();
if(!tag.equals(url))
return;
}catch(Exception e){
e.printStackTrace();
}
}
}
}
}
private static class AsyncPhotoTask extends AsyncTask<Void, Void, Bitmap>{
private ImageView imageView;
private String url;
private boolean checkTags;
public AsyncPhotoTask(ImageView imageView, String url, boolean checkTags){
this.imageView = imageView;
this.url = url;
this.checkTags = checkTags;
}
@Override
protected Bitmap doInBackground(Void... params) {
try{
if(checkTags)
imageView.setTag(url);
}catch(Exception e){
e.printStackTrace();
}
return memoryCache.get(url, mInSampleSize);
}
@Override
protected void onPostExecute(Bitmap result) {
try{
if(result!=null && !result.isRecycled()){
imageView.setImageBitmap(result);
}
else
{
imageView.setImageResource(PLACEHOLDER_IMAGE);
new AsyncPhotoLoad(imageView, url, checkTags).execute();
}
}catch(Exception e){
e.printStackTrace();
}
}
}
public static void clearCache() {
memoryCache.clear();
fileCache.clear();
}
public static void clearMemory(){
memoryCache.clear();
}
public static class MemoryCacheNew {
private HashMap<String, CachedBitmap> cache=new HashMap<String, CachedBitmap>();
public Bitmap get(String id, int sampleSize){
if(!cache.containsKey(id))
return null;
if(cache.get(id) == null)
return null;
if(cache.get(id).sampleSize != sampleSize)
return null;
SoftReference<Bitmap> ref = cache.get(id).softBitmap;
return ref.get();
}
public void put(String id, Bitmap bitmap, int sampleSize){
cache.put(id, new CachedBitmap(bitmap, sampleSize));
}
public void clear() {
cache.clear();
}
private static class CachedBitmap {
public SoftReference<Bitmap> softBitmap;
public int sampleSize;
public CachedBitmap(Bitmap bitmap, int sampleSize){
this.softBitmap = new SoftReference<Bitmap>(bitmap);
this.sampleSize = sampleSize;
}
}
}
}
public class FileCache {
private File cacheDir;
public FileCache(Context context){
//Find the dir to save cached images
if (android.os.Environment.getExternalStorageState().equals(android.os.Environment.MEDIA_MOUNTED))
cacheDir=new File(context.getExternalCacheDir(),Consts.STORE_CACHE);
else
cacheDir=context.getCacheDir();
if(!cacheDir.exists())
cacheDir.mkdirs();
}
public File getFile(String url){
//I identify images by hashcode. Not a perfect solution, good for the demo.
String filename=String.valueOf(url.hashCode());
File f = new File(cacheDir, filename);
return f;
}
public void clear(){
File[] files=cacheDir.listFiles();
for(File f:files)
f.delete();
}
}
你这样称呼它：
imageLoader.DisplayImage(url, holder.image);