我有一个带有适配器的listView，它通过ImageLoader类从URL加载图像。 问题是，在列表项滚动到屏幕外之前，屏幕上的图像不会显示/加载。
基本上，显示了listView，但在向下滚动并再次向上滚动之前，不会加载任何图像。这适用于列表中的所有项目，在第一次显示它们时，直到您向后滚动它们才会加载图像。
我自己没有写过ImageLoader类，我很难理解为什么图像在第一次显示时没有加载。在队列中尝试了很多不同的东西，但似乎没有任何帮助。
ImageLoader类：
public class ImageLoader {
//the simplest in-memory cache implementation. This should be replaced with something like SoftReference or BitmapOptions.inPurgeable(since 1.6)
private HashMap<String, Bitmap> cache=new HashMap<String, Bitmap>();
private File cacheDir;
public ImageLoader(Context context){
//Make the background thead low priority. This way it will not affect the UI performance
photoLoaderThread.setPriority(Thread.NORM_PRIORITY-1);
//Find the dir to save cached images
if (android.os.Environment.getExternalStorageState().equals(android.os.Environment.MEDIA_MOUNTED))
cacheDir=new File(android.os.Environment.getExternalStorageDirectory(),"LazyList");
else
cacheDir=context.getCacheDir();
if(!cacheDir.exists())
cacheDir.mkdirs();
}
final int stub_id=R.drawable.nopic;
public void DisplayImage(String url, Activity activity, ImageView imageView, ProgressBar progressBar)
{
if(cache.containsKey(url)){
imageView.setImageBitmap(cache.get(url));
if (progressBar != null){
progressBar.setVisibility(View.GONE);
imageView.setVisibility(View.VISIBLE);
}
}else{
queuePhoto(url, activity, imageView, progressBar);
if(progressBar != null){
imageView.setVisibility(View.GONE); //ADDED
progressBar.setVisibility(View.VISIBLE);
} else {
imageView.setImageResource(stub_id);
}
}
}
private void queuePhoto(String url, Activity activity, ImageView imageView, ProgressBar progressBar)
{
//This ImageView may be used for other images before. So there may be some old tasks in the queue. We need to discard them.
photosQueue.Clean(imageView);
PhotoToLoad p = new PhotoToLoad(url, imageView, progressBar);
synchronized(photosQueue.photosToLoad){
photosQueue.photosToLoad.push(p);
photosQueue.photosToLoad.notifyAll();
}
//start thread if it's not started yet
if(photoLoaderThread.getState()==Thread.State.NEW)
photoLoaderThread.start();
}
private Bitmap getBitmap(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);
//from SD cache
Bitmap b = decodeFile(f);
if(b!=null)
return b;
//from web
try {
Bitmap bitmap=null;
InputStream is=new URL(url).openStream();
OutputStream os = new FileOutputStream(f);
Utils.CopyStream(is, os);
os.close();
bitmap = decodeFile(f);
return bitmap;
} catch (Exception ex){
ex.printStackTrace();
return null;
}
}
//decodes image and scales it to reduce memory consumption
private Bitmap decodeFile(File f){
try {
//decode image size
BitmapFactory.Options o = new BitmapFactory.Options();
o.inJustDecodeBounds = true;
BitmapFactory.decodeStream(new FileInputStream(f),null,o);
//Find the correct scale value. It should be the power of 2.
int requiredSize = 100;
int width_tmp=o.outWidth, height_tmp=o.outHeight;
int scale=1;
while(true){
if(width_tmp/2<requiredSize || height_tmp/2<requiredSize)
break;
width_tmp/=2;
height_tmp/=2;
scale*=2;
}
//decode with inSampleSize
BitmapFactory.Options o2 = new BitmapFactory.Options();
o2.inSampleSize=scale;
return BitmapFactory.decodeStream(new FileInputStream(f), null, o2);
} catch (FileNotFoundException e) {}
return null;
}
//Task for the queue
private class PhotoToLoad
{
public String url;
public ImageView imageView;
public ProgressBar progressBar;
public PhotoToLoad(String u, ImageView i, ProgressBar p){
url=u;
imageView=i;
progressBar = p;
}
}
PhotosQueue photosQueue=new PhotosQueue();
public void stopThread(){
photoLoaderThread.interrupt();
}
//stores list of photos to download
class PhotosQueue
{
private Stack<PhotoToLoad> photosToLoad=new Stack<PhotoToLoad>();
//removes all instances of this ImageView
public void Clean(ImageView image)
{
for(int j=0 ;j<photosToLoad.size();){
if(photosToLoad.get(j).imageView==image)
photosToLoad.remove(j);
else
++j;
}
}
}
//XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
// PHOTOSLOADER: Thread
//XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
class PhotosLoader extends Thread {
public void run() {
try {
while(true)
{
//thread waits until there are any images to load in the queue
if(photosQueue.photosToLoad.size() == 0)
synchronized(photosQueue.photosToLoad){
photosQueue.photosToLoad.wait();
}
if(photosQueue.photosToLoad.size() != 0)
{
PhotoToLoad photoToLoad;
synchronized(photosQueue.photosToLoad){
photoToLoad = photosQueue.photosToLoad.pop(); photosQueue.photosToLoad.remove(photoToLoad);
}
Bitmap bmp = getBitmap(photoToLoad.url);
cache.put(photoToLoad.url, bmp);
Object tag = photoToLoad.imageView.getTag();
if(tag != null && ((String)tag).equals(photoToLoad.url)){
BitmapDisplayer bd = new BitmapDisplayer(bmp, photoToLoad.imageView, photoToLoad.progressBar);
Activity a = (Activity)photoToLoad.imageView.getContext();
a.runOnUiThread(bd);
}
}
if(Thread.interrupted())
break;
}
} catch (InterruptedException e) {
//allow thread to exit
}
}
}
PhotosLoader photoLoaderThread = new PhotosLoader();
//XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
// BITMAP DISPLAYER
//XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
//Used to display bitmap in the UI thread
class BitmapDisplayer implements Runnable{
Bitmap bitmap;
ImageView imageView;
ProgressBar progressBar;
public BitmapDisplayer(Bitmap b, ImageView i, ProgressBar p){
bitmap = b;
imageView = i;
progressBar = p;
}
public void run(){
if(bitmap != null){
imageView.setImageBitmap(bitmap);
if (progressBar != null){
imageView.setVisibility(View.VISIBLE);
progressBar.setVisibility(View.GONE);
}
}else{
if (progressBar != null){
progressBar.setVisibility(View.VISIBLE);
imageView.setVisibility(View.GONE);
}
}
}
}
public void clearCache() {
//clear memory cache
cache.clear();
//clear SD cache
File[] files = cacheDir.listFiles();
for(File f:files)
f.delete();
}
}
适配器：
public class Adapter_Agenda extends BaseAdapter {
// DEBUG
private final String TAG = this.getClass().getSimpleName();
private LayoutInflater inflater = null;
public ViewHolder holder;
private ArrayList<Agenda> agendas;
UnixTimeToStringConverter unixConverter;
ImageLoader imageLoader;
View vi;
public Adapter_Agenda(Context context, ArrayList<Agenda> agendas) {
this.agendas = agendas;
unixConverter = new UnixTimeToStringConverter();
imageLoader = new ImageLoader(context);
inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
}
public int getCount() {
return agendas.size();
}
@Override
public Agenda getItem(int position) {
return agendas.get(position);
}
public long getItemId(int position) {
return position;
}
public View getView(int position, View convertView, ViewGroup parent) {
vi = convertView;
if(convertView == null){
vi = inflater.inflate(R.layout.item_list_agenda, null);
holder = new ViewHolder();
holder.header = (TextView)vi.findViewById(R.id.item_list_header);
holder.image = (ImageView)vi.findViewById(R.id.item_list_image);
holder.time = (TextView)vi.findViewById(R.id.item_list_time);
holder.description = (TextView)vi.findViewById(R.id.item_list_description);
vi.setTag(holder);
holder = (ViewHolder)vi.getTag();
} else {
holder = (ViewHolder)vi.getTag();
}
holder.header.setText(agendas.get(position).getHeader());
imageLoader.DisplayImage(agendas.get(position).getImageURL(), activity, holder.image, null);
holder.time.setText(unixConverter.getUnixDateFormated(agendas.get(position).getStartTime()));
return vi;
}
//XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
// VIEWHOLDER
//XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
public class ViewHolder{
public TextView time;
public ImageView image;
public TextView header;
public TextView description;
}
}
答案 0 :(得分：3)
刚刚放
holder.image.setTag(agendas.get(position).getImageURL());
前
imageLoader.DisplayImage(agendas.get(position).getImageURL(), activity, holder.image, null);
答案 1 :(得分：2)
图像将在新线程中下载。所以你正在做的是异步文件下载。 问题是你＆＃34;使用＆＃34;您的图像在实际下载之前。结果，无法显示图像。
你需要做什么：
在下载图像时创建类似回调的内容。您可以通过创建一个由适配器实现的接口来实现此目的。
每当调用接口方法时，您知道已经下载了图像，您可以通过调用＆＃34; notifyDatasetChange（）＆＃34;来通知适配器。