我在远程服务器上有一个图像网址列表。我需要异步加载到网格视图而不使用任何第三方库。我已经建立了以下任务：
class BitmapWorkerTask extends AsyncTask<String, Void, Bitmap> {
private final WeakReference<ImageView> imageViewReference;
public BitmapWorkerTask(ImageView imageView) {
// Use a WeakReference to ensure the ImageView can be garbage collected
imageViewReference = new WeakReference<ImageView>(imageView);
}
// Decode image in background.
@Override
protected Bitmap doInBackground(String... params) {
if (isCancelled()) {
return null;
}
try {
URL url = new URL(params[0]);
return BitmapFactory.decodeStream(url.openConnection().getInputStream());
} catch (IOException e) {
e.printStackTrace();
return null;
}
}
// Once complete, see if ImageView is still around and set bitmap.
@Override
protected void onPostExecute(Bitmap bitmap) {
if (imageViewReference != null && bitmap != null) {
final ImageView imageView = imageViewReference.get();
if (imageView != null) {
imageView.setImageBitmap(bitmap);
}
}
}
}`
我正在调用onBindViewHolder如下：
@Override
public void onBindViewHolder(GridHolder holder, int position) {
final AnimalModel animal = list.get(position);
holder.image.setTag(holder);
holder.text.setText(animal.getName());
new BitmapWorkerTask(holder.image).execute(animal.getImagePath());
}
我面临几个我无法找到解决方法的问题：
1）滚动图像几乎每次都丢失了它们的位置
2）在configurationChanged需要时间来加载图像时，我假设有几个任务没有停止并且操作系统等待它们，我该如何取消所有任务？请帮忙。
答案 0 :(得分：3)
将这4个类添加到项目中
1
import android.content.Context;
import java.io.File;
public class FileCache {
private File cacheDir;
public FileCache(Context context){
//Find the dir at SDCARD to save cached images
if (android.os.Environment.getExternalStorageState().equals(
android.os.Environment.MEDIA_MOUNTED))
{
//if SDCARD is mounted (SDCARD is present on device and mounted)
cacheDir = new File(
android.os.Environment.getExternalStorageDirectory(),"LazyList");
}
else
{
// if checking on simulator the create cache dir in your application context
cacheDir=context.getCacheDir();
}
if(!cacheDir.exists()){
// create cache dir in your application context
cacheDir.mkdirs();
}
}
public File getFile(String url){
//Identify images by hashcode or encode by URLEncoder.encode.
String filename=String.valueOf(url.hashCode());
File f = new File(cacheDir, filename);
return f;
}
public void clear(){
// list all files inside cache directory
File[] files=cacheDir.listFiles();
if(files==null)
return;
//delete all cache directory files
for(File f:files)
f.delete();
}
}
2
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Handler;
import android.widget.ImageView;
import com.example.smartlaw.R;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Collections;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class ImageLoader {
// Initialize MemoryCache
MemoryCache memoryCache = new MemoryCache();
FileCache fileCache;
//Create Map (collection) to store image and image url in key value pair
private Map<ImageView, String> imageViews = Collections.synchronizedMap(
new WeakHashMap<ImageView, String>());
ExecutorService executorService;
//handler to display images in UI thread
Handler handler = new Handler();
public ImageLoader(Context context){
fileCache = new FileCache(context);
// Creates a thread pool that reuses a fixed number of
// threads operating off a shared unbounded queue.
executorService=Executors.newFixedThreadPool(5);
}
// default image show in list (Before online image download)
final int stub_id= R.drawable.book_background;
public void DisplayImage(String url, ImageView imageView)
{
//Store image and url in Map
imageViews.put(imageView, url);
//Check image is stored in MemoryCache Map or not (see MemoryCache.java)
Bitmap bitmap = memoryCache.get(url);
if(bitmap!=null){
// if image is stored in MemoryCache Map then
// Show image in listview row
imageView.setImageBitmap(bitmap);
}
else
{
//queue Photo to download from url
queuePhoto(url, imageView);
//Before downloading image show default image
imageView.setImageResource(stub_id);
}
}
private void queuePhoto(String url, ImageView imageView)
{
// Store image and url in PhotoToLoad object
PhotoToLoad p = new PhotoToLoad(url, imageView);
// pass PhotoToLoad object to PhotosLoader runnable class
// and submit PhotosLoader runnable to executers to run runnable
// Submits a PhotosLoader runnable task for execution
executorService.submit(new PhotosLoader(p));
}
//Task for the queue
private class PhotoToLoad
{
public String url;
public ImageView imageView;
public PhotoToLoad(String u, ImageView i){
url=u;
imageView=i;
}
}
class PhotosLoader implements Runnable {
PhotoToLoad photoToLoad;
PhotosLoader(PhotoToLoad photoToLoad){
this.photoToLoad=photoToLoad;
}
@Override
public void run() {
try{
//Check if image already downloaded
if(imageViewReused(photoToLoad))
return;
// download image from web url
Bitmap bmp = getBitmap(photoToLoad.url);
// set image data in Memory Cache
memoryCache.put(photoToLoad.url, bmp);
if(imageViewReused(photoToLoad))
return;
// Get bitmap to display
BitmapDisplayer bd=new BitmapDisplayer(bmp, photoToLoad);
// Causes the Runnable bd (BitmapDisplayer) to be added to the message queue.
// The runnable will be run on the thread to which this handler is attached.
// BitmapDisplayer run method will call
handler.post(bd);
}catch(Throwable th){
th.printStackTrace();
}
}
}
private Bitmap getBitmap(String url)
{
File f=fileCache.getFile(url);
//from SD cache
//CHECK : if trying to decode file which not exist in cache return null
Bitmap b = decodeFile(f);
if(b!=null)
return b;
// Download image file from web
try {
Bitmap bitmap=null;
URL imageUrl = new URL(url);
HttpURLConnection conn = (HttpURLConnection)imageUrl.openConnection();
conn.setConnectTimeout(30000);
conn.setReadTimeout(30000);
conn.setInstanceFollowRedirects(true);
InputStream is=conn.getInputStream();
// Constructs a new FileOutputStream that writes to file
// if file not exist then it will create file
OutputStream os = new FileOutputStream(f);
// See Utils class CopyStream method
// It will each pixel from input stream and
// write pixels to output stream (file)
Utils.CopyStream(is, os);
os.close();
conn.disconnect();
//Now file created and going to resize file with defined height
// Decodes image and scales it to reduce memory consumption
bitmap = decodeFile(f);
return bitmap;
} catch (Throwable ex){
ex.printStackTrace();
if(ex instanceof OutOfMemoryError)
memoryCache.clear();
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;
FileInputStream stream1=new FileInputStream(f);
BitmapFactory.decodeStream(stream1,null,o);
stream1.close();
//Find the correct scale value. It should be the power of 2.
// Set width/height of recreated image
final int REQUIRED_SIZE=85;
int width_tmp=o.outWidth, height_tmp=o.outHeight;
int scale=1;
while(true){
if(width_tmp/2 < REQUIRED_SIZE || height_tmp/2 < REQUIRED_SIZE)
break;
width_tmp/=2;
height_tmp/=2;
scale*=2;
}
//decode with current scale values
BitmapFactory.Options o2 = new BitmapFactory.Options();
o2.inSampleSize=scale;
FileInputStream stream2=new FileInputStream(f);
Bitmap bitmap=BitmapFactory.decodeStream(stream2, null, o2);
stream2.close();
return bitmap;
} catch (FileNotFoundException e) {
}
catch (IOException e) {
e.printStackTrace();
}
return null;
}
boolean imageViewReused(PhotoToLoad photoToLoad){
String tag=imageViews.get(photoToLoad.imageView);
//Check url is already exist in imageViews MAP
if(tag==null || !tag.equals(photoToLoad.url))
return true;
return false;
}
//Used to display bitmap in the UI thread
class BitmapDisplayer implements Runnable
{
Bitmap bitmap;
PhotoToLoad photoToLoad;
public BitmapDisplayer(Bitmap b, PhotoToLoad p){bitmap=b;photoToLoad=p;}
public void run()
{
if(imageViewReused(photoToLoad))
return;
// Show bitmap on UI
if(bitmap!=null)
photoToLoad.imageView.setImageBitmap(bitmap);
else
photoToLoad.imageView.setImageResource(stub_id);
}
}
public void clearCache() {
//Clear cache directory downloaded images and stored data in maps
memoryCache.clear();
fileCache.clear();
}
}
3
import android.graphics.Bitmap;
import android.util.Log;
import java.util.Collections;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Map.Entry;
public class MemoryCache {
private static final String TAG = "MemoryCache";
//Last argument true for LRU ordering
private Map<String, Bitmap> cache = Collections.synchronizedMap(
new LinkedHashMap<String, Bitmap>(10,1.5f,true));
//current allocated size
private long size=0;
//max memory cache folder used to download images in bytes
private long limit=1000000;
public MemoryCache(){
//use 25% of available heap size
setLimit(Runtime.getRuntime().maxMemory()/4);
}
public void setLimit(long new_limit){
limit=new_limit;
Log.i(TAG, "MemoryCache will use up to "+limit/1024./1024.+"MB");
}
public Bitmap get(String id){
try{
if(!cache.containsKey(id))
return null;
return cache.get(id);
}catch(NullPointerException ex){
ex.printStackTrace();
return null;
}
}
public void put(String id, Bitmap bitmap){
try{
if(cache.containsKey(id))
size-=getSizeInBytes(cache.get(id));
cache.put(id, bitmap);
size+=getSizeInBytes(bitmap);
checkSize();
}catch(Throwable th){
th.printStackTrace();
}
}
private void checkSize() {
Log.i(TAG, "cache size="+size+" length="+cache.size());
if(size>limit){
//least recently accessed item will be the first one iterated
Iterator<Entry<String, Bitmap>> iter=cache.entrySet().iterator();
while(iter.hasNext()){
Entry<String, Bitmap> entry=iter.next();
size-=getSizeInBytes(entry.getValue());
iter.remove();
if(size<=limit)
break;
}
Log.i(TAG, "Clean cache. New size "+cache.size());
}
}
public void clear() {
try{
// Clear cache
cache.clear();
size=0;
}catch(NullPointerException ex){
ex.printStackTrace();
}
}
long getSizeInBytes(Bitmap bitmap) {
if(bitmap==null)
return 0;
return bitmap.getRowBytes() * bitmap.getHeight();
}
}
4
import java.io.InputStream;
import java.io.OutputStream;
public class Utils {
public static void CopyStream(InputStream is, OutputStream os)
{
final int buffer_size=1024;
try
{
byte[] bytes=new byte[buffer_size];
for(;;)
{
//Read byte from input stream
int count=is.read(bytes, 0, buffer_size);
if(count==-1)
break;
//Write byte from output stream
os.write(bytes, 0, count);
}
}
catch(Exception ex){}
}
}
只需在Adapter getView（）方法中使用它就可以了
public View getView(final int position, View convertVie, ViewGroup parent) {
if (convertView == null) {
inflater = (LayoutInflater) mContext
.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
convertView = inflater.inflate(R.layout.book_item_to_download, parent, false);
holder = new Holder();
holder.BookImage = (ImageView) convertView.findViewById(R.id.bookBackground);
convertView.setTag(holder);
} else {
holder = (Holder) convertView.getTag();
}
ImageView image = holder.BookImage;
//DisplayImage function from ImageLoader Class
imageLoader.DisplayImage(url, image);
//Then do whatever you want here
return convertView;
}
这会将图像加载到后台的每个位置，这样就不会出现滚动问题，并且会将图像保留在缓存中，因此如果请求是面向电话或者我不会加载任何格式的