异步加载图片基本思想:
1. 先从内存缓存中获取图片显示(内存缓冲)
2. 获取不到的话从SD卡里获取(SD卡缓冲)
3. 都获取不到的话从网络下载图片并保存到SD卡同时加入内存并显示(视情况看是否要显示)
- public class LoaderAdapter extends BaseAdapter{
- private static final String TAG = "LoaderAdapter";
- private boolean mBusy = false;
- public void setFlagBusy(boolean busy) {
- this.mBusy = busy;
- }
- private ImageLoader mImageLoader;
- private int mCount;
- private Context mContext;
- private String[] urlArrays;
- public LoaderAdapter(int count, Context context, String []url) {
- this.mCount = count;
- this.mContext = context;
- urlArrays = url;
- mImageLoader = new ImageLoader(context);
- }
- public ImageLoader getImageLoader(){
- return mImageLoader;
- }
- @Override
- public int getCount() {
- return mCount;
- }
- @Override
- public Object getItem(int position) {
- return position;
- }
- @Override
- public long getItemId(int position) {
- return position;
- }
- @Override
- public View getView(int position, View convertView, ViewGroup parent) {
- ViewHolder viewHolder = null;
- if (convertView == null) {
- convertView = LayoutInflater.from(mContext).inflate(
- R.layout.list_item, null);
- viewHolder = new ViewHolder();
- viewHolder.mTextView = (TextView) convertView
- .findViewById(R.id.tv_tips);
- viewHolder.mImageView = (ImageView) convertView
- .findViewById(R.id.iv_image);
- convertView.setTag(viewHolder);
- } else {
- viewHolder = (ViewHolder) convertView.getTag();
- }
- String url = "";
- url = urlArrays[position % urlArrays.length];
- viewHolder.mImageView.setImageResource(R.drawable.ic_launcher);
- if (!mBusy) {
- mImageLoader.DisplayImage(url, viewHolder.mImageView, false);
- viewHolder.mTextView.setText("--" + position
- + "--IDLE ||TOUCH_SCROLL");
- } else {
- mImageLoader.DisplayImage(url, viewHolder.mImageView, true);
- viewHolder.mTextView.setText("--" + position + "--FLING");
- }
- return convertView;
- }
- static class ViewHolder {
- TextView mTextView;
- ImageView mImageView;
- }
- }
关键代码是ImageLoader的DisplayImage方法,再看ImageLoader的实现
- public class ImageLoader {
- private MemoryCache memoryCache = new MemoryCache();
- private AbstractFileCache fileCache;
- private Map<ImageView, String> imageViews = Collections
- .synchronizedMap(new WeakHashMap<ImageView, String>());
- // 线程池
- private ExecutorService executorService;
- public ImageLoader(Context context) {
- fileCache = new FileCache(context);
- executorService = Executors.newFixedThreadPool(5);
- }
- // 最主要的方法
- public void DisplayImage(String url, ImageView imageView, boolean isLoadOnlyFromCache) {
- imageViews.put(imageView, url);
- // 先从内存缓存中查找
- Bitmap bitmap = memoryCache.get(url);
- if (bitmap != null)
- imageView.setImageBitmap(bitmap);
- else if (!isLoadOnlyFromCache){
- // 若没有的话则开启新线程加载图片
- queuePhoto(url, imageView);
- }
- }
- private void queuePhoto(String url, ImageView imageView) {
- PhotoToLoad p = new PhotoToLoad(url, imageView);
- executorService.submit(new PhotosLoader(p));
- }
- private Bitmap getBitmap(String url) {
- File f = fileCache.getFile(url);
- // 先从文件缓存中查找是否有
- Bitmap b = null;
- if (f != null && f.exists()){
- b = decodeFile(f);
- }
- if (b != null){
- return b;
- }
- // 最后从指定的url中下载图片
- 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();
- OutputStream os = new FileOutputStream(f);
- CopyStream(is, os);
- os.close();
- bitmap = decodeFile(f);
- return bitmap;
- } catch (Exception ex) {
- Log.e("", "getBitmap catch Exception...\nmessage = " + ex.getMessage());
- return null;
- }
- }
- // decode这个图片并且按比例缩放以减少内存消耗,虚拟机对每张图片的缓存大小也是有限制的
- 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.
- final int REQUIRED_SIZE = 100;
- 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 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 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() {
- if (imageViewReused(photoToLoad))
- return;
- Bitmap bmp = getBitmap(photoToLoad.url);
- memoryCache.put(photoToLoad.url, bmp);
- if (imageViewReused(photoToLoad))
- return;
- BitmapDisplayer bd = new BitmapDisplayer(bmp, photoToLoad);
- // 更新的操作放在UI线程中
- Activity a = (Activity) photoToLoad.imageView.getContext();
- a.runOnUiThread(bd);
- }
- }
- boolean imageViewReused(PhotoToLoad photoToLoad) {
- String tag = imageViews.get(photoToLoad.imageView);
- if (tag == null || !tag.equals(photoToLoad.url))
- return true;
- return false;
- }
- // 用于在UI线程中更新界面
- 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;
- if (bitmap != null)
- photoToLoad.imageView.setImageBitmap(bitmap);
- }
- }
- public void clearCache() {
- memoryCache.clear();
- fileCache.clear();
- }
- public static void CopyStream(InputStream is, OutputStream os) {
- final int buffer_size = 1024;
- try {
- byte[] bytes = new byte[buffer_size];
- for (;;) {
- int count = is.read(bytes, 0, buffer_size);
- if (count == -1)
- break;
- os.write(bytes, 0, count);
- }
- } catch (Exception ex) {
- Log.e("", "CopyStream catch Exception...");
- }
- }
- }
先从内存中加载,没有则开启线程从SD卡或网络中获取,这里注意从SD卡获取图片是放在子线程里执行的,否则快速滑屏的话会不够流畅,这是优化一。于此同时,在adapter里有个busy变量,表示listview是否处于滑动状态,如果是滑动状态则仅从内存中获取图片,没有的话无需再开启线程去外存或网络获取图片,这是优化二。ImageLoader里的线程使用了线程池,从而避免了过多线程频繁创建和销毁,有的童鞋每次总是new一个线程去执行这是非常不可取的,好一点的用的AsyncTask类,其实内部也是用到了线程池。在从网络获取图片时,先是将其保存到sd卡,然后再加载到内存,这么做的好处是在加载到内存时可以做个压缩处理,以减少图片所占内存,这是优化三。
而图片错位问题的本质源于我们的listview使用了缓存convertView,假设一种场景,一个listview一屏显示九个item,那么在拉出第十个item的时候,事实上该item是重复使用了第一个item,也就是说在第一个item从网络中下载图片并最终要显示的时候其实该item已经不在当前显示区域内了,此时显示的后果将是在可能在第十个item上输出图像,这就导致了图片错位的问题。所以解决之道在于可见则显示,不可见则不显示。在ImageLoader里有个imageViews的map对象,就是用于保存当前显示区域图像对应的url集,在显示前判断处理一下即可。
下面再说下内存缓冲机制,本例采用的是LRU算法,先看看MemoryCache的实现
- public class MemoryCache {
- private static final String TAG = "MemoryCache";
- // 放入缓存时是个同步操作
- // LinkedHashMap构造方法的最后一个参数true代表这个map里的元素将按照最近使用次数由少到多排列,即LRU
- // 这样的好处是如果要将缓存中的元素替换,则先遍历出最近最少使用的元素来替换以提高效率
- private Map<String, Bitmap> cache = Collections
- .synchronizedMap(new LinkedHashMap<String, Bitmap>(10, 1.5f, true));
- // 缓存中图片所占用的字节,初始0,将通过此变量严格控制缓存所占用的堆内存
- private long size = 0;// current allocated size
- // 缓存只能占用的最大堆内存
- private long limit = 1000000;// max memory in bytes
- public MemoryCache() {
- // use 25% of available heap size
- setLimit(Runtime.getRuntime().maxMemory() / 10);
- }
- 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) {
- 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) {
- // 先遍历最近最少使用的元素
- 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() {
- cache.clear();
- }
- long getSizeInBytes(Bitmap bitmap) {
- if (bitmap == null)
- return 0;
- return bitmap.getRowBytes() * bitmap.getHeight();
- }
- }
首先限制内存图片缓冲的堆内存大小,每次有图片往缓存里加时判断是否超过限制大小,超过的话就从中取出最少使用的图片并将其移除,当然这里如果不采用这种方式,换做软引用也是可行的,二者目的皆是最大程度的利用已存在于内存中的图片缓存,避免重复制造垃圾增加GC负担,OOM溢出往往皆因内存瞬时大量增加而垃圾回收不及时造成的。只不过二者区别在于LinkedHashMap里的图片缓存在没有移除出去之前是不会被GC回收的,而SoftReference里的图片缓存在没有其他引用保存时随时都会被GC回收。所以在使用LinkedHashMap这种LRU算法缓存更有利于图片的有效命中,当然二者配合使用的话效果更佳,即从LinkedHashMap里移除出的缓存放到SoftReference里,这就是内存的二级缓存,有兴趣的童鞋不凡一试。