android列表下载图片listview缓存
在使用listview下载图片的时候，经常遇到图片错乱问题，快速滑动会导致图片位置不对，这里提供一种思路
实现效果图：
（1）activity_main.xml
<ListView android:id="@+id/list_view" android:layout_width="match_parent" android:layout_height="match_parent"/>
（2）images_item.xml
<ImageView android:id="@+id/image" android:layout_width="match_parent" android:layout_height="120dp" android:src="@mipmap/empty_photo" android:scaleType="fitXY"/>
（3）在清单文件AndroidManifest.xml里添加联网权限
<uses-permission android:name="android.permission.INTERNET"/> <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/> <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
（4）创建一个图片类Images，用于放置图片数组
public class Images { public final static String[] imageUrls = new String[] { "http://img.my.****.net/uploads/201508/05/1438760758_3497.jpg", "http://img.my.****.net/uploads/201508/05/1438760758_6667.jpg", "http://img.my.****.net/uploads/201508/05/1438760757_3588.jpg", "http://img.my.****.net/uploads/201508/05/1438760756_3304.jpg", "http://img.my.****.net/uploads/201508/05/1438760755_6715.jpeg", "http://img.my.****.net/uploads/201508/05/1438760726_5120.jpg", "http://img.my.****.net/uploads/201508/05/1438760726_8364.jpg", "http://img.my.****.net/uploads/201508/05/1438760725_4031.jpg", "http://img.my.****.net/uploads/201508/05/1438760724_9463.jpg", "http://img.my.****.net/uploads/201508/05/1438760724_2371.jpg", "http://img.my.****.net/uploads/201508/05/1438760707_4653.jpg", "http://img.my.****.net/uploads/201508/05/1438760706_6864.jpg", "http://img.my.****.net/uploads/201508/05/1438760706_9279.jpg", "http://img.my.****.net/uploads/201508/05/1438760704_2341.jpg", "http://img.my.****.net/uploads/201508/05/1438760704_5707.jpg", "http://img.my.****.net/uploads/201508/05/1438760685_5091.jpg", "http://img.my.****.net/uploads/201508/05/1438760685_4444.jpg", "http://img.my.****.net/uploads/201508/05/1438760684_8827.jpg", "http://img.my.****.net/uploads/201508/05/1438760683_3691.jpg", "http://img.my.****.net/uploads/201508/05/1438760683_7315.jpg", "http://img.my.****.net/uploads/201508/05/1438760663_7318.jpg", "http://img.my.****.net/uploads/201508/05/1438760662_3454.jpg", "http://img.my.****.net/uploads/201508/05/1438760662_5113.jpg", "http://img.my.****.net/uploads/201508/05/1438760661_3305.jpg", "http://img.my.****.net/uploads/201508/05/1438760661_7416.jpg", "http://img.my.****.net/uploads/201508/05/1438760589_2946.jpg", "http://img.my.****.net/uploads/201508/05/1438760589_1100.jpg", "http://img.my.****.net/uploads/201508/05/1438760588_8297.jpg", "http://img.my.****.net/uploads/201508/05/1438760587_2575.jpg", "http://img.my.****.net/uploads/201508/05/1438760587_8906.jpg", "http://img.my.****.net/uploads/201508/05/1438760550_2875.jpg", "http://img.my.****.net/uploads/201508/05/1438760550_9517.jpg", "http://img.my.****.net/uploads/201508/05/1438760549_7093.jpg", "http://img.my.****.net/uploads/201508/05/1438760549_1352.jpg", "http://img.my.****.net/uploads/201508/05/1438760548_2780.jpg", "http://img.my.****.net/uploads/201508/05/1438760531_1776.jpg", "http://img.my.****.net/uploads/201508/05/1438760531_1380.jpg", "http://img.my.****.net/uploads/201508/05/1438760530_4944.jpg", "http://img.my.****.net/uploads/201508/05/1438760530_5750.jpg", "http://img.my.****.net/uploads/201508/05/1438760529_3289.jpg", "http://img.my.****.net/uploads/201508/05/1438760500_7871.jpg", "http://img.my.****.net/uploads/201508/05/1438760500_6063.jpg", "http://img.my.****.net/uploads/201508/05/1438760499_6304.jpeg", "http://img.my.****.net/uploads/201508/05/1438760499_5081.jpg", "http://img.my.****.net/uploads/201508/05/1438760498_7007.jpg", "http://img.my.****.net/uploads/201508/05/1438760478_3128.jpg", "http://img.my.****.net/uploads/201508/05/1438760478_6766.jpg", "http://img.my.****.net/uploads/201508/05/1438760477_1358.jpg", "http://img.my.****.net/uploads/201508/05/1438760477_3540.jpg", "http://img.my.****.net/uploads/201508/05/1438760476_1240.jpg", "http://img.my.****.net/uploads/201508/05/1438760446_7993.jpg", "http://img.my.****.net/uploads/201508/05/1438760446_3641.jpg", "http://img.my.****.net/uploads/201508/05/1438760445_3283.jpg", "http://img.my.****.net/uploads/201508/05/1438760444_8623.jpg", "http://img.my.****.net/uploads/201508/05/1438760444_6822.jpg", "http://img.my.****.net/uploads/201508/05/1438760422_2224.jpg", "http://img.my.****.net/uploads/201508/05/1438760421_2824.jpg", "http://img.my.****.net/uploads/201508/05/1438760420_2660.jpg", "http://img.my.****.net/uploads/201508/05/1438760420_7188.jpg", "http://img.my.****.net/uploads/201508/05/1438760419_4123.jpg", }; }
（5）在MainActivity里面引用适配器，设置显示的内容
public class MainActivity extends AppCompatActivity { private ListView listView; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); setContentView(R.layout.activity_main); listView = (ListView) findViewById(R.id.list_view); ImageAdapter adapter = new ImageAdapter(this, 0, Images.imageUrls); listView.setAdapter(adapter); } }
（6）适配器ImageAdapter
public class ImageAdapter extends ArrayAdapter<String> { private ListView mListView; private Bitmap mLoadingBitmap; /** * 图片缓存技术的核心类，用于缓存所有下载好的图片，在程序内存达到设定值时会将最少最近使用的图片移除掉。 */ private LruCache<String, BitmapDrawable> mMemoryCache; public ImageAdapter(Context context, int resource, String[] objects) { super(context, resource, objects); mLoadingBitmap = BitmapFactory.decodeResource(context.getResources(), R.mipmap.empty_photo); // 获取应用程序最大可用内存 int maxMemory = (int) Runtime.getRuntime().maxMemory(); int cacheSize = maxMemory / 8; mMemoryCache = new LruCache<String, BitmapDrawable>(cacheSize) { @Override protected int sizeOf(String key, BitmapDrawable drawable) { return drawable.getBitmap().getByteCount(); } }; } @Override public View getView(int position, View convertView, ViewGroup parent) { if (mListView == null) { mListView = (ListView) parent; } String url = getItem(position); View view; if (convertView == null) { view = LayoutInflater.from(getContext()).inflate(R.layout.images_item, null); } else { view = convertView; } ImageView image = (ImageView) view.findViewById(R.id.image); BitmapDrawable drawable = getBitmapFromMemoryCache(url); if (drawable != null) { image.setImageDrawable(drawable); } else if (cancelPotentialWork(url, image)) { BitmapWorkerTask task = new BitmapWorkerTask(image); AsyncDrawable asyncDrawable = new AsyncDrawable(getContext() .getResources(), mLoadingBitmap, task); image.setImageDrawable(asyncDrawable); task.execute(url); } return view; } /** * 自定义的一个Drawable，让这个Drawable持有BitmapWorkerTask的弱引用。 */ class AsyncDrawable extends BitmapDrawable { private WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference; public AsyncDrawable(Resources res, Bitmap bitmap, BitmapWorkerTask bitmapWorkerTask) { super(res, bitmap); bitmapWorkerTaskReference = new WeakReference<>( bitmapWorkerTask); } public BitmapWorkerTask getBitmapWorkerTask() { return bitmapWorkerTaskReference.get(); } } /** * 获取传入的ImageView它所对应的BitmapWorkerTask。 */ private BitmapWorkerTask getBitmapWorkerTask(ImageView imageView) { if (imageView != null) { Drawable drawable = imageView.getDrawable(); if (drawable instanceof AsyncDrawable) { AsyncDrawable asyncDrawable = (AsyncDrawable) drawable; return asyncDrawable.getBitmapWorkerTask(); } } return null; } /** * 取消掉后台的潜在任务，当认为当前ImageView存在着一个另外图片请求任务时 * ，则把它取消掉并返回true，否则返回false。 */ public boolean cancelPotentialWork(String url, ImageView imageView) { BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView); if (bitmapWorkerTask != null) { String imageUrl = bitmapWorkerTask.imageUrl; if (imageUrl == null || !imageUrl.equals(url)) { bitmapWorkerTask.cancel(true); } else { return false; } } return true; } /** * 将一张图片存储到LruCache中。 * * @param key * LruCache的键，这里传入图片的URL地址。 * @param drawable * LruCache的值，这里传入从网络上下载的BitmapDrawable对象。 */ public void addBitmapToMemoryCache(String key, BitmapDrawable drawable) { if (getBitmapFromMemoryCache(key) == null) { mMemoryCache.put(key, drawable); } } /** * 从LruCache中获取一张图片，如果不存在就返回null。 * * @param key * LruCache的键，这里传入图片的URL地址。 * @return 对应传入键的BitmapDrawable对象，或者null。 */ public BitmapDrawable getBitmapFromMemoryCache(String key) { return mMemoryCache.get(key); } /** * 异步下载图片的任务。 * * @author guolin */ class BitmapWorkerTask extends AsyncTask<String, Void, BitmapDrawable> { String imageUrl; private WeakReference<ImageView> imageViewReference; public BitmapWorkerTask(ImageView imageView) { imageViewReference = new WeakReference<>(imageView); } @Override protected BitmapDrawable doInBackground(String... params) { imageUrl = params[0]; // 在后台开始下载图片 Bitmap bitmap = downloadBitmap(imageUrl); BitmapDrawable drawable = new BitmapDrawable(getContext().getResources(), bitmap); addBitmapToMemoryCache(imageUrl, drawable); return drawable; } @Override protected void onPostExecute(BitmapDrawable drawable) { ImageView imageView = getAttachedImageView(); if (imageView != null && drawable != null) { imageView.setImageDrawable(drawable); } } /** * 获取当前BitmapWorkerTask所关联的ImageView。 */ private ImageView getAttachedImageView() { ImageView imageView = imageViewReference.get(); BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView); if (this == bitmapWorkerTask) { return imageView; } return null; } /** * 建立HTTP请求，并获取Bitmap对象。 * * @param imageUrl * 图片的URL地址 * @return 解析后的Bitmap对象 */ private Bitmap downloadBitmap(String imageUrl) { Bitmap bitmap = null; HttpURLConnection con = null; try { URL url = new URL(imageUrl); con = (HttpURLConnection) url.openConnection(); con.setConnectTimeout(5 * 1000); con.setReadTimeout(10 * 1000); bitmap = BitmapFactory.decodeStream(con.getInputStream()); } catch (Exception e) { e.printStackTrace(); } finally { if (con != null) { con.disconnect(); } } return bitmap; } } }