需求：从数据库中得到图片的byte数据，编码成png图片，显示到ListView上
这里最大一个问题就是ListView滑动卡的问题，要解决卡，就需要用到异步加载数据库的图片；
原创博客，转载注明出处：/deng0zhaotai/article/details/50596332
程序第一次启动时会把assets中的数据库压缩文件复制到/data/data/packageName.../databases/目录下，这里使用的是网上找的一个工具类，详细代码可以在下面的链接下载，异步操作主要是在Adapter的getView()函数中处理，如果已经添加到缓存就直接取，否则会开启task去加载，下面看下ImageAdapter的详细代码，都有添加注释，应该不难理解
package com.dzt.listviewshow;import java.util.List;import com.dzt.listviewshow.AsyncImageLoader.OnLoadFinishListener;import com.dzt.listviewshow.utils.LogUtils;import android.content.Context;import android.graphics.Bitmap;import android.support.v4.util.LruCache;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import android.widget.BaseAdapter;import android.widget.ImageView;import android.widget.TextView;public class ImageAdapter extends BaseAdapter {private Context context;private List<CarClass> carClass;private LayoutInflater mInflater;private LruCache<String, Bitmap> mMemoryCache;public ImageAdapter(Context context, List<CarClass> carClass) {this.context = context;this.carClass = carClass;mInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);// LruCache通过构造函数传入缓存值，以KB为单位。int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);// 使用最大可用内存值的1/8作为缓存的大小。int cacheSize = maxMemory / 8;mMemoryCache = new LruCache<String, Bitmap>(cacheSize) {@Overrideprotected int sizeOf(String key, Bitmap bitmap) {// 重写此方法来衡量每张图片的大小，默认返回图片数量。return bitmap.getByteCount() / 1024;}};}@Overridepublic int getCount() {// TODO Auto-generated method stubreturn carClass.size();}@Overridepublic Object getItem(int position) {// TODO Auto-generated method stubreturn carClass.get(position);}@Overridepublic long getItemId(int position) {// TODO Auto-generated method stubreturn position;}@Overridepublic View getView(int position, View convertView, ViewGroup parent) {// TODO Auto-generated method stub// 1获取item,再得到控件// 2 获取数据// 3绑定数据到itemViewHolder hodler = null;CarClass cc = carClass.get(position);if (convertView == null) {convertView = mInflater.inflate(R.layout.item, null);hodler = new ViewHolder();hodler.tvView = (TextView) convertView.findViewById(R.id.tv_title);hodler.ivView = (ImageView) convertView.findViewById(R.id.iv_img);convertView.setTag(hodler);} else {hodler = (ViewHolder) convertView.getTag();}hodler.ivView.setTag(cc.index);final Bitmap bitmap = getBitmapFromMemCache("" + cc.index);if (bitmap == null) {LogUtils.i("getView-----bitmap == null");hodler.ivView.setImageResource(R.drawable.carback);// 异步的加载图片 (线程池 + Handler ) ---> AsyncTaskasyncloadImage(hodler.ivView, cc);} else {LogUtils.i("getView-----bitmap != null");hodler.ivView.setImageBitmap(bitmap);}hodler.tvView.setText("第 " + position + " 个 " + cc.carClass);return convertView;}private void asyncloadImage(ImageView iv_header, CarClass cc) {AsyncImageLoader task = new AsyncImageLoader(context, iv_header,cc.index, new LoadFinish());task.execute(cc.bitmap);}class ViewHolder {ImageView ivView;TextView tvView;}public void addBitmapToMemoryCache(String key, Bitmap bitmap) {if (getBitmapFromMemCache(key) == null) {mMemoryCache.put(key, bitmap);}}public Bitmap getBitmapFromMemCache(String key) {return mMemoryCache.get(key);}class LoadFinish implements OnLoadFinishListener {@Overridepublic void onLoadFinish(Bitmap bitmap, int index) {// TODO Auto-generated method stubLogUtils.i("onLoadFinish-----index = " + index);addBitmapToMemoryCache(String.valueOf(index), bitmap);}}}
为了防止滑动过快时错位显示，需要在getview函数为ImageView加上hodler.ivView.setTag(cc.index);，在异步加载时根据Tag是否显示
package com.dzt.listviewshow;import android.content.Context;import android.graphics.Bitmap;import android.graphics.BitmapFactory;import android.os.AsyncTask;import android.widget.ImageView;public class AsyncImageLoader extends AsyncTask<byte[], Void, Bitmap> {private Context mContext;private ImageView image;private int mIndex;private OnLoadFinishListener mListener;/*** 构造方法，需要把ImageView控件和LruCache 对象传进来* * @param image* 加载图片到此 {@code}TextView* @param lruCache* 缓存图片的对象*/public AsyncImageLoader(Context context, ImageView image, int index,OnLoadFinishListener listener) {mContext = context;this.image = image;mIndex = index;mListener = listener;}@Overrideprotected Bitmap doInBackground(byte[]... params) {// TODO Auto-generated method stubreturn getBitmap(params[0]);}@Overrideprotected void onPostExecute(Bitmap bitmap) {int inx = (int) image.getTag();if (mIndex == inx) {image.setImageBitmap(bitmap);if (mListener != null) {mListener.onLoadFinish(bitmap, mIndex);}}}// 解析图片private Bitmap getBitmap(byte[] icon) {return BitmapFactory.decodeByteArray(icon, 0, icon.length);}interface OnLoadFinishListener {public void onLoadFinish(Bitmap bitmap, int index);}}
关键代码：
int inx = (int) image.getTag();
if (mIndex == inx) {
image.setImageBitmap(bitmap);
if (mListener != null) {
mListener.onLoadFinish(bitmap, mIndex);
}
}
本文可以学习到异步加载图片、防止item显示错位、图片缓存、解压压缩文件
代码下载链接：/s/1qXuwilu
