android 异步加载图片缩略图

建一个AsyncLoadedImage类继承AsyncTask异步加载类，调用publishProgress方法更新onProgressUpdate贮存缩略图信息到Adapter。监听Adapter Change实现异步加载缩略图。

main.xml

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <GridView
        android:id="@+id/sdcard"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:gravity="center"
        android:horizontalSpacing="10dp"
        android:numColumns="auto_fit"
        android:stretchMode="columnWidth"
        android:verticalSpacing="10dp" />

</FrameLayout>

MFile.java

public class MFile extends Activity {
    private GridView sdcardImages;
    private FileAdapter fileAdapter;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
        setContentView(R.layout.main);

        setupViews();
        setProgressBarIndeterminateVisibility(true);
        new AsyncLoadedImage().execute();
    }

    /*
     * 释放Bitmap内存
     */
    protected void onDestroy() {
        super.onDestroy();
        final GridView grid = sdcardImages;
        final int count = grid.getChildCount();
        ImageView v = null;
        for (int i = 0; i < count; i++) {
            v = (ImageView) grid.getChildAt(i);
            ((BitmapDrawable) v.getDrawable()).setCallback(null);
        }
    }

    /*
     * 初始化文件浏览View
     */
    private void setupViews() {
        sdcardImages = (GridView) findViewById(R.id.sdcard);
        sdcardImages.setOnItemClickListener(new fileListener());
        fileAdapter = new FileAdapter(getApplicationContext());
        sdcardImages.setAdapter(fileAdapter);
    }

    /*
     * 刷新Adapter
     */
    private void addImage(LoadedImage... value) {
        for (LoadedImage image : value) {
            fileAdapter.addPhoto(image);
            fileAdapter.notifyDataSetChanged();
        }
    }

    /*
     * 点击监听
     */
    class fileListener implements OnItemClickListener {

        @Override
        public void onItemClick(AdapterView<?> paramAdapterView,
                View paramView, int paramInt, long paramLong) {
        }
    }

    /*
     * 异步加载缩略图到LoadedImage然后调用addImage方法更新Adapter
     */
    class AsyncLoadedImage extends AsyncTask<Object, LoadedImage, Object> {
        @Override
        protected Object doInBackground(Object... params) {
            String path = Environment.getExternalStorageDirectory()
                    .getAbsolutePath() + "/hfdatabase/3/img";
            File file = new File(path);
            if (!file.exists()) {
                file.mkdirs();
            } else {
                File[] files = file.listFiles();
                String[] paths = new String[files.length];
                Bitmap bitmap;
                Bitmap newBitmap;
                for (int i = 0; i < files.length; i++) {
                    paths[i] = files[i].getPath();
                    try {
                        BitmapFactory.Options options = new BitmapFactory.Options();
                        options.inSampleSize = 10;
                        bitmap = BitmapFactory.decodeFile(paths[i], options);
                        newBitmap = ThumbnailUtils.extractThumbnail(bitmap, 80,
                                80);
                        bitmap.recycle();
                        if (newBitmap != null) {
                            publishProgress(new LoadedImage(newBitmap));
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
            return null;
        }

        @Override
        public void onProgressUpdate(LoadedImage... value) {
            addImage(value);
        }

        @Override
        protected void onPostExecute(Object result) {
            setProgressBarIndeterminateVisibility(false);
        }
    }

    /*
     * Adapter
     */
    class FileAdapter extends BaseAdapter {

        private Context mContext;
        private ArrayList<LoadedImage> photos = new ArrayList<LoadedImage>();

        public FileAdapter(Context context) {
            mContext = context;
        }

        public void addPhoto(LoadedImage photo) {
            photos.add(photo);
        }

        public int getCount() {
            return photos.size();
        }

        public Object getItem(int position) {
            return photos.get(position);
        }

        public long getItemId(int position) {
            return position;
        }

        public View getView(int position, View convertView, ViewGroup parent) {
            final ImageView imageView;
            if (convertView == null) {
                imageView = new ImageView(mContext);
            } else {
                imageView = (ImageView) convertView;
            }
            imageView.setScaleType(ImageView.ScaleType.FIT_CENTER);
            imageView.setPadding(8, 8, 8, 8);
            imageView.setImageBitmap(photos.get(position).getBitmap());
            return imageView;
        }
    }

    /*
     * 这是个保存bitmap的类，加入Adapter的ArrayList中，随着addImage更新Adapter
     */
    private static class LoadedImage {
        Bitmap mBitmap;

        LoadedImage(Bitmap bitmap) {
            mBitmap = bitmap;
        }

        public Bitmap getBitmap() {
            return mBitmap;
        }
    }

    /*
     * 消息提示
     */
    private Toast toast;

    public void showMsg(String arg) {
        if (toast == null) {
            toast = Toast.makeText(this, arg, Toast.LENGTH_SHORT);
        } else {
            toast.cancel();
            toast.setText(arg);
        }
        toast.show();
    }
}