使用 asynctask 从内容提供者加载图像位图

Question

我一直在尝试通过 MediaStore 内容提供程序查询 sd 卡上的所有图像，并在 GridView 上显示它们的缩略图。

但是，如果我在主线程上加载图像缩略图，滚动会变得非常慢......

所以我尝试通过 asynctasks 加载位图： 滚动性能变得更好，但现在网格项目不断重新加载它们的缩略图，直到它得到正确的位图......

这是我的异步任务，它加载位图：

package x.y;

import java.lang.ref.WeakReference;

import android.content.ContentResolver;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory.Options;
import android.os.AsyncTask;
import android.provider.MediaStore;
import android.widget.ImageView;

public class ImageThumbnailLoader extends AsyncTask<Long, Void, Bitmap> {

    private final Options mOptions;

    private WeakReference<ImageView> mImageViewWeakReference;
    private ContentResolver mContentResolver;

        public ImageThumbnailLoader(ImageView imageView,
                ContentResolver cr) {
        mContentResolver = cr;
        mImageViewWeakReference = new WeakReference<ImageView>(imageView);
        mOptions = new Options();
        mOptions.inSampleSize = 4;
    }



    @Override
    protected Bitmap doInBackground(Long... params) {
        Bitmap result;
            result = MediaStore.Images.Thumbnails.getThumbnail(
                    mContentResolver, params[0],
                    MediaStore.Images.Thumbnails.MINI_KIND, mOptions);
        return result;
    }

    @Override
    protected void onPostExecute(Bitmap result) {
        super.onPostExecute(result);
            if (mImageViewWeakReference != null
                    && mImageViewWeakReference.get() != null)
                mImageViewWeakReference.get().setImageBitmap(result);
    }

}

这是我的自定义光标适配器：

package x.y;

import android.content.Context;
import android.database.Cursor;
import android.graphics.BitmapFactory.Options;
import android.provider.MediaStore;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.CursorAdapter;
import android.widget.ImageView;

public class MediaCursorAdapter extends CursorAdapter {

    private LayoutInflater mInflater;
    private final static int mColumnID = 0;
    private Options mOptions;

    public MediaCursorAdapter(Context context, Cursor c) {
        super(context, c);

        mInflater = LayoutInflater.from(context);
        mOptions = new Options();
        mOptions.inSampleSize = 4;
    }

    @Override
    public void bindView(View view, Context context, Cursor cursor) {

        ViewHolder holder = (ViewHolder) view.getTag();
        ImageThumbnailLoader imageLoader = new ImageThumbnailLoader(holder.thumbImg,
                context.getContentResolver());
        imageLoader.execute(cursor.getLong(mColumnID));
//      holder.thumbImg.setImageBitmap(MediaStore.Images.Thumbnails.getThumbnail(
//                  context.getContentResolver(), cursor.getLong(mColumnID),
//                  MediaStore.Images.Thumbnails.MINI_KIND, mOptions));
        Log.i("Prototype", "bindView : " + cursor.getPosition());
    }

    @Override
    public View newView(Context context, Cursor cursor, ViewGroup parent) {
        Log.i("Prototype", "newView : " + cursor.getPosition());
        View view = mInflater.inflate(R.layout.grid_item, null);
        ViewHolder holder = new ViewHolder(view);
        view.setTag(holder);
        return view;
    }


    private static class ViewHolder {
        ImageView thumbImg, dragImg;

        ViewHolder(View base) {
            thumbImg = (ImageView) base.findViewById(R.id.thumbImage);
            dragImg = (ImageView) base.findViewById(R.id.dragImage);
        }
    }

}

我用这段代码查询游标并将其发送到适配器：

query(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, new String[] { MediaStore.Images.Media._ID,
            MediaStore.Images.Media.DATA }, null, null,
    MediaStore.Images.Media._ID);

看起来我的自定义光标适配器上的 bindview() 被调用的频率比预期的要高...任何人都知道如何让我的 gridview 上的图像在保持滚动性能的同时停止重新加载？

提前致谢。

Answer 1

问题已解决，必须在 onPostExecute() 上检查异步任务开始时的图像是否与结束时的图像相同。

新的绑定视图：

@Override
    public void bindView(View view, Context context, Cursor cursor) {

        ViewHolder holder = (ViewHolder) view.getTag();
        holder.thumbImg.setId(cursor.getPosition());
        ImageThumbnailLoader imageLoader = new ImageThumbnailLoader(holder.thumbImg,
            context.getContentResolver());
        imageLoader.execute(cursor.getLong(mColumnID));
    Log.i("Prototype", "bindView : " + cursor.getPosition());
    }

新的异步：

public class ImageThumbnailLoader extends AsyncTask<Long, Void, Bitmap> {

    private final Options mOptions;

    private WeakReference<ImageView> mImageViewWeakReference;
    private ContentResolver mContentResolver;
    private int mPosition;


        public ImageThumbnailLoader(ImageView imageView,
            ContentResolver cr) {
        mContentResolver = cr;
        mImageViewWeakReference = new WeakReference<ImageView>(imageView);
        mOptions = new Options();
        mOptions.inSampleSize = 4;
        mPosition = imageView.getId();
    }



    @Override
    protected Bitmap doInBackground(Long... params) {
        Bitmap result;
            result = MediaStore.Images.Thumbnails.getThumbnail(
                mContentResolver, params[0],
                MediaStore.Images.Thumbnails.MINI_KIND, mOptions);
        return result;
    }

    @Override
    protected void onPostExecute(Bitmap result) {
        super.onPostExecute(result);
            if (mImageViewWeakReference != null
                    && mImageViewWeakReference.get() != null
                        && mPosition == mImageViewWeakReference.get().getId())
                mImageViewWeakReference.get().setImageBitmap(result);
    }

}