Question

我有这个适配器，我加载来自互联网的数据。如果我快速向下滚动，它会在显示正确的图像之前显示错误的图像。但是，图像旁边的相应文本不会这样做。当它下载更多数据时更明显。请帮我调试一下。以下是listview的样子：

The listview

使用仅包含textview数据的数据列表初始化categoryAdapter。当调用categoryAdapter的getView（）时，它会通过asynctask从url下载图像。当达到列表视图的底部时，OnScrollListener会再下载12篇文章（仅限文本）。这是getView（）。请注意，我认为回收旧视图是问题所在，所以我尝试在将imageview的drawable更改为新图像之前将图像快速设置为null，这样我就不会看到旧图像了。如果我快速向上滚动，这是因为没有看到错误，但如果我快速向下滚动则不行。可能是什么问题？

class CategoryAdapter extends BaseAdapter{...
@Override
public View getView(int i, View view, final ViewGroup viewGroup){
    // crate a new rowItem object here
    LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    View row;

    if (view == null) row = inflater.inflate(R.layout.single_row, viewGroup, false);
    else row = view;

    // Get the description, image and title of the row item
    TextView title = (TextView) row.findViewById(R.id.textView);
    TextView description = (TextView) row.findViewById(R.id.textView2);
    ImageView image = (ImageView) row.findViewById(R.id.imageView);
    ProgressBar pb = (ProgressBar) row.findViewById(R.id.progressBarSingleRow);

    /* On getting view, set this to invisible until loaded. (issue before: old image seen
       before new image on fast scroll) Mostly fixed by this, but on fast scroll down, still
       shows a little */
    image.setImageDrawable(null);

    // Set the values of the rowItem
    SingleRow rowTemp = articles.get(i);
    title.setText(rowTemp.title);
    description.setText(rowTemp.description);

    String s = "null";
    if (rowTemp.image != null) s = "not null";
    Log.e("ImageLog", "Item " + Integer.toString(i) + ", is " + s);

    // Load image into row element
    if (rowTemp.image == null) {    // download
        // Prepare prepped row objects in single holder object for fetchCategoryImageTask
        AdapterObject holder = new AdapterObject();
        holder.title = title;
        holder.description = description;
        holder.image = image;
        holder.pb = pb;

        new FetchCategoryImageTask(rowTemp, holder).execute();
    }
    else {                          // set saved image
        // Cropping image to preserve aspect ratio
        image.setScaleType(ImageView.ScaleType.CENTER_CROP);
        image.setCropToPadding(true);
        image.setImageBitmap(rowTemp.image);
    }

    return row;
}

以防万一，这里有一点fetchCategoryImageTask（）

@Override
protected void onPreExecute() {
    image.setVisibility(ImageView.INVISIBLE);
}

@Override
protected Bitmap doInBackground(Void... params) {
    return (sr.imageURL != null) ? downloadBitmap(sr.imageURL) : null;
}

@Override
protected void onPostExecute(final Bitmap b) {
    // imageView image
    // Preserve aspect ratio of image
    image.setScaleType(ImageView.ScaleType.CENTER_CROP);
    image.setCropToPadding(true);
    image.setImageBitmap(b);

    // Make image visible
    image.setVisibility(ImageView.VISIBLE);

    // Save image to SingleRow object for categoryAdapter's getView()
    sr.image = b;
}

Answer 1

这里发生的是ListViews重用视图。因此，它重用已有图像的视图。我建议您使用Volley库或ImageLoader库来解决此问题，在下载图像时很容易定义要显示的占位符。

因此，不要使用Asynctask，而是使用以下代码：

private void showImage(View view, String imageUrl) {
        DisplayImageOptions options = new DisplayImageOptions.Builder()//
                .showImageOnFail(R.drawable.image_placeholder) 
                .showImageOnLoading(R.drawable.image_placeholder) 
                .showImageForEmptyUri(R.drawable.image_placeholder) 
                .cacheInMemory(true) 
                .cacheOnDisk(true) 
                .build();
        ImageLoader.getInstance().displayImage(imageUrl, imageViewType, options);
    }

Universal Image Loader库：https://github.com/nostra13/Android-Universal-Image-Loader

Answer 2

这个答案来自评论，所以我会在这里发布。我每次使用LRU缓存获取行和缓存图像时都不会使用viewHolder模式来查看视图。以下是参考链接：ViewHolder，Caching bitmaps。

listview数据图像重复快速向下滚动

2 个答案: