Listview显示错误的图像

Question

我有一个带有ArrayAdapter的ListView，其中包含带有图像和字符串的行。直到我确定图像的加载速度变慢，这样我才能在显示列表之前无法加载图像，这样效果很好。因此，我开始使用加载图像到单独的线程中AsyncTask。

在开始滚动列表之前，我对结果感到非常满意。加载了错误的图像，看起来好像不是图像稍后出现的问题。如果我尝试对列表进行排序，问题将变得非常严重，并且没有图像在右行。

关于我在做什么错的任何想法？

public View getView(int position, View convertView, ViewGroup parent) {
        View v = convertView;
        ImageView imageView;
        TextView textView;
        if (v == null) {
            LayoutInflater vi = (LayoutInflater) getContext().getSystemService(
                    Context.LAYOUT_INFLATER_SERVICE);
            v = vi.inflate(R.layout.drink_list_row, null);
        }

        Drink drink = allItems.get(position);
        if (drink != null && v != null) {
            imageView = (ImageView) v.findViewById(R.id.picture);
            textView = (TextView) v.findViewById(R.id.drinkName);
            imageView.setVisibility(View.GONE);
            loadImageBitmap(drink, imageView);
            textView.setText(drink.getName());

            if (subItems != null && subItems.contains(drink)) {
                textView.setVisibility(View.VISIBLE);
                imageView.setVisibility(View.VISIBLE);
            } else {
                textView.setVisibility(View.GONE);
                imageView.setVisibility(View.GONE);
            }
        }
        return v;
    }

Answer 1

问题来自于您的convertView：它是整个列表中使用的同一实例，因此，当异步加载完成时，当listview尝试使用相同的convertView绘制其他项目时，图像会更改（或在这种情况下） ，其子imageView）。

painting position 1, uses placeholder, starts loading image 1 asynchronously
painting position 2, uses placeholder, starts loading image 2 asynchronously
image 1 loading is complete, calling setImageBitmap on imageView
painting position 3, uses image 1, starts loading image 3 asynchronously
etc.

但是，您可以做的是在listadapter中保留位图缓存。像这样：

private Bitmap[] bitmapList;
private Bitmap bitmapPlaceholder;  

private void initBitmapListWithPlaceholders(){ 
// call this whenever the list size changes
// you can also use a list or a map or whatever so you 
// don't need to drop all the previously loaded bitmap whenever 
// the list contents are modified
    int count = getListCount();
    bitmapList = new Bitmap[count];
    for(int i=0;i<count;i++){
         bitmapList[i]=bitmapPlaceholder
    }
}

private void onBitmapLoaded(int position, Bitmap bmp){
// this is your callback when the load async is done
    bitmapList[position] = bmp;
}

public View getView(int position, View convertView, ViewGroup parent) {
    View v = convertView;
    ImageView imageView;
    TextView textView;
    if (v == null) {
        LayoutInflater vi = (LayoutInflater) getContext().getSystemService(
                Context.LAYOUT_INFLATER_SERVICE);
        v = vi.inflate(R.layout.drink_list_row, null);
    }

    Drink drink = allItems.get(position);
    if (drink != null && v != null) {
        imageView = (ImageView) v.findViewById(R.id.picture);
        textView = (TextView) v.findViewById(R.id.drinkName);
        imageView.setVisibility(View.GONE);
        imageView.setImageBitmap(bitmapList[position]);
        loadImageBitmap(drink, position); // this should call onBitmapLoaded(int position, Bitmap bmp) when finished to update the bitmapList and replace the placeholder
        textView.setText(drink.getName());

        if (subItems != null && subItems.contains(drink)) {
            textView.setVisibility(View.VISIBLE);
            imageView.setVisibility(View.VISIBLE);
        } else {
            textView.setVisibility(View.GONE);
            imageView.setVisibility(View.GONE);
        }
    }
    return v;
}