使用bitmapTools的display方法在ListView中加载多张图片时，存在上下滑动时中间部分的图片会消失的问题 about volley HOT 4 OPEN

private Context context;
private ArrayList<String> strings;

public RecycleAdapter(Context context, ArrayList<String> strings) {
    this.context = context;
    this.strings = strings;
}

@Override
public RecycleViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    View view = LayoutInflater.from(context).inflate(R.layout.recycle_item, parent, false);
    RecycleViewHolder holder = new RecycleViewHolder(view);
    return holder;
}

@Override
public void onBindViewHolder(RecycleViewHolder holder, int position) {
    MyApplication.getInstance().getBitmapTools().display(holder.img, strings.get(position));
}

@Override
public int getItemCount() {
    return strings.size();
}

public class RecycleViewHolder extends RecyclerView.ViewHolder {

    ImageView img;

    public RecycleViewHolder(View itemView) {
        super(itemView);
        img = (ImageView) itemView.findViewById(R.id.img);
    }
}

private RecyclerView recyclerView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    ArrayList<String> strings = new ArrayList<>();
    for (int i = 0; i < 3; i++) {
        strings.add("http://g.hiphotos.baidu.com/image/w%3D310/sign=972b9e33324e251fe2f7e2f99786c9c2/0824ab18972bd407e3e5eff479899e510fb3099c.jpg");
        strings.add("http://g.hiphotos.baidu.com/image/w%3D310/sign=0c1aeb23aad3fd1f3609a43b004f25ce/38dbb6fd5266d01687b5e625952bd40735fa350a.jpg");
        strings.add("http://h.hiphotos.baidu.com/image/w%3D310/sign=42c9b52c9925bc312b5d07996ede8de7/9e3df8dcd100baa1a22d27594710b912c9fc2eca.jpg");
        strings.add("http://a.hiphotos.baidu.com/image/w%3D310/sign=7b4616d4a964034f0fcdc4079fc27980/b999a9014c086e062d28e74200087bf40ad1cb33.jpg");
        strings.add("http://c.hiphotos.baidu.com/image/w%3D310/sign=3915003f9045d688a302b4a594c37dab/024f78f0f736afc383d9f669b119ebc4b64512e1.jpg");
        strings.add("http://e.hiphotos.baidu.com/image/w%3D310/sign=6366c3f20846f21fc9345852c6256b31/96dda144ad345982061b689b0ef431adcbef8422.jpg");
        strings.add("http://c.hiphotos.baidu.com/image/w%3D310/sign=612b97dcfd039245a1b5e70eb795a4a8/aa64034f78f0f7360d262d100b55b319eac413c2.jpg");
    }
    recyclerView = (RecyclerView) findViewById(R.id.recycle);
    recyclerView.setLayoutManager(new StaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL));
    recyclerView.setAdapter(new RecycleAdapter(this, strings));
}