Question

我尝试使用CardView实现RecyclerView，卡片只是带有textview的图像视图。一开始一切都按预期工作，但如果你向下滚动，然后在recyclerView中备份，那么imageViews将为空，textview很好。

发生了什么的GIF

gif of what happens

适配器：

    public class NewsCardAdapter : RecyclerView.Adapter
{
    private List<RSSItem> items;

    public NewsCardAdapter()
    {
        HasStableIds = true;
    }

    public override RecyclerView.ViewHolder OnCreateViewHolder(ViewGroup parent, int viewType)
    {
        var view = LayoutInflater.From(parent.Context).Inflate(Resource.Layout.NewsItem, parent, false);
        NewsViewHolder viewHolder = new NewsViewHolder(view);
        return viewHolder;
    }

    public override long GetItemId(int position)
    {
        return items[position].Id;
    }

    public override async void OnBindViewHolder(RecyclerView.ViewHolder viewHolder, int position)
    {
        var item = items[position];
        var holder = viewHolder as NewsViewHolder;
        if (item.ThumbNail != null)
        {
            holder.ImgThumbnail.SetImageBitmap(await BitmapFactory.DecodeStreamAsync(item.ThumbNail));
        }
        else
        {
            // Place holder in case item doesn't have thumbnail
            holder.ImgThumbnail.SetImageResource(Resource.Drawable.Icon);
        }
        holder.Title.Text = item.Title;
    }

    public override int ItemCount => items.Count;

    public List<RSSItem> Items
    {
        get { return items; }
        set { items = value; }
    }
}

public class NewsViewHolder : RecyclerView.ViewHolder
{
    private ImageView imgThumbnail;
    private TextView title;

    public NewsViewHolder(View itemView) : base(itemView)
    {
        imgThumbnail = itemView.FindViewById<ImageView>(Resource.Id.img_thumbnail);
        title = itemView.FindViewById<TextView>(Resource.Id.news_title);
    }

    public ImageView ImgThumbnail
    {
        get { return imgThumbnail; }
        set { imgThumbnail = value; }
    }

    public TextView Title
    {
        get { return title; }
        set { title = value; }
    }
}

模型（RSSItem）：

 public class RSSItem : INotifyPropertyChanged
{
    [PrimaryKey, AutoIncrement]
    public long Id { get; set; }
    public string Title { get; set; }
    private Stream thumbnail;

    [Ignore]
    public Stream ThumbNail
    {
        get { return thumbnail; }
        set
        {
            thumbnail = value;
            OnPropertyChanged();
        }
    }
    // ...
}

我的猜测 - 它与将imageView源分配给从流解码的位图而不是在模型中存储位图本身有关。但是我不能这样做，因为System.Drawable不是PCL的一部分。

Answer 1

尝试将Stream定位为在解码前启动，如Stream.Position = 0; 此外，不确定您的流是什么，您可以使用MemoryStreams在PCL级别缓存图像，或在UI级别缓存图像（通过存储BitmapFactory.DecodeStreamAsync结果）。

Answer 2

我建议您使用Picasso库来检索图像。它的性能更高（主要是在内存中）并且会解决您的问题。

Xamarin.Droid RecyclerView只显示一次图像

2 个答案: