UWP ListView 未加载项目

Question

我将以最简单的方式解释我的问题。 我一直在处理listview和gridview以及绑定很长时间，但现在我遇到了一个无法解释的问题，所以我真的需要一些帮助。

下面是我的listview的xaml代码。

 <ListView Name="OtherVideosList"  ItemsSource="{x:Bind VideoFiles}" SelectionChanged="OtherVideosList_SelectionChanged">
    <ListView.ItemTemplate>
        <DataTemplate x:DataType="data:VideoFile">
            <StackPanel Orientation="Horizontal">
                <Image Source="{x:Bind Thumbnail}"/>
                <StackPanel>
                    <TextBlock Text="{x:Bind FileName}"/>
                    <TextBlock Text="{x:Bind Duration}"/>
                </StackPanel>
            </StackPanel>
        </DataTemplate>
    </ListView.ItemTemplate>
</ListView>

我将它绑定到 ObservableCollection 下面是它的数据类型类。

public class VideoFile
{
    public string FileName { get; set; }
    public string Duration { get; set; }
    public StorageFile File { get; set; }
    public BitmapImage Thumbnail { get; set; }
}

我正在使用此类创建项目源

public ObservableCollection<VideoFile> VideoFiles { get; set; }

我使用一个按钮打开多个文件，然后将它们放到项目源中，以便稍后在媒体元素上播放它们。下面是事件处理程序的代码。

private async void OpenClick(object sender, RoutedEventArgs e)
    {
        try
        {
            var p = new FileOpenPicker();
            foreach (var item in videoTypes)
            {
                p.FileTypeFilter.Add(item);
            }
            //Curentplayingfiles is IReadOnlyList<StorageFiles>
            CurrentlyPlayingFiles = await p.PickMultipleFilesAsync();
            if (CurrentlyPlayingFiles.Count != 0)
            {
                if (CurrentlyPlayingFiles.Count == 1)
                {   //this if block works absolutely fine
                    CurrentlyPlayingFile = CurrentlyPlayingFiles[0];
                    var s = await CurrentlyPlayingFile.OpenReadAsync();
                    ME.SetSource(s, CurrentlyPlayingFile.ContentType);
                }
                else
                {
                    VideoFiles = new ObservableCollection<VideoFile>();
                    foreach (var file in CurrentlyPlayingFiles)
                    {
                        //Thumbnail and GetDuration are my own static methods to get thumbnail
                        //and duration property of the file respectively
                        VideoFiles.Add(new VideoFile { Thumbnail = await Thumbnail(file), Duration = await GetDuration(file), File = file, FileName = file.DisplayName });
                    }
                    //exception occurs on this very line below, because here OtherVideosList has zero items.
                    OtherVideosList.SelectedIndex = 0;
                }

            }

        }
        catch (Exception s){ var dr = s.Message; }
    }

我已经为你们提到了cmets中的关键点。 任何帮助将不胜感激，非常感谢..

Answer 1

在您的代码中，您似乎绑定到一个可观察的视频文件集合。在添加项目之前不要将其设置为新的。如果集合已经绑定，这将破坏您的绑定。改为清除列表中的所有项目

Answer 2

所以，您的页面没有实现InotifyPropertyChanged，您可以通过两种方式修复它：

1. 您可以在 costructor 中初始化您的 VideoFiles 集合，一切都会正常工作。

2. 另一种方式，实现INotifyPropertyChanged接口。 对了，默认x:Bind模式是OneTime所以这一步需要把Mode改成OneWay。

C#

    public sealed partial class MainPage : Page, INotifyPropertyChanged
    {
        private ObservableCollection<VideoFile> _videoFiles { get; set; }

        public MainPage()
        {
            this.InitializeComponent();
        }

        public ObservableCollection<VideoFile> VideoFiles
        {
            get
            {
                return _videoFiles;
            }
            set
            {
                if (_videoFiles != value)
                {
                    _videoFiles = value;
                    RaisePropertyChanged(nameof(VideoFiles));
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        private void OpenClick(object sender, RoutedEventArgs e)
        {
            VideoFiles = new ObservableCollection<VideoFile>();
            VideoFiles.Add(new VideoFile()
            {
                Duration = "02:00",
                FileName = "file name",
                Thumbnail = new BitmapImage(new Uri("http://s.ill.in.ua/i/news/630x373/298/298656.jpg"))
            });

        }

        private void RaisePropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

XAML：

<ListView Name="OtherVideosList"  
          ItemsSource="{x:Bind VideoFiles, Mode=OneWay}">

Answer 3

您只监听CollectionChanged 事件，而不是VideoFiles 属性本身的更改。试试这个：

// Assuming you're using C# 6. If not, assign it in constructor
public ObservableCollection<VideoFile> VideoFiles { get; set; } = new ObservableCollection<VideoFile>();

然后在 else 子句中：

else
{
    VideoFiles.Clear();
    foreach (var file in CurrentlyPlayingFiles)
    {
        //Thumbnail and GetDuration are my own static methods to get thumbnail
        //and duration property of the file respectively
        VideoFiles.Add(new VideoFile { Thumbnail = await Thumbnail(file), Duration = await GetDuration(file), File = file, FileName = file.DisplayName });
    }
    //exception occurs on this very line below, because here OtherVideosList has zero items.
    OtherVideosList.SelectedIndex = 0;
}

当您将新集合分配给 VideoFiles 时，它会破坏绑定，您将不会收到任何有关内容已更改的进一步通知。