Xamarin UWP 似乎绑定到错误的视图模型

Question

我有一个适用于 Android 和 UWP 的 Xamarin 项目。这个问题似乎只发生在 UWP 上。

在我的 Xamarin 项目中，我有 ContentPage 与绑定为上下文的视图模型。在这个 ViewModel 中有一个 ObservableCollection 和另一种视图模型。当我创建这个底层 ViewModel 的新实例并添加到我的 ObservableCollection 时，有时ContentPage 会按预期工作，在我的 ListView 中显示一个项目。但有时会添加一个空元素，将鼠标悬停在列表上时可以看到。发生这种情况时，我会在“输出”选项卡中收到一堆警告。

我的DownloadsPage：

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:local="clr-namespace:Downloader.Converters"
             mc:Ignorable="d"
             x:Class="Downloader.Views.DownloadsPage">

    <ContentPage.Resources>
        <ResourceDictionary>
            <local:DownloadStatusToColorConverter x:Key="downloadStatusToColor" />
        </ResourceDictionary>
    </ContentPage.Resources>

    <RefreshView IsRefreshing="{Binding IsBusy, Mode=TwoWay}" Command="{Binding LoadItemsCommand}">
        <ListView x:Name="DownloadsListView" SelectionMode="None" ItemsSource="{Binding Downloads}" RowHeight="70">
            <ListView.ItemTemplate>
                <DataTemplate>
                    <ViewCell>
                        <StackLayout Padding="10" BackgroundColor="{Binding DownloadStatus, Converter={StaticResource downloadStatusToColor}}">
                            <Label Text="{Binding Name}"
                                   d:Text="{Binding .}"
                                   LineBreakMode="NoWrap"
                                   Style="{DynamicResource ListItemTextStyle}"
                                   FontSize="16" />
                            <Grid Grid.Row="0" Grid.Column="0" Padding="10,0,10,0">
                                <ProgressBar BackgroundColor="Transparent"  Progress="{Binding PercentDownloaded}" HorizontalOptions="FillAndExpand" HeightRequest="20">
                                </ProgressBar>
                                <Label Text="{Binding PercentString}" HorizontalTextAlignment="Center"></Label>
                            </Grid>
                        </StackLayout>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </RefreshView>

</ContentPage>

DownloadsViewModel 在代码隐藏中设置为上下文，如下所示：

public partial class DownloadsPage : ContentPage
{
    private readonly DownloadsViewModel _viewModel;

    public DownloadsPage()
    {
        InitializeComponent();

        BindingContext = _viewModel = new DownloadsViewModel();

        Device.StartTimer(TimeSpan.FromSeconds(1), () =>
        {
            Device.BeginInvokeOnMainThread(() => _viewModel.RefreshDownloads());
            return true;
        });
    }
}

绑定DownloadsViewModel：

public class DownloadsViewModel : BaseViewModel
{
    public ObservableCollection<DownloadViewModel> Downloads { get; set; } = new ObservableCollection<DownloadViewModel>();
 
    public Command LoadItemsCommand { get; set; }

    public DownloadsViewModel()
    {
        Title = "Downloads";
        LoadItemsCommand = new Command(() => {
            IsBusy = true;
            Downloads.Clear();
            RefreshDownloads();
            IsBusy = false;
        });
    }

    public void RefreshDownloads()
    {
        foreach (var download in DownloadManager.GetDownloads())
        {
            var existingDownload = Downloads.FirstOrDefault(d => d.Id == download.Id);

            if (existingDownload != null)
            {
                existingDownload.UpdateValues(download);
            }
            else
            {
                Downloads.Add(new DownloadViewModel(download));
            }
        }
    }
}

ObservableCollection 包含 DownloadViewModel，看起来像这样：

public class DownloadViewModel : BaseViewModel
{
    private IDownload _download;
    public DownloadViewModel(IDownload download)
    {
        UpdateValues(download);
    }

    private string _id;
    public string Id
    {
        get { return _id; }
        set { SetProperty(ref _id, value); }
    }

    private string _name;
    public string Name
    {
        get { return _name; }
        set { SetProperty(ref _name, value); }
    }

    private DownloadStatus _status;
    public DownloadStatus DownloadStatus
    {
        get { return _status; }
        set { SetProperty(ref _status, value); }
    }

    public double PercentDownloaded
    {
        get
        {
            return _download.DownloadedBytes == -1
                ? 0f
                : (double)_download.DownloadedBytes / _download.TotalBytes;
        }
    }

    public string PercentString { get => $"{(int)(PercentDownloaded * 100)} %"; }

    public void UpdateValues(IDownload download)
    {
        _download = download;
        Id = _download.Id;
        Name = _download.Name;
        DownloadStatus = _download.Status;
    }
}

我有时会遇到导致 ListView 中的项目为空的错误：

Binding: 'DownloadStatus' property not found on 'Downloader.ViewModels.DownloadsViewModel', target property: 'Xamarin.Forms.StackLayout.BackgroundColor'
Binding: 'Name' property not found on 'Downloader.ViewModels.DownloadsViewModel', target property: 'Xamarin.Forms.Label.Text'
Binding: 'PercentDownloaded' property not found on 'Downloader.ViewModels.DownloadsViewModel', target property: 'Xamarin.Forms.ProgressBar.Progress'
Binding: 'PercentString' property not found on 'Downloader.ViewModels.DownloadsViewModel', target property: 'Xamarin.Forms.Label.Text'

在调试时，我已确认该项目已按预期添加到我的 ObservableCollection 中。

为什么有时它会在 DownloadsViewModel 而不是 DownloadViewModel 上查找 DownloadStatus、Name、PercentDownloaded 和 PercentString？

Answer 1

Xamarin UWP 似乎绑定到错误的视图模型

我检查了您的代码示例，它按预期工作。但是我发现进度值没有自动更新导致listview项无法显示，我已经更新了IDownload接口添加PercentDownloaded属性。对于测试，它可以在uwp平台上运行。

问题在于 ViewModel 没有为所有属性实现 INotifyPropertyChanged 的​​设置器。源代码在on Github，修复问题的提交是this one。