[英]Xamarin UWP seems to bind to the wrong view model
我有一個 Xamarin 項目,用於 Android 和 UWP。 這個問題似乎只發生在 UWP 上。
在我的 Xamarin 項目中,我將ContentPage
與視圖 model 綁定為上下文。 在這個 ViewModel 中有一個帶有另一種視圖 model 的ObservableCollection
。 當我創建這個底層 ViewModel 的新實例並添加到我的 ObservableCollection 時,有時ContentPage
會按預期工作,在我的 ListView 中顯示一個項目。 但有時會添加一個空元素,將鼠標懸停在列表上時可以看到。 發生這種情況時,我會在 Output 選項卡中收到一堆警告。
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
在代碼隱藏中設置為上下文,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;
});
}
}
綁定的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
包含如下所示的DownloadViewModel
:
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;
}
}
我有時遇到的錯誤會導致我的 ListView 中的項目為空:
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'
調試時,我已確認該項目已按預期添加到我的 ObservableCollection 中。
為什么有時它會在 DownloadsViewModel 而不是 DownloadViewModel 上尋找 DownloadStatus、Name、PercentDownloaded 和 PercentString?
Xamarin UWP 似乎綁定到錯誤的視圖 model
我檢查了您的代碼示例,它按預期工作。 但是我發現進度值沒有自動更新導致listview項無法顯示,我已經更新了IDownload接口添加PercentDownloaded屬性。 對於測試,它可以在 uwp 平台上工作。
問題是 ViewModel 沒有為所有屬性實現 INotifyPropertyChanged 的設置器。 源代碼在 Github 上可用,修復問題的提交就是這個。
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