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问题[mvvm](coding)

Martin Hope
infinitesimal
Asked: 2024-12-17 02:14:55 +0800 CST
  • 5

我正在尝试在 BindableProperty 中使用具有泛型类型的自定义控件，如下所示：

public partial class Overview : ContentView
{
    public static readonly BindableProperty AnimalsProperty =
            BindableProperty.Create(nameof(AnimalsProperty), typeof(ObservableRangeCollection<IAnimal>), typeof(Overview), null);

    public ObservableRangeCollection<IAnimal> Animals
    {
        get { return (ObservableRangeCollection<IAnimal>)GetValue(AnimalsProperty); }
        set { SetValue(AnimalsProperty, value); }
    }
}

public interface IAnimal {}
public partial class Dog: ObservableObject, IAnimal {}

DogOverview xaml：

<ContentView xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="DogOverview"
            >
    <Grid>
        <local:Overview
            Animals="{Binding Dogs}"
        />
    </Grid>
</ContentView>

public partial class DogViewModel: ObservableObject 
{
    [ObservableProperty]
    private ObservableRangeCollection<Dog> _dogs = [];
}

当我使用 BindableContext 时DogOverview：

public partial class DogOverview : ContentView
{
    public DogOverview(DogViewModel vm)
    {
        InitializeComponent();
        BindingContext = vm;
    }
}

行中有一个错误BindingContext = vm：**System.InvalidCastException:** 'Object must implement IConvertible.'。我检查过，这是因为我IAnimal在 BindableProprety 中使用了它。我只想创建具有泛型类型的自定义控件并在另一个中使用它ContentPage，这可能吗？

mvvm
Martin Hope
miniHessel
Asked: 2024-09-27 22:44:28 +0800 CST
  • 5

在 mvvm 中创建可观察属性的正确方法是

  [ObservableProperty]
  private string _name;

但是，在某些情况下，您需要更改集合。如何在不声明 Public 的情况下正确执行此操作？

private double _currentProgressPct;

public double CurrentProgressPct
{
    get => _currentProgressPct;
    set
    {
        _dispatcherQueue.TryEnqueue(DispatcherQueuePriority.High, () =>
        {
            SetProperty(ref _currentProgressPct, value);
        });
    }
}

据我了解，根据 mvvm，第一种方法是正确的，但后者不正确？

另外，根据本文，您需要实现 INotifyPropertyChanged 接口。但是，使用 [ObservableProperty] 时这是否必要？据我了解，社区工具包需要该类为可观察对象？

我之所以问最后几个问题，是因为使用具有内存焦点的性能分析器检查时，绑定似乎没有被释放。根据另一个线程，我发现这可能是因为未实现 INotifyProperyChanged？

mvvm
Martin Hope
Brett Simonds
Asked: 2024-04-07 09:38:46 +0800 CST
  • 5

我是 .NET MAUI/MVVM/SQLite 和 C# 的新手。我正在尝试利用 ViewModel 来绑定模型类中的数据。我可以使用模型命名空间作为我的绑定属性，它会在我的列表视图中显示数据。然而，一旦我尝试使用我的 ViewModel，它就不再显示。

CRUDView XAML 工作

        <Frame Grid.Row="6" Grid.Column="1" Grid.ColumnSpan="3" BackgroundColor="#B1C0C9" BorderColor="#7989A3">
            <ScrollView Orientation="Vertical">
                <ListView x:Name="listView" 
                          BackgroundColor="Gray" 
                          HasUnevenRows="True"
                          SeparatorVisibility="Default"
                          ItemTapped="listView_ItemTapped"
                          FlexLayout.Grow="1"
                          >
                    <ListView.ItemTemplate>
                        <DataTemplate>
                            <ViewCell>
                                <VerticalStackLayout Padding="5">
                                    <Label Text="{Binding Question}"
                                           FontSize="17" 
                                           FontFamily="OCRA" 
                                           TextColor="Black"/>
                                    <Grid>
                                        <Grid.ColumnDefinitions>
                                            <ColumnDefinition Width="50"/>
                                            <ColumnDefinition Width="*"/>
                                            <ColumnDefinition Width="*"/>
                                            <ColumnDefinition Width="Auto"/>
                                        </Grid.ColumnDefinitions>
                                        <Label Text="{Binding Id}"
                                               FontSize="10" 
                                               FontFamily="OCRA" 
                                               TextColor="Black"
                                               Grid.Column="0"
                                               />
                                        <Label Text="{Binding Answer}"
                                               FontSize="15" 
                                               FontFamily="OCRA" 
                                               TextColor="White"
                                               Grid.Column="1"
                                               Padding="10"/>
                                        <Label Text="{Binding Difficulty}"
                                               FontSize="15" 
                                               FontFamily="OCRA" 
                                               TextColor="Black"
                                               Grid.Column="3"
                                               Padding="10"/>
                                    </Grid>
                                    
                                </VerticalStackLayout>
                            </ViewCell>
                        </DataTemplate>
                    </ListView.ItemTemplate>
                </ListView>
            </ScrollView>

        </Frame>

在 ListView 中显示数据

当我恢复使用 ViewModel 进行数据绑定时：

CRUD查看XAML

<Frame Grid.Row="6" Grid.Column="1" Grid.ColumnSpan="3" BackgroundColor="#B1C0C9" BorderColor="#7989A3">
    <ScrollView Orientation="Vertical">
        <ListView x:Name="listView" 
                  BackgroundColor="Gray" 
                  HasUnevenRows="True"
                  SeparatorVisibility="Default"
                  ItemTapped="listView_ItemTapped"
                  FlexLayout.Grow="1"
                  ItemsSource="{Binding Flashcards}">
            <ListView.ItemTemplate>
                <DataTemplate x:DataType="vm:CRUDViewModel">
                    <ViewCell>
                        <VerticalStackLayout Padding="5">
                            <Label Text="{Binding Flashcard.Question}"
                                   FontSize="17" 
                                   FontFamily="OCRA" 
                                   TextColor="Black"/>
                            <Grid>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition Width="50"/>
                                    <ColumnDefinition Width="*"/>
                                    <ColumnDefinition Width="*"/>
                                    <ColumnDefinition Width="Auto"/>
                                </Grid.ColumnDefinitions>
                                <Label Text="{Binding Flashcard.Id}"
                                       FontSize="10" 
                                       FontFamily="OCRA" 
                                       TextColor="Black"
                                       Grid.Column="0"
                                       />
                                <Label Text="{Binding Flashcard.Answer}"
                                       FontSize="15" 
                                       FontFamily="OCRA" 
                                       TextColor="White"
                                       Grid.Column="1"
                                       Padding="10"/>
                                <Label Text="{Binding Flashcard.Difficulty}"
                                       FontSize="15" 
                                       FontFamily="OCRA" 
                                       TextColor="Black"
                                       Grid.Column="3"
                                       Padding="10"/>
                            </Grid>
                            
                        </VerticalStackLayout>
                    </ViewCell>
                </DataTemplate>
            </ListView.ItemTemplate>
        </ListView>
    </ScrollView>

</Frame>

视图模型

namespace cadflash.ViewModels
{
    public partial class CRUDViewModel : ObservableObject
    {
        [ObservableProperty]
        public ObservableCollection<Flashcard> flashcards = new();

        [ObservableProperty]
        public Flashcard flashcard = new();
        
        public CRUDViewModel()
        {
            Flashcard = new Flashcard();
            Flashcards = [];
        }

毛伊岛计划

MauiProgram

    public static class MauiProgram
    {
        public static MauiApp CreateMauiApp()
        {
            var builder = MauiApp.CreateBuilder();
            builder
                .UseMauiApp<App>()
                .UseMauiCommunityToolkit()
                .ConfigureFonts(fonts =>
                {
                    fonts.AddFont("OpenSans-Regular.ttf", "OpenSansRegular");
                    fonts.AddFont("OpenSans-Semibold.ttf", "OpenSansSemibold");
                    fonts.AddFont("OCRAEXT.ttf", "OCRA");
                });

            builder.Services.AddSingleton<LocalDbService>();
            builder.Services.AddTransient<HomeView>();
            builder.Services.AddTransient<DifficultyView>();
            builder.Services.AddTransient<CRUDView>();
            builder.Services.AddTransient<CRUDViewModel>();
            builder.Services.AddTransient<DifficultyViewModel>();
            builder.Services.AddTransient<HomeViewModel>();


#if DEBUG
            builder.Logging.AddDebug();
#endif

            return builder.Build();
        }
    }
}

CRUDView.cs

CRUDView.xaml.cs

public partial class CRUDView : ContentPage
{
    CRUDViewModel viewModel;
    private readonly LocalDbService localDbService; // This is the service that is used to interact with the local database
    private int _editFlashcardId; // This is the ID of the flashcard that is being edited
    public CRUDView(LocalDbService localDbService)
    {
        InitializeComponent();
        BindingContext = new CRUDViewModel();
        this.localDbService = localDbService;
        Task.Run(async () => await localDbService.GetFlashcardsAsync());
    }

    protected override async void OnAppearing()
    {
        base.OnAppearing();
        listView.ItemsSource = await localDbService.GetFlashcardsAsync();
    }

ListView中没有数据

为了让数据显示在列表视图中，我们经历了一周的痛苦。今天我弄清楚了如何直接从模型类绑定数据。理想情况下，我想使用 ViewModel 绑定......

mvvm