从外部页面添加到ObservableCollection的新元素不会持久化

Question

我有两页。在第一个部分中，我希望看到ObservableCollection中的每个元素，而第二个元素则希望向ObservableCollection中添加新元素。当我给出断点时，我会看到ObservableCollection有新元素，但是当我返回到第一页时，新元素就不存在了。我该怎么办？

有代码

        public ListViewFlascardViewModel()
    {
        GoToAddFlashcard = new Command(goToAddFlashcard);
        Fiszka = new ObservableCollection<Flashcard>();
        Fiszka.Add(new Flashcard {Name = "hello" });         
    }
    
    
    public Command GoToAddFlashcard { get; }
    Flashcard flashcard = new Flashcard();        

    async void goToAddFlashcard()
    {
        await Shell.Current.GoToAsync(nameof(View.NoweFiszki));;            
    }

    public ObservableCollection<Flashcard> Fiszka { get; set; }
}

，还有第二页：

 class NoweFiszkiViewModel
{
    public Command SaveFlashcard { get; }
    public NoweFiszkiViewModel()
    {
       
        SaveFlashcard = new Command(save);
    }


    ListViewFlascardViewModel list = new ListViewFlascardViewModel();
   

    private async void save()
    {
        list.Fiszka.Add(new Flashcard { Name = "Bye" });            
         await Shell.Current.GoToAsync("..");
    }
}

我尝试了很多事情，但没有什么帮助。我是新来的C#，我将感谢每一个帮助。

我加了整个代码。

有视图

ListViewFlashcard

<ContentPage 
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
xmlns:viewmodels="clr-namespace:MVVM.ViewModel" 
xmlns:model="clr-namespace:MVVM.Model"
x:Class="MVVM.View.ListViewFlashcard"   
x:DataType="viewmodels:ListViewFlascardViewModel"
>

<ContentPage.BindingContext>
    <viewmodels:ListViewFlascardViewModel/>        
</ContentPage.BindingContext>
<StackLayout>
    
    <Button 
            Command="{Binding GoToAddFlashcard}"
            Text="Click Me"/>
    
    <ListView
        ItemsSource="{Binding Fiszka}">
        <ListView.ItemTemplate>
            <DataTemplate x:DataType="model:Flashcard">
                <TextCell Text="{Binding Name}"/>
            </DataTemplate>
        </ListView.ItemTemplate>

    </ListView>        
</StackLayout>

NoweFiszki

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
         xmlns:viewmodels="clr-namespace:MVVM.ViewModel"
         x:Class="MVVM.View.NoweFiszki">
<ContentPage.BindingContext>
    <viewmodels:NoweFiszkiViewModel/>
</ContentPage.BindingContext>
<ContentPage.Content>
    <StackLayout>
        <Entry Text="{Binding Text, Mode=TwoWay}"/>
        <Button Text="Save"
                Command="{Binding SaveFlashcard}"/>
    </StackLayout>
</ContentPage.Content>

与模型

闪存卡

    public class Flashcard
{
    public string Name { get; set; }
}

和AppShell

<Shell xmlns="http://xamarin.com/schemas/2014/forms"
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
         x:Class="MVVM.AppShell"
   xmlns:local="clr-namespace:MVVM.View"
   >   
<FlyoutItem Title="Add Flashcard" Icon="icon_about.png">
    <ShellContent ContentTemplate="{DataTemplate local:ListViewFlashcard}"/>
</FlyoutItem>

    public partial class AppShell : Xamarin.Forms.Shell
{
    public AppShell()
    {
        InitializeComponent();
        Routing.RegisterRoute(nameof(NoweFiszki), typeof(NoweFiszki));            
    }

}

这是我所写的一切。

Answer 1

有几件事必须在您的代码中纠正：

在NoweFiszkiViewModel中，您每次都要创建一个新的ListViewFlascardViewModel实例，并在该新实例上添加元素，这不会影响ListViewFlashcard实际绑定到的实例。

要解决这个问题，您必须在ListViewFlashcard.xaml.cs中创建一个public static ListViewFlascardViewModel并将绑定上下文设置为它，如下所示

ListViewFlashcard.xaml.cs

public static ListViewFlascardViewModel vm { get; set; }

public ListViewFlashcard()
{
    InitializeComponent();
    vm = new ListViewFlascardViewModel();
}

protected override void OnAppearing()
{
    base.OnAppearing();
            
    BindingContext = vm;
}

然后更正ListViewFlashcard.xaml，如下所示

ListViewFlashcard.xaml

<!--<ContentPage.BindingContext>
        <viewmodels:ListViewFlascardViewModel/>
    </ContentPage.BindingContext>-->
<StackLayout>
    <Button 
        Command="{Binding GoToAddFlashcard}"
        Text="Click Me"/>

    <ListView ItemsSource="{Binding Fiszka}">
        <ListView.ItemTemplate>
            <DataTemplate x:DataType="viewmodels:Flashcard">
                <TextCell Text="{Binding Name}"/>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>
</StackLayout>

最后，您必须更正NoweFiszkiViewModel.cs，如下所示，将新元素添加到public static视图模型中：

NoweFiszkiViewModel.cs

public Command SaveFlashcard { get; }

public NoweFiszkiViewModel()
{
    SaveFlashcard = new Command(save);
}

//ListViewFlascardViewModel list = new ListViewFlascardViewModel();

private async void save()
{
    ListViewFlashcard.vm.Fiszka.Add(new Flashcard { Name = "Bye" });
    await Shell.Current.GoToAsync("..");
}