iOS ListView 上下文操作不会在 Xamarin Forms 中自动向后滑动

Question

在Xamarin Forms 中（使用Prism，虽然不一定需要），我想从ListView 中的项目动态更改MenuItem ContextActions：

MainPage.xaml

<ContentPage
x:Class="XamarinProject.Views.MainPage"
xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
Title="MainPage"
prism:ViewModelLocator.AutowireViewModel="True">
<StackLayout HorizontalOptions="Center" VerticalOptions="Center">
    <ListView
        x:Name="mList"
        ItemsSource="{Binding Items}"
        SelectedItem="{Binding SelectedItem}">

        <ListView.ItemTemplate>
            <DataTemplate>
                <ViewCell>
                    <ViewCell.ContextActions>
                        <MenuItem Command="{Binding CurrentCommand}" Text="{Binding Cycle}" />
                    </ViewCell.ContextActions>
                    <Label Text="{Binding State}" TextColor="Navy" />
                </ViewCell>
            </DataTemplate>
        </ListView.ItemTemplate>
    </ListView>
</StackLayout>
</ContentPage>

MainPageViewModel.cs

public class MainPageViewModel : BindableBase, INavigationAware
{
    public ObservableCollection<MyItemViewModel> Items { get; set; } = new ObservableCollection<MyItemViewModel>();

    public void OnNavigatedTo(NavigationParameters parameters)
    {
        Items.Add(new MyItemViewModel(_pageDialogService));
    }
}

MyItemViewModel

public class MyItemViewModel : BindableBase
{
    private string _cycle;

    public string Cycle
    {
        get { return _cycle; }
        set { SetProperty(ref _cycle, value); }
    }

    private DelegateCommand _currentCommand;
    private readonly DelegateCommand[] _commands;
    public DelegateCommand CurrentCommand
    {
        get { return _currentCommand; }
        set { SetProperty(ref _currentCommand, value); }
    }

    public MyItemViewModel(IPageDialogService dialogService)
    {
        _commands = new DelegateCommand[]
        {
            new DelegateCommand(show1),
            new DelegateCommand(show2),
            new DelegateCommand(show3)
        };
        show1();
    }

    private void show1()
    {
        show("1", 1, "I show 1");
    }

    private void show2()
    {
        show("2", 2, "I show 2");
    }

    private void show3()
    {
        show("3", 0, "I show 3");
    }

    private void show(string cycle, int newIndex, string message)
    {
        Cycle = cycle;
        CurrentCommand = _commands[newIndex];
        // dialogService available through Unity, it's available
        _dialogService.DisplayAlertAsync("Alert", message, "Ok");
    }
}

但是，在 iOS 上，当我滑动行时，我看到 ContextAction(s)，然后勾选它，它会执行 Command，但 ContextActionMenu/row 不会自动向后滑动，因为它应该是.如果我将它绑定到单个DelegateCommand，而不动态地将其引用更改为另一个DelegateCommand，它就可以正常工作（就像在XAML 中这样：Command={Binding MyNonChangingCommand}）。如何解决这个问题，以便该行自动“向后滑动”（就像它应该的那样）？或者它只是 Prism 或 Xamarin Forms 中的一个错误？

Answer 1

Prism 是一个 MVVM 框架，不负责 UI 演示问题，但有时您可能需要对 MasterDetailPages 和 NavigationPages 进行一些技巧，以使它们以您希望使用 Prism 的方式显示。

关于您的代码，您可能需要进行一些更改。

• ViewModel 和 Model 之间存在差异。例如，您可能有一个 Monkey 模型、一个 MonkeyListViewModel 和一个 MonkeyDetailViewModel。 MonkeyListViewModel 将包含一个 ObservableCollection<Monkey> 和一个或多个要为特定 Monkey 执行的命令
• 您需要创建一个DataTemplateSelector，其中包含模型的业务逻辑，并且可以选择合适的选择器（见下文）

型号

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

    public bool SomeProperty { get; set; }
}

视图模型

public class SomeItemListPageViewModel : BindableBase
{
    public ObservableCollection<SomeItem> SomeItems { get; set; }

    public DelegateCommand<SomeItem> MyCommand { get; }

    private void OnMyCommandExecuted( SomeItem item )
    {
        // Do Foo
    }
}

数据模板选择器

public class MyItemCellSelector : DataTemplateSelector
{
    protected override DataTemplate OnSelectTemplate( object item, BindableObject container )
    {
        var myItem = item as MyItem;

        if( myItem.SomeProperty == true )
            return Foo;

        return Bar;
    }

    public DataTemplate Foo { get; set; }

    public DataTemplate Bar { get; set; }
}

查看

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:selector="clr-namespace:MyApp.Selectors;assembly=MyApp"
             xmlns:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
             prism:ViewModelLocator.AutowireViewModel="True"
             x:Name="page"
             x:Class="MyApp.Views.SomeItemListPage">
  <ContentPage.Resources>
    <ResourceDictionary>
      <!-- Sets up the Foo Template that has no Context Actions -->
      <DataTemplate x:Key="fooTemplate">
        <TextCell Text="{Binding Name}" />
      </DataTemplate>

      <!-- Sets up the Bar Template that has the Do Foo Context Action which executes
            MyCommand from the ViewModel -->
      <DataTemplate x:Key="barTemplate">
        <TextCell Text="{Binding Name}">
          <TextCell.ContextActions>
            <MenuItem Text="Do Foo" 
              Command="{Binding BindingContext.MyCommand,Source={x:Reference page}}" 
              CommandParameter="{Binding .}" />
          </TextCell.ContextActions>
        </TextCell>  
      </DataTemplate>

      <!-- Sets up the Selector to us the defined Templates for my Foo and Bar conditions -->
      <selector:MyItemCellSelector x:Key="myItemSelector"
                                   Default="{StaticResource fooTemplate}"
                                   PendingUpload="{StaticResource barTemplate}" />
    </ResourceDictionary>
  </ContentPage.Resources>

    <ListView ItemsSource="{Binding SomeItems}"
              ItemTemplate="{StaticResource localInspectionSelector}">
    </ListView>

</ContentPage>