Question

我正在使用Prism Library和Xamarin Forms并尝试使用描述here的EventToCommandBehavior将ListTapped行为附加到Listview上。

执行ItemTappedCommand委托时，事件参数为null，这意味着我无法提取ListView项。

以下是我的ViewModel代码：

 private DelegateCommand<ItemTappedEventArgs> _itemTappedCommand;
    public RecipeListViewModel(INavigationService navigationService) : base(navigationService)
    {

        RefreshDataCommand = new Command(async () => await RefreshData());
        _itemTappedCommand = new DelegateCommand<ItemTappedEventArgs>(args => {
            var b = args; // args is null
            _navigationService.NavigateAsync(nameof(Recipe));
        });
    }

public DelegateCommand<ItemTappedEventArgs> ItemTappedCommand { get { return _itemTappedCommand; } }

这是我的XAML

<ListView ItemsSource="{Binding Recipes}"
            HasUnevenRows="false"
            IsPullToRefreshEnabled="true"
            CachingStrategy="RecycleElement"
            IsRefreshing="{Binding IsBusy, Mode=OneWay}"
            RefreshCommand="{Binding RefreshDataCommand}"
            x:Name="lvRecipes">
        <ListView.Behaviors>
            <b:EventToCommandBehavior EventName="ItemTapped" Command="{Binding ItemTappedCommand}" />
        </ListView.Behaviors>
        <ListView.Header>..........
.....</ListView>

Answer 1

正如布莱恩所说，你说错了。例如，假设我有一个字符串集合只是为了简化它，但它可以是任何东西的集合。

public class MainPageViewModel
{
    public MainPageViewModel()
    {
        People = new ObservableRangeCollection<string>()
        {
            "Fred Smith",
            "John Thompson",
            "John Doe"
        };

        ItemTappedCommand = new DelegateCommand<string>(OnItemTappedCommandExecuted);
    }

    public ObservableRangeCollection<string> People { get; set; }

    public DelegateCommand<string> ItemTappedCommand { get; }

    private void OnItemTappedCommandExecuted(string name) =>
        System.Diagnostics.Debug.WriteLine(name);
}

我的ListView只需要执行以下操作：

<ListView ItemsSource="{Binding People}">
    <ListView.Behaviors>
        <b:EventToCommandBehavior Command="{Binding ItemTappedCommand}" 
                                  EventName="ItemTapped"
                                  EventArgsParameterPath="Item" />
    </ListView.Behaviors>
</ListView>

ViewModel中的命令应该期望您绑定到ItemsSource的{{1}}的集合中的确切Object类型。要正确使用它，您只需在ListView中指定路径，在这种情况下只需EventArgs。

Answer 2

Control EventArgs不会传递给ViewModel。这打破了MVVM模式。只需使用EventArgsParameterPath或EventArgsConverter即可将您所需的内容准确传递给ViewModel。

如何使用Xamarin Prism EventToCommand行为

2 个答案: