mvvm - 如何针对Xamarin Forms在Prism 7.2.0.1367的IDialogAware中使用RequestClose

Question

我一直在关注Prism 7.2.0.1367 release notes.我能够使用dialogService.ShowDialog(NavigationStrings.MyViewModel)导航到对话框ViewModel。但是，当我关闭它时，我在RequestClose上得到一个空引用异常。

在我的Xamarin Forms项目中，我有一个像这样的ViewModel:

public class MyViewModel : BaseViewModel, IDialogAware, IAutoInitialize
{
    private DelegateCommand _closeCommand;

    public DelegateCommand CloseCommand => _closeCommand ?? (_closeCommand = new DelegateCommand(Close));

    public event Action<IDialogParameters> RequestClose;

    public MyViewModel(INavigationService navigationService) : base(navigationService) { }

    public override void OnAppearing()
    {
        base.OnAppearing();
    }

    private void Close()
    {
        RequestClose(null);
    }

    public bool CanCloseDialog() => true;

    public void OnDialogClosed()
    {
        Console.WriteLine("The Demo Dialog has been closed...");
    }

    public void OnDialogOpened(IDialogParameters parameters)
    {
        // No need to do anything as IAutoInitialize will take care of what we need here...
    }
}

我应该将RequestClose设置为阻止它为null吗？没有文档说明必须设置此事件。

Answer 1

RequestClose是null，因为您从未订阅此事件。
您应该在调用RequestClose之前测试RequestClose(null)是否为null。

如果要使用自定义行为来处理关闭，可以订阅此事件。

通过检查 Action 是否为null进行编辑:

public class MyViewModel : BaseViewModel, IDialogAware, IAutoInitialize
{
    private DelegateCommand _closeCommand;

    public DelegateCommand CloseCommand => _closeCommand ?? (_closeCommand = new DelegateCommand(Close));

    public event Action<IDialogParameters> RequestClose;

    public MyViewModel(INavigationService navigationService) : base(navigationService) { }

    public override void OnAppearing()
    {
        base.OnAppearing();
    }

    private void Close()
    {
        RequestClose?.Invoke(null);
    }

    public bool CanCloseDialog() => true;

    public void OnDialogClosed()
    {
        Console.WriteLine("The Demo Dialog has been closed...");
    }

    public void OnDialogOpened(IDialogParameters parameters)
    {
        // No need to do anything as IAutoInitialize will take care of what we need here...
    }
}