Xamarin.Forms中的自定义EventHandler在xaml中不起作用

在Xamarin.Forms中，在PCL中，我具有自定义控件，带有自定义事件:

In Xamarin.Forms, in PCL, I have custom control, with custom event:

public class TestGrid : Grid
{
    public EventHandler OnTapped;

    public TestGrid()
    {
        var tgr = new TapGestureRecognizer { NumberOfTapsRequired = 1 };
        tgr.Tapped += Tgr_Tapped;
        this.GestureRecognizers.Add(tgr);
    }

    private void Tgr_Tapped(object sender, EventArgs e)
    {
        OnTapped?.Invoke(sender, e);
    }
}

在我的起始页中，我有:

in my StartPage I have:

public partial class StartPage : ContentPage
{
    public StartPage()
    {
        InitializeComponent();
        BindingContext = new StartPageViewModel();
        MainGrid.OnTapped += OnGridTapped;
    }

    private void OnGridTapped(object sender, EventArgs e)
    {
        Debug.WriteLine("Tapped!");
    }
}

我的XAML看起来像这样，并且可以正常工作:

My XAML looks like that, and it works:

<v:TestGrid x:Name="MainGrid" />

但是！

当我删除MainGrid.OnTapped += OnGridTapped;时 并在xaml中添加事件，在这里:

when I remove MainGrid.OnTapped += OnGridTapped; and add event in xaml, here:

<v:TestGrid x:Name="MainGrid" OnTapped="OnGridTapped">

我不工作.它说..找不到OnTapped事件:

I doens't work. It says.. OnTapped event not found:

未处理的异常:

Unhandled Exception:

Xamarin.Forms.Xaml.XamlParseException:位置6:33.无属性 找到名字OnTapped

Xamarin.Forms.Xaml.XamlParseException: Position 6:33. No Property of name OnTapped found

我的问题是:为什么?有什么区别?

My question is: Why? What's the difference?