Xamarin Forms ReactiveUI

Question

代码：

文件： MainPageViewModel.cs

using System.Reactive;
using ReactiveUI;
using ReactiveUI.Fody.Helpers;

namespace App2
{
    public class MainPageViewModel: ReactiveObject
    {
        public ReactiveCommand<Unit, Unit> OnChange { get; }
        
        [Reactive]
        public string Greeting { get; set; }

        public MainPageViewModel()
        {
            Greeting = "Hello world!";
            OnChange = ReactiveCommand.Create(() =>
            {
                Greeting = "Go C#!";
            });
        }
    }
}

页面：

文件： MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<xamForms:ReactiveContentPage x:TypeArguments="vm:MainPageViewModel" xmlns="http://xamarin.com/schemas/2014/forms"
                              xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                              xmlns:xamForms="clr-namespace:ReactiveUI.XamForms;assembly=ReactiveUI.XamForms"
                              xmlns:vm="clr-namespace:App2;assembly=App2"
                              x:Class="App2.MainPage">
    <ContentPage.BindingContext>
        <vm:MainPageViewModel></vm:MainPageViewModel>
    </ContentPage.BindingContext>

    <StackLayout>
        <Label Text="{Binding Greeting}" 
           HorizontalOptions="Center"
           VerticalOptions="CenterAndExpand" />
        
        <Button Command="{Binding OnChange}" Text="Click"></Button>
    </StackLayout>

</xamForms:ReactiveContentPage>