如何正确绑定和更新Xamarin ?表格视图

使用MVVM模式，我有一个模型，ViewModel和视图，其中包含一个ListView。ListView绑定到ViewModel的一个成员，该成员是Model类的ObservableCollection。我可以使初始显示的绑定工作，并且可以在对视图进行操作时更新Model类上对应行的属性，但是我无法使视图刷新，无法从ObservableCollection中的Model类中提取数据。ListView类不包含使无效或强制刷新的方法，这将解决我的问题。如何让ListView在更新模型上的数据后刷新?

下面是我尝试做的一个简单示例:每行包含一个Button和Label。在点击一个按钮后，我可以更新标签，这将反映在屏幕上。我需要做的是更新Model，这反过来应该强制更新视图。然而，我不能让它工作。在实际的应用程序中，模型的更新将在业务逻辑层中进行，而不是在视图中，之后我需要强制刷新ListView。

示例代码:

using System;
using System.Collections.ObjectModel;
using Xamarin.Forms;
namespace ListViewTest
{   
    public class Model
    {
        public static int ids = 0;
        public Model(string count) 
        {
            Id = +1;
            Count = count;
        }
        public int Id { get; set; }
        public string Count { get; set; }
    }
    public class ModelList : ObservableCollection<Model>
    {
    }
    public class ViewModel
    {
        ModelList list = new ModelList();
        public ModelList ViewModelList
        {
            get { return list; }
            set 
            { 
                list = value; 
            }
        }
    }
    public partial class MainPage : ContentPage
    {   
        public ViewModel viewModel;
        public class DataCell : ViewCell
        {
            public DataCell()
            {
                var Btn = new Button();
                Btn.Text = "Click";
                var Data = new Label();
                Data.SetBinding(Label.TextProperty,"Count");
                Btn.Clicked += (object sender, EventArgs e) => 
                {
                    Model model = (Model)(((Button)sender).Parent.BindingContext);
                    int count = Convert.ToInt32(model.Count);
                    count++;
                    model.Count = count.ToString();
                    // Need to refresh ListView from data source here... How???
                };
                StackLayout s = new StackLayout();
                s.Orientation = StackOrientation.Horizontal;
                s.Children.Add(Btn);
                s.Children.Add(Data);
                this.View = s;
            }
        }
        public MainPage ()
        {
            viewModel = new ViewModel();
            viewModel.ViewModelList.Add(new Model("0"));
            viewModel.ViewModelList.Add(new Model("0"));
            InitializeComponent();
        }
        public void InitializeComponent()
        {
            ListView listView = new ListView
            {
                ItemsSource = viewModel.ViewModelList,
                ItemTemplate = new DataTemplate(() =>
                    {
                        return new DataCell();
                    })
            };
            Content = listView;
        }
    }
}

public class Model : INotifyPropertyChanged
{
    // boiler-plate
    public event PropertyChangedEventHandler PropertyChanged;
    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = PropertyChanged;
        if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
    }
    protected bool SetField<T>(ref T field, T value, string propertyName)
    {
        if (EqualityComparer<T>.Default.Equals(field, value)) return false;
        field = value;
        OnPropertyChanged(propertyName);
        return true;
    }
    // props
    private string _count;
    public string Count 
    {
        get { return _count; }
        set { SetField(ref _count, value, "Count"); }
    }
}