ListView源没有从NavigationParameters更新

这是我的代码在我的Xamarin表单应用程序，它应该填充与DetaledActivity的Activities列表从上一页填充的xaml ListView。当我逐步执行和调试代码时，_activities包含许多项目，但它们没有显示在xaml页面中。

如果我在此页面的构造函数中使用项目填充列表，那么我可以单击这些单元格，但其中没有显示文本。

public class DetailedActivityPageViewModel : _ViewModelBase
{
    INavigationService _navigationService;
    public DetailedActivityPageViewModel(INavigationService navigationService)
    {
        _navigationService = navigationService;
    }
    public override void OnNavigatedTo(NavigationParameters parameters)
    {
        base.OnNavigatedTo(parameters);
        if (parameters.ContainsKey("properties"))
        {
            var localAcs = new List<DetailedActivity>();
            var properties = (Dictionary<string, object>)parameters["properties"];
            foreach (var prop in properties)
            {
                if (prop.Value != null)
                    localAcs.Add(new DetailedActivity { Key = prop.Key.ToString(), Value = prop.Value.ToString() });
            }
            _activities = localAcs;
        }
    }
    private IEnumerable<DetailedActivity> _activities;
    public IEnumerable<DetailedActivity> Activities { get { return _activities; } set { SetProperty(ref _activities, value, "Activities"); } }
}

public class DetailedActivity
{
    public string Key;
    public string Value;
}

---

<ListView ItemsSource="{Binding Activities}">
    <ListView.ItemTemplate>
        <DataTemplate>
            <ViewCell>
                <StackLayout Orientation="Horizontal">
                    <Label Text="{Binding Key}" />
                    <Label Text="{Binding Value}" />
                </StackLayout>
            </ViewCell>
        </DataTemplate>
    </ListView.ItemTemplate>
</ListView>

public override void OnNavigatedTo(NavigationParameters parameters)
{
    base.OnNavigatedTo(parameters);
    if (parameters.ContainsKey("properties"))
    {
        var localAcs = new List<DetailedActivity>();
        _activities = localAcs;
        var properties = (Dictionary<string, object>)parameters["properties"];
        foreach (var prop in properties)
        {
            if (prop.Value != null)
                localAcs.Add(new DetailedActivity { Key = prop.Key.ToString(), Value = prop.Value.ToString() });
        }
    }
}