Xaml Xamarin i命令未开火

我是MVVM新手，在我的Xamarin.Forms应用程序中出现了一些错误

下面是我在sessionpage.xml中看到的内容

<Grid.Children>
    <Image Source="zero.jpg" Grid.Row="0" Grid.Column="0">
    <Image.GestureRecognizers>
        <TapGestureRecognizer
            Command="{Binding TapCommand}"
            CommandParameter="0" />
     </Image.GestureRecognizers>

而ViewModel类是：

public class TapViewModel : INotifyPropertyChanged
{
    int taps = 0;
    ICommand tapCommand;

    public event PropertyChangedEventHandler PropertyChanged;

    public TapViewModel()
    {
        // configure the TapCommand with a method
        tapCommand = new Command(OnTapped);
    }

    public ICommand TapCommand
    {
        get { return tapCommand; }
    }
    void OnTapped(object s)
    {
        taps++;
        Debug.WriteLine("parameter: " + s);
    }
    //region INotifyPropertyChanged code omitted
}

事件没有触发，怎么了？

我回答自己

我的错误是我必须分配

this.BindingContext=新的TapViewModel（）

public class TapViewModel : INotifyPropertyChanged
{
    int taps = 0;
    ICommand tapCommand;

    public event PropertyChangedEventHandler PropertyChanged;

    public TapViewModel()
    {
        // configure the TapCommand with a method
        tapCommand = new Command(OnTapped);
    }

    public ICommand TapCommand
    {
        get { return tapCommand; }
    }
    void OnTapped(object s)
    {
        taps++;
        Debug.WriteLine("parameter: " + s);
    }
    //region INotifyPropertyChanged code omitted
}