MVVMCross Android按钮绑定将禁用它

我正在尝试用MvvmCross将简单命令绑定到Android的按钮。 问题是,在我绑定命令之后,按钮将变为禁用状态。

已删除绑定 local:MvxBind="Click GotoScreen2Command"

为什么会发生这种情况,我如何解决它?

MainContainerViewModel.cs :

public class MainContainerViewModel : BaseViewModel
{
    readonly IMvxNavigationService _navigationService;

    public IMvxAsyncCommand GotoScreen2Command { get; set; }

    public MainContainerViewModel(IMvxNavigationService navigationService)
    {
        _navigationService = navigationService;

        GotoScreen2Command = new MvxAsyncCommand(NavigateToScreen2Async);
    }

    private Task NavigateToScreen2Async()
    {
        return _navigationService.Navigate<Screen2ViewModel>();
    }
}

fragment_main.axml :

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:local="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/btn_goToScreen2"
        android:text="Go to Screen2"
        android:gravity="center_horizontal"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        local:MvxBind="Click GotoScreen2Command"/>

</android.support.constraint.ConstraintLayout> 

以下是我的Android观点。

MainFragment.cs

[MvxFragmentPresentation(typeof(MainContainerViewModel), Resource.Id.content_frame)]
public class MainFragment : BaseFragment<MainViewModel>
{
    protected override int FragmentLayoutId => Resource.Layout.fragment_main;
}

以及 MainContainerActivity.cs :

[Activity(
        Theme = "@style/AppTheme",
        WindowSoftInputMode = SoftInput.AdjustResize | SoftInput.StateHidden)]
    public class MainContainerActivity : BaseActivity<MainContainerViewModel>, IDrawerActivity
    {
        protected override int ActivityLayoutId => Resource.Layout.activity_main_container;                         
    }