Xamarin.Android and Spinner binding with ReactiveU

问题:

I'm trying to bind to a Spinner using ReactiveUI in a Xamarin.Android application. To add items to the Spinner, I need to use ArrayAdapter. But ArrayAdapter needs Android.Content.Context. Should I pass it into the ViewModel?

Anyone know about an application written in Xamarin.Android, which uses ReactiveUI, where I could look inspiration? The ReactiveUI documentation only has a reference to a sample application written for iOS.

View

    <?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
    <TextView
        android:id="@+id/Label"
        android:text="Zařízení:"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

    <Spinner
        android:id="@+id/Devices"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_toRightOf="@id/Label"/>

    <Button
        android:id="@+id/EditCommand"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_toRightOf="@+id/Devices"
        android:text="Edit"/>
</RelativeLayout>

Activity

namespace Test.Droid
{
    [Activity(Label = "Test.Droid", MainLauncher = true)]
    public class MainActivity : ReactiveActivity, IViewFor<MainViewModel>
    {
        public Spinner Devices { get; private set; }

        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);

            SetContentView(Resource.Layout.Main);
            ViewModel = new MainViewModel();
            this.WireUpControls();

            // Bindings

            this.Bind(this.ViewModel, ) // ?
        }


        private MainViewModel _viewModel;
        public MainViewModel ViewModel
        { get => _viewModel; set { this.RaiseAndSetIfChanged(ref _viewModel, value); } }

        object IViewFor.ViewModel
        { get => ViewModel; set { ViewModel = (MainViewModel)value; } }
    }
}

ViewModel

namespace Test.Droid.ViewModels
{
    public class MainViewModel : ReactiveObject
    {
        // How to databind Spinner Devices ?


        public MainViewModel()
        {           
        }
    }
}

回答1:

I haven't done any Xamarin.Android development, but in general you don't want to pass details about the view into the ViewModel - it should not know anything about the view.

I would expose the list of items as a collection (e.g. IList<Item>) and use a converter on the binding to create an ArrayAdapter:

this.OneWayBind(this.ViewModel.Devices, this.View.Property, devices => new ArrayAdapter(devices));

this.View.Property should refer to the property that changes whenever the list of devices changes. The third parameter (devices => new ArrayAdapter()) receives the property from the ViewModel as an argument, you then return a value that can be set on the this.View.Property.

For example:

• ViewModel.Count is a string
• View.Property is an int

Bind like this:

this.OneWayBind(this.ViewModel.Count, this.View.Property, count => int.Parse(count));

The third parameter can be a function or lambda that accepts an argument of the type of the ViewModel property and returns a value of the type of the view property.

回答2:

I'd said I'd given in providing any feedback on Stackoverflow but anyway... we have a collection of binding extensions, one of which is for a Spinner. in typical usage it looks like this

        // bind the options for the distance 
        this.BindSpinner(ViewModel,
            vm => vm.TravelLimitSelected,
            vm => vm.CurrentTravelLimit,
            vm => vm.TravelLimitChoices.Distances,
            f => f.travelLimitSpinner,
            (items) =>
                    new ActionSpinnerAdapter<DistanceChoiceModel>((c, p) => new DistanceLimitViewHost(c, p), items));

where in this case, the extension method looks like

public static IDisposable BindSpinner<TView, TViewModel, TCommandProp, TSpinnerViewModel>(
             this TView view,
             TViewModel viewModel,
             Expression<Func<TViewModel, TCommandProp>> viewModelCommandName,
             Expression<Func<TViewModel, int>> viewModelSelectedPropertyName,
             Expression<Func<TViewModel, IList<TSpinnerViewModel>>> viewModelSourceItemsName,
             Expression<Func<TView, Spinner>> spinnerControl, Func<IList<TSpinnerViewModel>, ISpinnerAdapter> adapterFactory) where TViewModel : RxViewModel
             where TCommandProp : ICommand
             where TView : class, IViewFor<TViewModel>
             where TSpinnerViewModel : class
        {