Execute ReactiveUI Command from Event in Xamarin Forms - xamarin.forms

Question

I have entry fields in a Xamarin Forms page that I want to trigger a ReactiveUI command when the user is finished entering text into them. I am using ReactiveUI.Events.XamForms and am trying to trigger a command based off of the Unfocused event, but I am not sure how to set up the command to get that to work.
Here is my XAML:
<?xml version="1.0" encoding="utf-8" ?>
<rxui:ReactiveContentPage
x:Class="XamarinReactiveUISwipeView.MainPage"
x:TypeArguments="vm:MainPageViewModel"
xmlns:vm="clr-namespace:XamarinReactiveUITest.ViewModel;assembly=XamarinReactiveUITest"
xmlns:rxui="clr-namespace:ReactiveUI.XamForms;assembly=ReactiveUI.XamForms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
xmlns:ios="clr- namespace:Xamarin.Forms.PlatformConfiguration.iOSSpecific;assembly=Xamarin.Forms.Core"
xmlns="http://xamarin.com/schemas/2014/forms"
ios:Page.UseSafeArea="true">
<StackLayout>
<StackLayout Margin="10,0,0,0" Orientation="Horizontal">
<Label Text="Task ID: " />
<Entry x:Name="EntryTaskID" />
</StackLayout>
<StackLayout Margin="10,0,0,0" Orientation="Horizontal">
<Label Text="Task Name: " />
<Entry x:Name="EntryTaskName" />
</StackLayout>
</StackLayout>
</rxui:ReactiveContentPage>
Here is my code behind:
public partial class MainPage : ReactiveContentPage<MainPageViewModel>
{
public MainPage()
{
InitializeComponent();
ViewModel = new MainPageViewModel();
this.WhenActivated(disposable =>
{
this.Bind(ViewModel, vm => vm.TheTaskItem.TaskID, page => page.EntryTaskID.Text)
.DisposeWith(disposable);
this.Bind(ViewModel, vm => vm.TheTaskItem.TaskName, page => page.EntryTaskName.Text)
.DisposeWith(disposable);
EntryTaskName.Events().Unfocused.InvokeCommand(ViewModel, vm => vm.TheCommand);
});
}
}
here is my model:
public class TaskItem
{
public TaskItem() { }
public string TaskID { get; set; }
public string TaskName { get; set; }
}
and here is my view model:
public class MainPageViewModel : ReactiveObject
{
public MainPageViewModel()
{
TheTaskItem = new TaskItem { TaskID = "1", TaskName = "TheTaskName" };
TheCommand = ReactiveCommand.Create<FocusEventArgs, Unit>(ExecuteTheCommand);
}
public ReactiveCommand<FocusEventArgs, Unit> TheCommand { get; }
private void ExecuteTheCommand(FocusEventArgs args)
{
//do something
}
private TaskItem _theTaskItem;
public TaskItem TheTaskItem
{
get => _theTaskItem;
set => this.RaiseAndSetIfChanged(ref _theTaskItem, value);
}
}
In the view model above, it won't compile, but I can't figure out how to set up the ExecuteTheCommand method. The error is:
'void MainPageViewModel.ExecuteTheCommand(FocusEventArgs)' has the wrong return type
But in looking at examples, it looked like methods with void returns use the Unit type.
What do I need to do here to set up the command properly to get this to work?

Answer 1

In comment above, OP says that this works if change:
TheCommand = ReactiveCommand.Create<FocusEventArgs, Unit>(ExecuteTheCommand);
to:
TheCommand = ReactiveCommand.Create<FocusEventArgs>(ExecuteTheCommand);
Unexpectedly, the type of the variable that holds the result of Create, does not use that same generic type signature. It needs to be:
public ReactiveCommand<FocusEventArgs, Unit> TheCommand { get; }
According to the doc in link by Rodney Littles in comment, Unit is used to represent "void return type". (Generic type "TOutput" would be "void"; but "void" is not a valid Generic type.)