Binding CalendarView DateChange event with MvvmCross - xamarin

Question

I have a CalendarView that looks like this:
<CalendarView
android:layout_width="match_parent"
android:layout_height="300dp"
android:id="#+id/createReservationCalendarView" />
Here is how I handle the DateChange event without MvvmCross:
protected override void OnCreate(Bundle savedInstanceState)
{
... Code ...
calendar.DateChange += (s, args) =>
{
var year = args.Year;
var month = args.Month + 1;
var dayOfMont = args.DayOfMonth;
var date = new DateTime(year, month, dayOfMont);
var myReservations = new Intent(this, typeof(CreateReservationTimeslotScreen));
myReservations.PutExtra("selectedDate", date.Ticks);
StartActivity(myReservations);
};
}
Now that I have switched to MvvmCross, I would like to have my ViewModel start the new activity instead.
Im not sure how to do this, since the ViewModel should be OS and UI agnostic.
The "args" argument is of type CalendarView.DateChangeEventArgs, which is Android specific, so I cant use that in the ViewModel. It derives from System.EventArgs, so maybe I could use that instead. I am thinking that there must be a simpler way.
A thought that I had was if it is possible to update a property on the ViewModel from the activity, and then execute the switch to the new Activity from there? I'm not sure how this could be accomplished since activites dont have references to their ViewModels.
Any suggestions?
Thanks.

Answer 1

MvvmCross does give you access to your ViewModel from your View. The relationship between your View (e.g. Activity/fragment in Android) and your ViewModel, and their ability to share data (models) in both directions is a core characteristic a Mvvm framework.
In order to setup an Activity to be used with MvvmCross you need to make sure to inherit from MvxActivity or MvxAppCompatActivity (If using Android Support Library). Following which you need to link your Activity to its corresponding ViewModel using one of the possible conventions (See link, for basic sample of each registration offered by the MvxViewModelViewTypeFinder). A simple example would be to use the concrete type based registration using the type parameter overload.
public class FirstActivity : MvxAppCompatActivity<FirstViewModel>
Now that you have access to your ViewModel from your View you can create a command that can be used to execute the navigation:
CalendarViewModel (ViewModel linked to the current Activity in question)
Create a command that requires a DateTime parameter, which in turn will pass the value when navigation (see MvvmCross Navigation docs for alternative navigation and parameter passing conventions).
public class CalendarViewModel : MvxViewModel
{
IMvxCommand _goToMyReservationCommand;
public IMvxCommand GoToMyReservationCommand =>
_goToMyReservationCommand ??
(_goToMyReservationCommand = new MvxCommand<DateTime>(NavigateToMyReservation));
void NavigateToMyReservation(DateTime reservationDate)
{
ShowViewModel<MyReservationViewModel>(
new GoToMyReservationParameter
{
ReservationTicks = reservationDate.Ticks
});
}
}
Navigation Parameter Class
Holds the values and type information used for navigation.
public class GoToMyReservationParameter
{
public long ReservationTicks { get; set; }
}
MyReservationViewModel
The ViewModel that will receive the value passed.
public class MyReservationViewModel : MvxViewModel
{
public void Init(GoToMyReservationParameter parameters)
{
var reservationTicks = parameters.ReservationTicks;
// Do what you need with the parameters
}
}
View
Execute the command on the ViewModel and pass through the DateTime object.
public class CalendarActivity : MvxAppCompatActivity<CalendarViewModel>
{
protected override void OnCreate(Bundle bundle)
{
... Code...
calendar.DateChange += (s, args) =>
{
var year = args.Year;
var month = args.Month + 1;
var dayOfMont = args.DayOfMonth;
var date = new DateTime(year, month, dayOfMont);
ViewModel.GoToMyReservationCommand.Execute(date);
};
}
}