Setting passed parameter before page loads - mvvm

Question

I have moved xamrin.form.maps to MVVM and have set my xaml to
<viewModels:CustomMap IsShowingUser="True" x:Name="customMap" MapPosition="{Binding MyPosition}" CustomPins="{Binding PinCollection}" />
My customMap Class
public static readonly BindableProperty MapPositionProperty = BindableProperty.Create(
nameof(MapPosition),
typeof(Position),
typeof(CustomMap),
new Position(0, 0),
propertyChanged: (b, o, n) =>
{
Console.WriteLine("here");
((CustomMap)b).MoveToRegion(MapSpan.FromCenterAndRadius(
(Position)n, Distance.FromMiles(.05)));
});
public Position MapPosition
{
get { return (Position)GetValue(MapPositionProperty); }
set { SetValue(MapPositionProperty, value); }
}
From page one, I am sending a lat lng to the map. If I set MapPosition in my constructor in my VM it works perfect, the problem is getting the lat and lng to my constructor before everything is bound, other wise it will not update.
In the view model I am collecting the parameters to set the position.
public override async void OnNavigatedTo(INavigationParameters parameters)
{
if (parameters.ContainsKey("mapLocation"))
{
var mapLocation = parameters.GetValue<MapLocation>("mapLocation");
myPosition = new Position(mapLocation.Lat, mapLocation.Lng);
}
}

Answer 1

This has nothing to do with the time when your property is updated.
You bound MapPosition to MyPosition, which I'd guess is a property. For MVVM to work, this MapPosition has to notify that it has changed, usually this looks something like (the the documentation of INotifyPropertyChanged.PropertyChanged)
public MapLocation MyPosition
{
get => myPosition;
set
{
if(myPosition == value)
{
return;
}
myPosition = value;
OnPropertyChanged();
}
}
private void OnPropertyChanged([CallerMemberName] string memberName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
Your view does know that if the bound objects implements INotifyPropertyChanged it has to subscribe to PropertyChanged and can update its state accordingly if this event is raised.
Now why doesn't it work in your case?
You are not setting MyPosition, allowing the setter to raise PropertyChanged and thus the view to update its state, but you are directly setting the backing field myPosition. In this case, the setter never is called and the view has no chance to know that the value of MyPosition has changed, therefor won't update its state.
If your setter implements the pattern shown above, just change
myPosition = new Position(mapLocation.Lat, mapLocation.Lng);
to
MyPosition = new Position(mapLocation.Lat, mapLocation.Lng);
and it should work. Otherwise implement the change notification pattern in MyPosition, too.