Xamarin - How to pass variable to viewmodel - class

Question

I have 2 Views. One has an event, that passes two variables to the second page and loads the page up:
private void CollectionView_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
var item = (GalleryListEntry)e.CurrentSelection.FirstOrDefault();
Navigation.PushModalAsync(new Gallery(item.PhotographerCode, item.GalleryCode));
}
in the second page I have this:
public Gallery(string photographerCode, string galleryCode)
{
InitializeComponent();
}
The second page, has a Collection view that has its own Bindingsource.
For this bindingsource i have a Model, a Service, and a ViewModel. The service is called by the Viewmodel, and returns a List of images to be shown in the second page's collection view.
Inside this service class, I would need to access to the two variables passed above (photograperCodeand galleryCode) but I cannot figure out how to pass the variables to the ViewModel so i can then forward it to the class.
ViewModel:
using GalShare.Model;
using GalShare.Service;
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Text;
namespace GalShare.ViewModel
{
class GalleryViewModel
{
public ObservableCollection<Gallery> Galleries { get; set; }
public GalleryViewModel()
{
Galleries = new GalleryService().GetImageList();
}
}
}
I tried like this
((GalleryViewModel)this.BindingContext).pCode = photographerCode;
((GalleryViewModel)this.BindingContext).gCode = galleryCode;
but I get this error: System.NullReferenceException: 'Object reference not set to an instance of an object.'
BindingContext is Null, but in the Xaml file i have this:
<ContentPage.BindingContext>
<vm:GalleryViewModel/>
</ContentPage.BindingContext>

Answer 1

This should work fine. First in your Gallery
public Gallery(string photographerCode, string galleryCode)
{
InitializeComponent();
BindingContext = new GalleryViewModel(photographerCode, galleryCode);
}
And now in the ViewModel
class GalleryViewModel
{
public ObservableCollection<Gallery> Galleries { get; set; }
public GalleryViewModel(string pCode, string gCode)
{
this.pCode = pCode;
this.gCode = gCode;
Galleries = new GalleryService().GetImageList();
}
}