Xamarin Forms MVVM Databinding failing when I'm binding to a single object - mvvm

Question

I'm having an issue with data not binding correctly on a details page when I have clicked through from a ListView via a button. The ListView binds perfectly and the object gets passed through to the details page. The Id of the object is read and a full version of the object is called from an API and set to a new instance of the object. When I add a breakpoint, the full object is available, but Labels on the view aren't populated. Here is the ViewModel:
DetailsViewModel.cs
public class DetailsViewModel
{
public Deal Deal { get; set; }
public int DealId { get; set; }
public DetailsViewModel(int id)
{
Deal = new Deal();
DealId = id;
}
public async void GetDeal()
{
var deal = await Deal.GetDeal(DealId);
if(deal != null)
{
Deal = deal;
}
}
}
The codebehind looks like this:
DetailPage.Xaml.cs
DetailsViewModel viewModel;
int dealId;
public DetailPage(int id)
{
InitializeComponent();
dealId = id;
viewModel = new DetailsViewModel(dealId);
BindingContext = viewModel;
}
protected override void OnAppearing()
{
base.OnAppearing();
viewModel.GetDeal();
}
And the Xaml file is
DetailPage.Xaml
<ContentPage.Content>
<ScrollView>
<StackLayout x:Name="detailsLayout">
<Label Text="{Binding Deal.Name}" />
</StackLayout>
</ScrollView>
</ContentPage.Content>
When I put a breakpoint in Deal = deal on DetailsViewModel, the Deal object exists and has the correct data, but I just get a blank screen. I have tried Labels with Text="{Binding Name}" and Text="{Binding Deal.Name}".
I have also tried manually creating a deal in the GetDeal function of the ViewModel and still nothing is bound.

Answer 1

1) Ensure your property Notifies the UI of a change implementing the INotifyPropertyChanged interface. See https://learn.microsoft.com/en-us/xamarin/xamarin-forms/xaml/xaml-basics/data-bindings-to-mvvm
2) Ensure the set is done on the UI thread using Device.BeginInvokeOnMainThread. https://learn.microsoft.com/fr-fr/dotnet/api/xamarin.forms.device.begininvokeonmainthread?view=xamarin-forms
using System.ComponentModel;
using System.Runtime.CompilerServices;
using Xamarin.Forms;
namespace YourNamespace
{
public class DetailsViewModel : INotifyPropertyChanged
{
private Deal _deal;
public Deal Deal
{
get => _deal;
set
{
if (_deal != value)
{
_deal = value;
OnPropertyChanged();
}
}
}
public int DealId { get; set; }
public DetailsViewModel(int id)
{
//!! useless assignation
//Deal = new Deal();
DealId = id;
}
public async void GetDeal()
{
var deal = await Deal.GetDeal(DealId);
if (deal != null)
{
//Ensure we are on UI thread
Device.BeginInvokeOnMainThread(() => Deal = deal);
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
}