Xamarin View not Binding from viewModel after Constructor - mvvm

Question

I have a simple View that displays a label with a Question that is being bound from my ViewModel. now if I set the property in my constructor I see the Label displaying whatever I set it to. if I populated from my command function I do not see the label changed. The funny thing is that if I set the Title property (a simple string that has a get and set), then that changes no matter where I set it. but for some reason this particular property does not want to show the changes to it. I have tried simplifying this as much as I can. I tried to define a public string property in my ViewModel and again if I set it in the Constructor than it binds other wise if it is being set in my Command Function then it does not change.
here is my XAML
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
x:Class="Pre.MyPage"
Title="{Binding Title}"
Icon="about.png">
<StackLayout VerticalOptions="Center" HorizontalOptions="Center" >
<Label Text="{Binding MyClassObj.Question, Mode=TwoWay}"/>
</StackLayout>
</ContentPage>
Here is my Code behind
public partial class MyPage : ContentPage
{
MyViewModel vm;
MyViewModel ViewModel => vm ?? (vm = BindingContext as MyViewModel);
public MyPage()
{
InitializeComponent();
BindingContext = new MyViewModel(Navigation);
}
protected override void OnAppearing()
{
base.OnAppearing();
ViewModel.LoadQuestionCommand.Execute("1");
}
}
Here is my ViewModel
public class MyViewModel : ViewModelBase
{
public MyClass MyClassObj {get;set;}
ICommand loadQuestionCommand;
public ICommand LoadQuestionCommand =>
loadQuestionCommand ?? (loadQuestionCommand = new Command<string>(async (f) => await LoadQuestion(f)));
public MyViewModel(INavigation navigation) : base(navigation)
{
Title = "My Title";
}
async Task<bool> LoadQuestion(string id)
{
if (IsBusy)
return false;
try
{
IsBusy = true;
MyClassObj = await StoreManager.QuestionStore.GetQuestionById(id);
//MyClassObject is populated when I break here
}
catch (Exception ex)
{
Debug.WriteLine(ex.Message);
}
finally
{
IsBusy = false;
}
return true;
}

Answer 1

I don't see where you are firing the INofityPropertyChanged event for your MyClassObj property.
Instead of just:
public MyClass MyClassObj {get;set;}
you should have something like:
MyClass myClassObj;
public MyClass MyClassObj
{
get {return myClassObj;}
set
{
//if they are the same you should not fire the event.
//but since it's a custom object you will need to override the Equals
// of course you could remove this validation.
if(myClassObj.Equals(value))
return;
myClassObj = value;
//This method or something has to be in your VieModelBase, similar.
NotifyPropertyChanged(nameof(MyClassObj));
}
}
Where the last method
NotifyPropertyChanged(nameof(MyClassObj));
is who notifies the View about the changes.