Should ItemSource and BindingContext both be set when using MVVM (Xamarin.Froms ListView)? - mvvm

Question

Model:
public class Question : INotifyPropertyChanged
{
private float? _answer;
public float? Answer
{
get => _answer;
set
{
_answer = value;
NotifyPropertyChanged();
}
}
protected void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged;
}
View model:
public class QuestionViewModel
{
private ObservableCollection<Question> _questions;
public ObservableCollection<Question> Questions
{
get => _questions;
set
{
if (_questions != value)
{
_questions = value;
}
}
}
}
XAML:
<ListView x:Name="ListViewQuestions" SelectionMode="Single" HasUnevenRows="True" HeightRequest="250" VerticalOptions="FillAndExpand">
<ListView.ItemTemplate>
<DataTemplate>
<ViewCell>
<Entry x:Name="EntryAnswer" Text="{Binding Answer,Mode=TwoWay}" Keyboard="Numeric" FontSize="Medium" VerticalOptions="End"
HorizontalOptions="FillAndExpand" Grid.Row="0" Grid.Column="1" >
<Entry.Behaviors>
<behaviors:EntryMaxValueBehavior MaxValue="{Binding MaxVal}" BindingContext="{Binding BindingContext, Source={x:Reference EntryAnswer}}" />
<behaviors:EntryMinValueBehavior MinValue="{Binding MinVal}" BindingContext="{Binding BindingContext, Source={x:Reference EntryAnswer}}" />
</Entry.Behaviors>
</Entry>
</ViewCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
In my page OnAppearing method, I set the ListViewQuestions like this:
var questions = await DataStore.GetQuestions(_inspection.Id);
var questionsViewModel = new QuestionViewModel { Questions = new ObservableCollection<Question>(questions) };
ListViewQuestions.ItemsSource = null;
ListViewQuestions.ItemsSource = questionsViewModel.Questions;
However, when values are entered into EntryAnswer, the setter in the Question model is not called, as I would expect. I thought that maybe this was because the BindingContext for the ListView needed to be set, so I set it like this:
ListViewQuestions.BindingContext = questionsViewModel;
However, the setter in the Question model is still not called. I also tried implementing INotifyPropertyChanged in the QuestionViewModel, but still no joy. I checked that the ObservableCollection in the View Model is set correctly, with actual data, and it is. Can anyone spot what might be going wrong here?
Edit 1: I also tried not setting the ItemSource, but only setting the ListViewQuestions.BindingContext to the view model, but then the ListView was not being populated with any data.

Answer 1

Here is how this works together.
The BindingContext is the object that will be the scope for whatever bindings that are in the page or it's children, unless you specify a different context for a certain child object, but let's not overcomplicate things for now.
This means, that when you have set the BindingContext, all Bindings will now start looking into the object referenced in the BindingContext. In your case, you set the BindingContext to an instance of QuestionViewModel.
You want your ListView, to get its items from the QuestionViewModel.Questions property. So, you set a binding like this:
<ListView x:Name="ListViewQuestions" ItemsSource="{Binding Questions}" ...>.
Questions needs to be a public property in the BindingContext, in our case QuestionViewModel. You got this right already.
Now, whenever you assign something to Questions this should also propagate to your ListView because of the binding.
Inside your ListView you are using a ViewCell, now note, that the scope does change here. Each cell represents an instance of an object inside the ItemsSource. In our case, each cell will hold a Question. You are using this:
<Entry x:Name="EntryAnswer" Text="{Binding Answer,Mode=TwoWay}" ...>
This means Answer needs to be a public property inside Question. You got this right already.
When you implement it like this, basically the only thing you do is fill your view model and assign that to the BindingContext of your page. If you are using an MVVM framework, this might happen automatically.
At some point, you might run into some trouble that the UI doesn't update and you will have to implement the INotifyPropertyChanged interface. Have a close look at what object doesn't update on screen and implement the interface on that object along with the needed plumbing, but from what I can see in this code, this isn't needed right now. And besides, you have implemented it the right way in your Question right now.
I hope this makes sense! It's a bit hard to wrap your head around the first time, but once you get the swing of it, it is pretty easy!

Answer 2

In your Answer Setter try:
set
{
float? temp = null;
if(float.TryParse(value, out temp)
{
_answer = temp;
NotifyPropertyChanged("Answer");
}
}
It seems like for this to work though your setter would have to be called, and you indicate that it is not, so I think it must be the min, max binding where this is kicking out the error. For now perhaps get rid of that and see if the setter will get called.
In WPF using a converter is typical and I think will work with the Xamarin as well. See this for a good example of how to implement IValueConverter.