Custom iOS UIView Class MvvmCross Constraints - Draw(CGRect rect) Method Never Called - ios

Question

I have a situation that I cannot seem to work out Where the specific problem lies and I'm hoping for a little input.
I have a very simple view that at the moment just has a UIScrollView. In the ViewDidLoad method I register to listen for the view models PropertyChanged event like this:
public override void ViewDidLoad()
{
base.ViewDidLoad();
ViewModel.PropertyChanged += ViewModel_PropertyChanged;
}
private void ViewModel_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
{
if (e.PropertyName == "MyCollection")
{
BuildUi();
}
}
When that event is fired it kicks off building the view since the data is now available within the view model. In the BuildUi method I have this code:
View Code
private void BuildUi()
{
_mainScrollView = new UIScrollView(View.Frame)
{
ShowsHorizontalScrollIndicator = false,
AutoresizingMask = UIViewAutoresizing.All
};
Add(_mainScrollView);
View.SubviewsDoNotTranslateAutoresizingMaskIntoConstraints();
View.AddConstraints(
_mainScrollView.AtLeftOf(View),
_mainScrollView.AtTopOf(View),
_mainScrollView.WithSameWidth(View),
_mainScrollView.WithSameHeight(View));
foreach(var ctx in ViewModel.MyCollection)
{
var contextScoreControl = new ContextScoreView(new CGRect(100,100,100,100), ctx);
_mainScrollView.Add(contextScoreControl);
//var btn = ViewHelpers.CreateDefaultButton("test");
//_mainScrollView.Add(btn);
}
_mainScrollView.SubviewsDoNotTranslateAutoresizingMaskIntoConstraints();
var constraints = _mainScrollView.VerticalStackPanelConstraints(new Margins(20, 10, 20, 10, 5, 5), _mainScrollView.Subviews);
_mainScrollView.AddConstraints(constraints);
}
Control Code
[Register("MyControl")]
public class MyControl : UIView
{
private MyViewModel _vm;
private UILabel _ctxTitle;
private UITextView _explanationText;
private UISlider _scoreSlider;
public MyControl(MyViewModel vm)
{
_vm = vm;
BackgroundColor = UIColor.Cyan;
}
public MyControl(IntPtr handle) : base(handle) { }
public MyControl(CGRect rect, MyViewModel vm)
{
_vm = vm;
BackgroundColor = UIColor.Cyan;
}
public MyControl(CGRect rect) : base(rect)
{
BackgroundColor = UIColor.Cyan;
}
public override void Draw(CGRect rect)
{
base.Draw(rect);
_ctxTitle = ViewHelpers.CreateTitleLabel();
_ctxTitle.Text = _vm.Text;
_explanationText = ViewHelpers.CreateAutoExpandingReadOnlyTextView();
_explanationText.Text = _vm.ScoringHint;
_scoreSlider = ViewHelpers.CreateDefaultSlider();
_scoreSlider.Value = _vm.Score;
AddSubview(_ctxTitle);
AddSubview(_explanationText);
AddSubview(_scoreSlider);
this.AddConstraints(
_ctxTitle.AtTopOf(this, IosConstants.UIPadding),
_ctxTitle.AtLeftOf(this, IosConstants.UIPadding),
_ctxTitle.WithSameWidth(this).Minus(IosConstants.UIPadding * 2),
_explanationText.Below(_ctxTitle, IosConstants.UIPadding),
_explanationText.WithSameLeft(_ctxTitle),
_explanationText.WithSameRight(_ctxTitle),
_scoreSlider.Below(_explanationText, IosConstants.UIPadding),
_scoreSlider.WithSameLeft(_explanationText),
_scoreSlider.WithSameRight(_explanationText),
_scoreSlider.AtBottomOf(this)
);
}
}
The issue I'm seeing right now is that the Draw method is never called which means nothing appears in the view.
I've tried lots and lots of permutations of the constructors you can see in the control code. Using CGRect.Empty and as is in the code a manually generated CGRect but no matter what I try I cannot seem to get this flow of code to work.
As you can see from the code if I swap out my custom control for a simple UIButton everything works as expected so the main view code where the UIScrollView is added with constraints is working and laying itself out as expected and desired. The problem seem to lie either in the control code itself or the hierarchy of the constraints themselves. I just cannot figure out what needs to happen to make this work.
Under certain circumstances I was seeing the following error:
Unable to simultaneously satisfy constraints.
I've used this "pattern" of code many times in other views within the app and it works in those situations which is why I'm slightly confused about what is going on in this situation. The difference is the looping over a dynamic number of controls in this situation.
Without changing the current structure of the view (to say using a UITableView instead) how can I get this playing nicely and render the custom UIView controls?