Binding to Properties of Nested CustomViews in a ViewController - xamarin.ios

Question

Given I have the following setup (simplified version, removed logic to add to parent view and constraints etc).
public class TestViewModel : MvxViewModel
{
string _text;
public string Text
{
get => _text;
set
{
_text = value;
RaisePropertyChanged(() => Text);
}
}
}
public class TestViewController : MvxViewController<TestViewModel>
{
CustomViewA customViewA;
public TestViewController()
{
}
public override void ViewDidLoad()
{
base.ViewDidLoad();
var bindingSet = this.CreateBindingSet<TestViewController, TestViewModel>();
bindingSet
.Bind(customViewA)
.For(v => v.Text)
.To(vm => vm.Text);
bindingSet.Apply();
}
}
public class CustomViewA : UIView
{
CustomViewB customViewB;
public string Text
{
get => customViewB.Text;
set => customViewB.Text = value;
}
}
public class CustomViewB : UIView
{
UITextField textField;
public string Text
{
get => textField.Text;
set => textField.Text = value;
}
}
Why is it that the bindings do not work? Only if I would make the UITextField in CustomViewB public and directly bind to it in the ViewController rather than the public property that directs to the Text property it seems to work. Like so:
bindingSet
.Bind(customViewA.customViewB.textField)
.For(v => v.Text)
.To(vm => vm.Text);
What am I missing here?

Answer 1

It depends on the requirements you have.
Binding in one direction should work (view model-to-view), I have tested your code and when the ViewModel property changes, the change is propagated to CustomViewA and from there to CusomViewB and finally to the UITextField.
However, the problem is with the opposite direction (view-to-view model). When the user updates the text field, its Text property changes. However, there is nothing notified about this change.
Although the property Text points to the text field, it is not "bound" to it, so when TextField's Text changes, the property itself doesn't know about it and neither does the MvvmCross binding.
In fact, MvvmCross binding in the control-to-view model direction is based on the ability to observe an event that tells the binding to check the new value of the bining source. This is already implemented for UITextField's Text, and it hooks up the EditingChanged event (see source code).
You can still make custom bindings work in the view-to-view model direction by implementing them manually. This is described in the documentation.