How to reference and bind a property at the same time? - xamarin.forms

Question

Here is what I mean by the title. I have an Entry with a property called "IsValid" (bool) which is bound to a behaviour (it is called Validator) which checks if the input is in range (0 - 10 in this case) and colour the background of the entry red or transparent. That works fine. However, as I have the same logic in my ViewModel (to check if the input is in range) and show a dialog message if it is not, I wanted to directly bind to the validator's IsValid and use the bind fild in my viewModel (IsBinReferenceValid) abd thus remove the locig from the vm. Currently, the property in my vm IsBinReferenceValid is not changed in any way which indicates that the binding does not work.
Here is the xaml code:
<userControl:DetailedEntry
PlaceholderLabel="{x:Static locale:BinPrintLang.BinRef}"
Text="{Binding BinTextEntry}"
TextColor="{StaticResource PrimaryColor}"
BgColor="White"
BorderColor="{StaticResource DisableColor}"
VerticalOptions="CenterAndExpand"
IsLabelVisible="True"
Label="Bin Reference"
IsImportant="True"
IsValid="{Binding Source={x:Reference InputLengthValidator}, Path=IsValid}">
<userControl:DetailedEntry.EntryBehavior>
<ui:InputLengthValidator x:Name="InputLengthValidator"
MinValue="0"
MaxValue="10"
IsValid="{Binding Source=IsBinReferenceValid, Mode=OneWayToSource}"/>
</userControl:DetailedEntry.EntryBehavior>
</userControl:DetailedEntry>
Any ideas how I can reference and bind to a property at the same time, is that even possible (That is, if that is where the problem is coming from)?
Base validator code:
public class ValueInRangeValidator : Validator<Entry>
{
private static BindableProperty MinValueProperty =
BindableProperty.Create("MinValue", typeof(decimal?), typeof(ValueInRangeValidator));
public decimal? MinValue
{
get { return (decimal?) GetValue(MinValueProperty); }
set
{
SetValue(MinValueProperty, value);
OnPropertyChanged();
}
}
public static BindableProperty MaxValueProperty =
BindableProperty.Create("MaxValue", typeof(decimal?), typeof(ValueInRangeValidator));
public decimal? MaxValue
{
get { return (decimal?) GetValue(MaxValueProperty); }
set
{
SetValue(MaxValueProperty, value);
OnPropertyChanged();
}
}
public virtual void Bindable_TextChanged(object sender, TextChangedEventArgs e)
{
decimal i = 0;
IsValid = decimal.TryParse(e.NewTextValue, out i);
IsValid = IsValid && (MinValue == null ? i >= decimal.MinValue : i >= MinValue);
IsValid = IsValid && (MaxValue == null ? i <= decimal.MaxValue : i <= MaxValue);
}
protected override void OnAttachedTo(Entry bindable)
{
bindable.TextChanged += Bindable_TextChanged;
}
protected override void OnDetachingFrom(Entry bindable)
{
bindable.TextChanged -= Bindable_TextChanged;
}
}
InputLengthValidator code:
public class InputLengthValidator : ValueInRangeValidator
{
public override void Bindable_TextChanged(object sender, TextChangedEventArgs e)
{
var max = (int) MaxValue;
var min = (int) MinValue;
var textLenght = e.NewTextValue.Length;
IsValid = textLenght >= min && textLenght < max;
}
}

Answer 1

I managed to get the validation working by subscribing to another (custom) bindable property of my DetailedEntry control called IsWarning.
<userControl:DetailedEntry
rid.Row="1"
Grid.Column="0"
Label="{x:Static locale:GoodsReceiptLang.NumLabels}"
Text="{Binding NumberOfLabels, Mode=TwoWay}"
TextColor="{StaticResource PrimaryColor}"
Keyboard="Numeric"
IsImportant="True"
IsWarning="{Binding ShowWarning}">
</userControl:DetailedEntry>
My VM:
private bool CanPrint()
{
var errors = new List<string>();
ShowWarning = false;
if (SelectedPrinter == null)
errors.Add(CommonLang.SelectPrinterErrorMsg);
if (string.IsNullOrEmpty(NumberOfLabels) || !int.TryParse(NumberOfLabels, out int numLabels))
{
ShowWarning = true;
errors.Add(CommonLang.NotValidInput);
}
if (errors.Any())
{
ShowErrorMessage(string.Join(" ", errors.ToArray()));
return false;
}
return true;
}