Binding to an expression

Question

I have an onscreen numeric keypad to type a PIN. What I want to do is disable the buttons when four digits of PIN are entered. I can certainly do this with code pretty easily, but it seems to me to be the sort of thing that should be done with binding.

Something like:

<Button Style="Whatever" IsEnabled="{Binding ElementName=PinBox ???}"/>

It seems there isn't a way to do that (which to be honest seems rather primitive to me.) So I considered the alternative, which is a plain property on the underlying Window class. But I'm not sure how to bind to it. Do I need to specify the class itself as its own data context, or do I need to extract the PIN string into a View Model?

And subsequently, how do I get the plain property to update the GUI?

I suppose I could defined a view model class and have a dependency property called "ButtonsEnabled" but it seems kind of heavyweight for such a simple problem.

Let me know if I am missing something.

Answer 1

You can write a converter which return boolean depending on digits in TextBox

The XAML fo r button would be

<Button Content="Test" IsEnabled="{Binding ElementName=PinBox,Path=Text,Converter={StaticResource DigitsToBoolConverter}}" Grid.Row="1" Height="20" Width="100"></Button>

where PinBox is the textbox name used to enter pin.

The Converter function is

 public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
 {
     return value.ToString().Length >= 4;
 }

Answer 2

Another way using commands:

XAML:

<Button Content="2" Style="Whatever" Command={Binding MyCommand} CommandParamater="2"/>

ViewModel:

public ICommand MyCommand { get; private set; }
public string PinNumber { get; private set; }
public void Init()
{
  MyCommand = new RelayCommand(
    param => AddPinNumberDigit(param),
    param => CanAddPin);
}
private void AddPinNumberDigit(string digit)
{
  PinNumber += digit;
}
public bool CanAddPin { 
  get
  {
    return PinNumber.Length < 3;
  }
}