public CalculatorViewModel()
{
NumberCommand = new RelayCommand(param => { calculator.Number(Convert.ToInt32(param)); UpdateDisplay(); },
param => calculator.CanDoNumber());
PlusCommand = new RelayCommand(param => { calculator.Plus(); UpdateDisplay(); },
param => calculator.CanDoSign());
MinusCommand = new RelayCommand(param => { calculator.Minus(); UpdateDisplay(); },
param => calculator.CanDoSign());
TimesCommand = new RelayCommand(param => { calculator.Times(); UpdateDisplay(); },
param => calculator.CanDoSign());
OverCommand = new RelayCommand(param => { calculator.Over(); UpdateDisplay(); },
param => calculator.CanDoSign());
EqualsCommand = new RelayCommand(param => { calculator.Equals(); UpdateDisplay(); },
param => calculator.CanDoEquals());
ClearCommand = new RelayCommand(param => { calculator.Clear(); UpdateDisplay(); },
param => calculator.CanDoClear());
}