How to bind ValueChanged of DecimalUpDown Control on ViewModel? - wpf

Question

<xctk:DecimalUpDown Value="{Binding TotalAmount, Mode=TwoWay}"
HorizontalAlignment="Right"
Grid.Column="1"
VerticalAlignment="Center"
Width="120" Height="21" Margin="5 0 0 0" />

Answer 1

In MVVM, you would put the ValueChanged logic in the setter of the TotalAmount source property as this property gets set then when value is changed, e.g.:
public decimal TotalAmount
{
get { return _totalAmout; }
set { _totalAmount = value; PropertyBoundToDecimalUpDown3 = _totalAmount + _otherAmount; }
}
There is no reason to handle the event itself when you bind to a source property.

Answer 2

You must write your logic inside the callback of each property. The ValueChanged event handler approach requires tightly coupled code behind and it will break the MVVM pattern.
If you prefer the DP way:
public decimal Value1
{
get { return (decimal)GetValue(Value1Property); }
set { SetValue(Value1Property, value); }
}
public static readonly DependencyProperty Value1Property =
DependencyProperty.Register("Value1", typeof(decimal), typeof(MyVM),
new PropertyMetadata(0m, (d, e) =>
{
var vm = d as MyVM;
vm.TotalValue = e.NewValue + vm.Value2;
}));
public decimal Value2
{
get { return (decimal)GetValue(Value2Property); }
set { SetValue(Value2Property, value); }
}
public static readonly DependencyProperty Value2Property =
DependencyProperty.Register("Value2", typeof(decimal), typeof(MyVM),
new PropertyMetadata(0m, (d, e) =>
{
var vm = d as MyVM;
vm.TotalValue = e.NewValue + vm.Value1;
}));
Or if you prefer the INotifyPropertyChanged way:
public decimal Value1
{
get => _value1;
set {
_value1 = value;
TotalValue = value + Value2;
NotifyPropertyChanged();
}
}
public decimal Value2
{
get => _value2;
set
{
_value2 = value;
TotalValue = value + Value1;
NotifyPropertyChanged();
}
}
Note that writing TotalValue = ... will handle its own NotifyPropertyChanged. However writing _totalValue = ... does not.