Update XAML Binding after a property changes - xaml

Question

I am using MVVM light on a Windows 8.1 Store app. I have a view model that has several properties on it that have no setters (their value is derived from a different property). Here is part of my ViewModel:
public float TotalAmount
{
get
{
if (!LineItems.Any())
return 0.0f;
return SubTotal + TotalFees - TotalDiscounts;
}
}
public float SubTotal
{
get
{
return !LineItems.Any() ? 0.0f : LineItems.Select(i => i.Price.Amount).Sum();
}
}
public float TotalFees
{
get
{
return !Fees.Any() ? 0.0f : Fees.Select(f => f.Amount).Sum();
}
}
public float TotalDiscounts
{
get
{
return !Discounts.Any() ? 0.0f : Discounts.Select(d => d.Amount).Sum();
}
}
public ObservableCollection<Discount> Discounts { get; set; }
public ObservableCollection<Fee> Fees { get; set; }
public Customer Customer { get; set; }
public ObservableCollection<OrderLineItem> LineItems { get; set; }
In the XAML, I bind to each of these properties, and want to update them when an Item is added to LineItems.
Here is the relevant part of the XAML:
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="3*" />
<ColumnDefinition Width="1*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<TextBlock Text="SubTotal:" Style="{StaticResource OrderDetailsLabel}" />
<TextBlock Grid.Column="1" Style="{StaticResource OrderDetailsValue}"
Text="{Binding SubTotal, Converter={StaticResource MoneyConverter}, Mode=TwoWay}" />
<TextBlock Grid.Row="1" Text="Fees:" Style="{StaticResource OrderDetailsLabel}" />
<TextBlock Grid.Row="1" Style="{StaticResource OrderDetailsValue}" Grid.Column="1"
Text="{Binding TotalFees, Converter={StaticResource MoneyConverter}, Mode=TwoWay}" />
<TextBlock Grid.Row="2" Text="Discounts:" Style="{StaticResource OrderDetailsLabel}" />
<TextBlock Grid.Row="2" Style="{StaticResource OrderDetailsValue}" Grid.Column="1"
Text="{Binding TotalDiscounts, Converter={StaticResource MoneyConverter}, Mode=TwoWay}" />
<TextBlock Grid.Row="3" Text="Order Total:" Style="{StaticResource OrderDetailsLabel}" />
<TextBlock Grid.Row="3" Style="{StaticResource OrderDetailsValue}" Grid.Column="1"
Text="{Binding TotalAmount, Converter={StaticResource MoneyConverter}, Mode=TwoWay}" />
</Grid>
My Binding to the LineItems is working as expected, but the fields that are populated by doing math on the line items are not.
Line Item Binding functioning as expected:
<ListView ItemsSource="{Binding LineItems, Mode=TwoWay}"
SelectionMode="Single"
HorizontalAlignment="Stretch">
<ListView.ItemContainerStyle>
....
How can I get SubTotal, Total, etc. to update when LineItems changes?

Answer 1

Pretty simple:
Register the ObservableCollection<T>.CollectionChanged event for each of those collection. Since your desired properties are based on some calculation of the collection.
On event fire Raise the OnPropertyChanged on those properties.
Example how to raise the OnPropertyChanged/inotifypropertychanged
MSDN: inotifypropertychanged Example

Answer 2

Something like this in your constructor:
LineItems.CollectionChanged += (s, e) =>
{
RaisePropertyChanged("SubTotal");
RaisePropertyChanged("TotalFees");
RaisePropertyChanged("TotalDiscounts");
}
assuming your ViewModel implements INotifyPropertyChanged, and has a 'RaisePropertyChanged' method that raises the PropertyChanged event (e.g. You're using MVVM-Light's ViewModelBase as your baseclass).

Answer 3

I ended up solving this by raising the OnPropertyChanged event on in the handler for my AddLineItems message:
public NewOrderViewModel()
{
//Subscribe to messages
MessengerInstance.Register<OrderItemAdded>(this, o => AddOrderItem(o.LineItem));
MessengerInstance.Register<SelectedCustomer>(this, c => Customer = c.Customer);
Discounts = new ObservableCollection<Discount>();
Fees = new ObservableCollection<Fee>();
LineItems = new ObservableCollection<OrderLineItem>();
}
private void AddOrderItem(OrderLineItem lineItem)
{
//add Line Item to collection
LineItems.Add(lineItem);
//Raise events for properties derived from collection
RaisePropertyChanged("TotalAmount");
RaisePropertyChanged("SubTotal");
}