Update Binding on Composite Property when Child Property Changes - wpf

Question

Let's say I have a property called Greeting that is principally composed of two bound properties: LastName and FirstName. Can I subscribe to updates on first and last name so I can force a refresh with OnPropertyChanged() for my Greeting property. Here's a simple example:
View
<TextBox Text="{Binding FirstName}" />
<TextBox Text="{Binding LastName}" />
<TextBlock Text="{Binding Greeting}" />
ViewModel
Public Property FirstName() As String
Get
Return _firstName
End Get
Set(ByVal value As String)
_firstName = value
OnPropertyChanged("FirstName")
End Set
End Property
'... Omitting LastName for brevity ...
Public ReadOnly Property Greeting() As String
Get
Return String.Format("Hello {0} {1}", Firstname, LastName)
End Get
End Property
The way this is currently set up, nothing will ever update the Greeting binding. I could put OnPropertyChanged("Greeting") in the setter for FirstName and LastName, but this feels wrong. In a more complex example, I'd rather each object just take care of refreshing itself when something changes.
Q:) Can I force an update for a ReadOnly Property when one of the Properties it's composed of changes?

Answer 1

You can call PropertyChange of Greetings from setter of FirstName and LastName
Public Property FirstName() As String
Get
Return _firstName
End Get
Set(ByVal value As String)
_firstName = value
OnPropertyChanged("FirstName")
OnPropertyChanged("Greeting")
End Set
End Property
OR
You can subscribe to PropertyChanged of your ViewModel in itself
AddHandler this.PropertyChanged, AddressOf PropertyChanged
and in PropertyChanged you can check which property is changed depending on that you can RaisePropertyChanged for Greeting

Answer 2

Borrowing from nit's answer, just to round it out a little. Here's what I did to fire an update on the Greeting property when FirstName or LastName changes:
Private Sub UpdateGreeting(ByVal sender As Object, ByVal e As PropertyChangedEventArgs) _
Handles Me.PropertyChanged
If e.PropertyName = "FirstName" OrElse e.PropertyName="LastName" Then
OnPropertyChanged("Greeting")
End If
End Sub
It handles the PropertyChanged event that is already implemented as part of the INotifyPropertyChanged interface on the ViewModel.
Then it checks if the PropertyName value of the event argument is equal to "FirstName" or "LastName".
If so, it manually raises the OnPropertyChanged() method for the Greeting property.