#121 – Being Notified When the Value of a Dependency Property Changes

A class that implements a dependency property can optionally ask to be notified when the value of the property changes.  The class specifies a PropertyChangedCallback when registering the property.

            PropertyMetadata ageMetadata =
                new PropertyMetadata(
                    18,     // Default value
                    new PropertyChangedCallback(OnAgeChanged),   // ** call when property changes
                    new CoerceValueCallback(OnAgeCoerceValue));

            // Register the property
            AgeProperty =
                DependencyProperty.Register(
                    "Age",                 // Property's name
                    typeof(int),           // Property's type
                    typeof(Person),        // Defining class' type
                    ageMetadata,           // Defines default value & callbacks  (optional)
                    new ValidateValueCallback(OnAgeValidateValue));   // validation (optional)

The class can use this callback to perform some logic whenever the property value changes, e.g. automatically setting the value of another property.

        private static void OnAgeChanged
            (DependencyObject depObj, DependencyPropertyChangedEventArgs e)
        {
            Person p = (Person)depObj;
            p.AARPCandidate = (int)e.NewValue > 60 ? true : false;
        }