Observal is a library that helps you to manage complex, hierarchical object models in your WPF applications.

Getting started

We start by creating an observer, which tracks a set of items:

var observer = new Observer();

To put the observer to use, we use extensions. For example, the PropertyChangedExtension allows us to be notified any time a property changes on a tracked object:

observer.Extend(new PropertyChangedExtension())
   .WhenPropertyChanges(x => Console.WriteLine("Property {0} of object {1} changed", x.PropertyName, x.Source));

We can now add items to the observer for tracking:

observer.Add(customer1);
observer.Add(customer2);
observer.Add(customer3);

The extension automatically attaches to added items - in this case, it will print whenever an item property is changed.

customer2.Age = 23;   // Prints 'Property Age of object Customer 2 changed'

The power of Extensions

Extensions can be combined together to create powerful reactive applications. The out-of-the-box extensions are:

• PropertyChangedExtension, which attaches to the INotifyPropertyChanged.PropertyChanged event
• TraverseExtension, which traverses object hierarchies and adds child property values to the observer
• CollectionExpansionExtension, which adds items in ObservableCollections to the observer
• ItemsChangedExtension, which invokes a callback when items are added or removed

For example, suppose we have a tree of employees:

var orgChart = new Employee("Pointy Haired Boss",
    new Employee("Catbert",
       new Employee("Dilbert"),
       new Employee("Wally"),
       new Employee("Alice")));

We can use the TraverseExtension to expand the tree of employees automatically:

observer.Extend(new TraverseExtension())
    .Follow<Employee>(x => x.DirectReports);

Since DirectReports is a collection, we can use the CollectionExpansionExtension to add all items in the collection to the observer:

observer.Extend(new CollectionExpansionExtension());

We can then use PropertyChangedExtension to be notified whenever any employee's property changes:

observer.Extend(new PropertyChangedExtension())
   .WhenPropertyChanges(x => Console.WriteLine("Employee {0} changed: {1}", x.Source, x.PropertyName));

Now if any employee's property changes, we'll know:

orgChart.DirectReports[0].DirectReports[1].Title = "Programmer";   // Prints

That goes for newly added objects:

var newGuy = new Employee();
orgChart.DirectReports[0].DirectReports[1].Add(newGuy);
newGuy.IsFired = true;     // Prints