As a WPF or Silverlight developer chances are you have implemented the INotifyPropertyChanged-interface more than once. The interface is simple enough, consisting of a single PropertyChanged-event. The data binding infrastructure in WPF and Silverlight will look for this interface and subscribe to the PropertyChanged-event. As a developer you have to write code in your property setters to raise the event whenever some property is changed. This is boring and repetitive code to write, that often contains magic strings (property names) and can really clutter your ViewModels. Finding an elegant implementation of the INotifyPropertyChanged-interface has become one of the holy-grails of WPF and Silverlight developers.
An idea I had some time back where to use AOP-techniques and dynamic proxies to get INotifyPropertyChanged implemented automatically. With the recent beta release of Castle Dynamic Proxy 2.2 with official support for Silverlight, combined with Christmas holiday, I figured this would be a good time to take a stab at this problem.
After doing some research I found a couple implementations of INPC using AOP. In August 2008 Gian Mari Ricci published two blog posts about automatic implementation of INPC. The first one he used Reflection.Emit and dynamic code generation, the second was a much simpler implementation using Castle Dynamic Proxy.
In June Ray Houston published an interesting post on how they had implemented auto wiring of INPC as part of the Fluent Silverlight framework. Fluent Silverlight is an open source framework by Ray Houston and Gabriel Schenker that provides a fluent interface for configuring data binding in a strongly typed manner using C#. Their implementation of automatic wiring of INPC is based on Castle Dynamic Proxy, and is really simple and elegant. Instead of trying to reinvent the wheel I decided to re-implement the automatic INPC parts from Fluent Silverlight, and walk you through the code in this blog post. I do not want to take credit for Ray and Gabriel’s work, and must emphasize that this should be considered a walkthrough of their implementation.
The first part of the implementation is a new IAutoNotifyPropertyChanged interface that extends the INotifyPropertyChanged interface with one new method. This interface will be implemented for any ViewModel where we want automatic change notification. The interface serves as both a marker interface for the interceptor, as well as providing a method that the interceptor can call to raise the PropertyChanged-event.
To create a new ViewModel we simply implement the interface, and make any property we want change notification for as virtual. The properties need to be virtual so that the generated proxy class can override the property setter method and call the interceptor. The implementation of the interface can off course be moved down to a base class, but I have included it in this sample to make it clear what is required by the ViewModel for automatic change notification.
With a ViewModel implementing the IAutoNotifyPropertyChanged-interface in place we can now write an interceptor that will add change notification for the properties on the ViewModel. Interceptors are a concept from AOP that enables us to intercept method calls and add new behavior dynamically. This is helpful as we can move cross-cutting concerns out of the business logic. Typical use cases for AOP often include authentication, logging, exception handling and other technical concerns. By having these concerns implemented outside our class we can keep our business logic nice and clean. In our case we are going to apply some of these techniques to keep the ViewModel free from change notification code cluttering the class.
The interceptor is defined as a generic class that intercepts classes implementing the IAutoNotifyPropertyChanged-interface. The interceptor implements IInterceptor-interface which defines a method that will be called for every method invocation, providing us a hook to add addition behavior to the method call. Our interceptor will start by letting the original method call (the property setter) run before checking if we should raise change notification. If the property being set isn’t marked for exclusion we call the OnPropertyChanged method on the IAutoNotifyPropertyChanged-interface, passing in the property name as the parameter.
The code to determine if we should raise change notification starts by checking if the method being invoked is in fact a property setter. This is done by checking the IsSpecialName-property on the MethodInfo object, as well as checking that the name of the method starts with “set_“. The second step is to check if the property is marked with a DoNotNotifyChanges-attribute. This is a marker attribute that can be applied on properties to prevent automatic change notification. The third step is to check if the property being set is of type ICommand. A common pattern when creating ViewModels is to expose operations as Commands hanging of the ViewModel as properties. You do not need change notification for commands, so they are ignored by the interceptor.
Now that we have our change notification interceptor the only thing missing is a way to create the proxy object. To do this we create a new proxy creator. The proxy creator has a generic method that creates proxied instances of any class implementing the IAutoNotifyPropertyChanged-interface.
This diagram illustrates how the View calls the property setter on the proxied ViewModel. The property setter gets intercepted, and the interceptor lets the original setter execute before raising the PropertyChanged-event.
Now that we got the proxy creator it is really straight forward to create an instance of our ViewModel that will have automatic change-notification. I have included the tests documenting the behavior of the automatic change notification interceptor. The tests documents change notification for simple properties, as well as exclusion of commands and properties marked with the DoNotNotifyChanges-attribute.
Using dynamic proxies for automatic change notification is a good way to keep your ViewModels nice and tidy as well as not having to hand-write tedious, repetitive change notification code. In my next blog post I will explore how we can integrate the change-notification interceptor with the Ninject IoC-container. By integrating the interceptor with an IoC-container we can have any ViewModel retrieved from the container support automatic change notification.
The example code for this blog post requires Silverlight 3, and includes the latest beta of Castle Dynamic Proxy 2.2. The code is experimental, and not intended for production use.