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Mini Rx: Observable Extensions

The Reactive Extensions (Rx) provide LINQ style querying capabilities for events in C#, VB.Net and JavaScript. Rx implements extension methods over IObservable<T>, just as LINQ to Objects provides a set of extension methods over IEnumerable<T>,


There has been a fair amount of Microsoft publicity on the Rx library in blogs, videos and talks. Demand for Rx skills in IT jobs has grown over the last 12 months. 

Reactive Extensions Demand Trend

That said, the current implementation of the Rx library has at least a couple of issues:

Given that LINQ is based partly on higher-order functions from functional programming perhaps it’s not surprising F# supported querying over events back in 2006. It’s also relatively trivial to expose this functionality to C# by defining compatible extension methods using the ExtensionAttribute e.g.

type ObservableExtensions private () =
   static member Select<'T,'U>(source:IObservable<'T>,selector:Func<'T,'U>) =
       source |> selector.Invoke
   static member Where<'T>(source:IObservable<'T>,predicate:Func<'T,bool>) =
       source |> Observable.filter predicate.Invoke
   static member Subscribe<'T>(source:IObservable<'T>, action:Action<'T>) =
       source |> Observable.subscribe action.Invoke

This is already enough to provide basic LINQ syntax in C# for types implementing IObservable<T>:

var leftPressedMove =
    from e in mouseMove
    where e.LeftButton == MouseButtonState.Pressed
    select e;

F# custom events implement IObservable<’T> by default and F# provides modules with higher-order functions for both .Net Events and the IObservable<’T> interface.

For C# a mechanism is needed to convert .Net events to an object that implements IObservable<T>. This can be achieved fairly concisely in F# using object expressions:

let FromEvent<'TEventArgs, 'TDelegate when 'TEventArgs:> EventArgs>
            removeHandler:Action<'TDelegate>)  =
    { new IObservable<'TEventArgs> with
        member this.Subscribe(observer:IObserver<_>) =
            let handler = Action<_>(observer.OnNext) |> conversion.Invoke
            addHandler.Invoke handler
            let remove () = removeHandler.Invoke handler
            { new IDisposable with member this.Dispose() = remove () }

Although converting from the event in C# feels a little convoluted:

var mouseMove =
    Observable.FromEvent<MouseEventArgs, MouseEventHandler>(
        f => new MouseEventHandler((sender, args) => f(args)),
        handler => MouseMove += handler,
        handler => MouseMove -= handler);

Again for C# a mechanism is required for directly creating objects that can be both a source of events and be used to observe events. Rx follows the Observer pattern and provides a type called Subject that implements both IObserver<T> and IObservable<T>.

Earlier in the year I put up 2 simple types on F# Snippets, that are functionally equivalent to Rx’s Subject<T> and ReplaySubject<T>:

The ReplaySubject implementation uses F# Agents to simplify concurrency..

The types can be used easily from C#:

var s = new Subject<int>();

For Silverlight and WPF we need a mechanism for invoking methods on the UI thread, which I implemented in F# back in 2010: Implementing IObservable and extending Observable 

    .Select(e => e.GetPosition(canvas))
    .Delay(closure * 100)
    .Subscribe(pos =>
        Canvas.SetLeft(label, pos.X + closure * 10);
        Canvas.SetTop(label, pos.Y);

Putting it altogether, the inevitable "time flies like an arrow" Silverlight demo:

In case you’d like to have a play yourself, I’ve put up a preview release on CodePlex:

Exposing F# Dynamic Lookup to C#, WPF & Silverlight

Like C#, F# is primarily a statically typed programming language. That said both languages provide support for dynamic typing.

Sometimes dynamic typing can be a pragmatic way of solving a specific problem. For example say you’re a solutions provider with a core product and have a number of clients with bespoke requirements. One client asks for a product rating feature. This can be relatively easily achieved using dynamic properties:

  • a bunch of client specific properties are read from the database, including the rating value, which is then set as a dynamic property on the product object
  • zero changes are required to the core code
  • at the UI, on WPF, the dynamic property can simply be bound through XAML like any other object property
  • on Silverlight 4,0 direct binding is not currently possible, however there is a simple workaround - specifying the dynamic property as a parameter for a value converter (example later)

C# and F# use slightly different approaches for dynamic properties:

  • C# 4.0 provides a dynamic type, which tells the compiler that member lookup on the object should be deferred to run time
  • F# employs a dynamic lookup operator, which when overloaded defines the behaviour at runtime. This means that in F# dynamic lookup is explicit (the ? operator), and can be mixed with static lookup on the same object (the . operator)

Is it is still possible to implement dynamic properties in F# that can be consumed by other .Net languages like C#, VB.Net, IronRuby or IronPython; plus WPF and Silverlight. The trick is to inherit from .Net 4.0’s System.Dynamic.DynamicObject type and implement the System.ComponentModel.INotifyPropertyChanged interface:

open System.Dynamic

/// Dynamic Lookup type
type DynamicLookup () =
    inherit DynamicObject ()
    /// Synchronization object
    let sync = obj()
    /// Property Changed event
    let propertyChanged = Event<_,_>()    
    /// Properties
    let mutable properties = Map.empty
    /// Gets property value
    member private this.GetValue name = 
        Map.tryFind name properties
    /// Sets property value, creating a new property if none exists
    member private this.SetValue (name,value) =
        /// Atomically writes new properties reference
        let Write () = 
            properties <-
                |> Map.remove name 
                |> Map.add name value
        // Synchronize property writes
        lock sync Write
        // Trigger property changed event
        |> propertyChanged.Trigger    
    override this.TryGetMember(binder:GetMemberBinder,result:obj byref ) =     
        match this.GetValue binder.Name with
        | Some value -> result <- value; true
        | None -> false
    override this.TrySetMember(binder:SetMemberBinder, value:obj) =        
    override this.GetDynamicMemberNames() =
        properties |> (fun pair -> pair.Key)
    member this.PropertyChanged = propertyChanged.Publish
    interface System.ComponentModel.INotifyPropertyChanged with
        member this.PropertyChanged = propertyChanged.Publish     
    static member (?) (lookup:#DynamicLookup,name:string) =
        match lookup.GetValue name with
        | Some(value) -> value
        | None -> raise (new System.MemberAccessException())        
    static member (?<-) (lookup:#DynamicLookup,name:string,value:'v) =
        lookup.SetValue (name,value)
    static member GetValue (lookup:DynamicLookup,name) =

F# usage:

/// Product type inherits dynamic lookup
type Product (name,price) =
    inherit DynamicLookup ()
    member this.Name = name
    member this.Price = price
// Initiate product object with dynamic rating value
let p = Product("F# for Scientists",49.95M)
do p?Stars <- 5
// Access product's properties
let stars = System.Convert.ToInt32(p?Stars)
do printf "%s...%M %s" p.Name p.Price (System.String('*',stars))

C# usage:

// Create Product type with dynamic stars value
Product product = new Product("Expert F#",54.99M);
((dynamic)product).Stars = 5;       
// Read product properties
dynamic p = product;
string s = 
    string.Format("{0}...{1} {2}",
        p.Name, p.Price, new String('*', (int) p.Stars));

Use from Silverlight 4.0

<t:PropertyLookup x:Key="DynamicConverter"/>


<TextBox Text="{Binding Converter={StaticResource DynamicConverter},


dynamic product = new Product("Real World FP", 35.99);
product.Stars = 5;
DataContext = product;


public class PropertyLookup : IValueConverter
    public object Convert(
        object value, 
        Type targetType, 
        object parameter, 
        CultureInfo culture)
        return DynamicLookup.GetValue(
            (DynamicLookup) value,
            (string) parameter);            

    public object ConvertBack(object value, 
        Type targetType, 
        object parameter, 
        CultureInfo culture)
        throw new NotImplementedException();

Note: Silverlight 4.0 requires Microsoft.CSharp.dll to use dynamic types.

MVMMC – MVVM grows a Controller

Ray Booysen does it again! The first time I saw Ray do a talk was at NxtGen in Cambridge, when he gave me a timely introduction to MVVM just as I was starting to play with WPF. MVVM (Model-View-View Model) is an architectural pattern, that improves on code behind, making it possible to unit test your UI code (View Model), a good thing. Over a year later and I’m just starting to play with Silverlight, Ray delivers another timely talk on Silverlight gotchas at the Edge UG in London, based on his real-world development experience (developing Financial applications). This time he introduces among other good things MVVMC (MVVM plus Controller). Here the controller takes responsibility for communication between the Model and View Model, leaving the View Model with the single responsibility of providing data to the View.

Somewhat shell shocked I was left with a few questions on implementation details, again Ray to the rescue via Twitter:

ptrelford Excellent @raybooysen Silverlight gotchas talk - MVVMC (C for controller), Network is on UI thread, browser connection limits #edgeug

ptrelford @raybooysen really liked Fat Controller slide, of Thomas the Tank Engine; controller takes responsiblity for marshalling threads etc #edgeug

raybooysen @ptrelford fat controller should be in all slide decks. :)

raybooysen Talk finished at @edge_ug. Think it went ok. Feel like I rambled a little.

ptrelford So with MVVMC the Model is observed by a Controller, which constructs & updates a View Model, which the (XAML) View binds to

raybooysen @ptrelford thanks for coming tonight. :)

ptrelford @raybooysen using MVVMC the Controller observes the Model & updates View Model, which observes user input and passes messages to the Model?

raybooysen @ptrelford viewmodel observes and notifies the the controller.

ptrelford @raybooysen thanks! So View Model takes a reference to the parent controller?

raybooysen @ptrelford no. Can surface the ui interaction via events. Or use rx if you're in for some fun.

ptrelford @raybooysen Cool thanks again! That makes sense, View Model exposes IObservable<T> properties. BTW your talk blew me away (hence questions)!

raybooysen @ptrelford no worries. Iobservable feels quite right in these scenarios. Since controller essentially owns the viewmodel

Finally, this was the same talk that Ray delivered at DDD Scotland, so the slides are already available here