{"id":878,"date":"2009-10-20T16:50:13","date_gmt":"2009-10-20T14:50:13","guid":{"rendered":"http:\/\/www.navision-blog.de\/2009\/10\/20\/iobservableiobserver-using-the-reactive-framework-with-f\/"},"modified":"2009-10-22T09:56:45","modified_gmt":"2009-10-22T07:56:45","slug":"iobservableiobserver-using-the-reactive-framework-with-f","status":"publish","type":"post","link":"http:\/\/www.navision-blog.de\/blog\/2009\/10\/20\/iobservableiobserver-using-the-reactive-framework-with-f\/","title":{"rendered":"IObservable\/IObserver – Using the Reactive Framework with F#"},"content":{"rendered":"

One of the nice new features in .NET 4.0 beta 2 is the IObservable<T><\/a>\/IObserver<T> support from the Reactive Framework (\u201cRx Framework\u201d or sometimes \u201cLinqToEvents\u201d). It is a really powerful way to use reactive programming in .NET and especially in F# developed by Erik Meijer and his team. <\/p>\n

If you want to see some of the beautiful math behind the Reactive Framework you should definitely watch this<\/a> Expert to Expert video on Channel 9. You can see Brian Beckman and Erik Meijer showing that IObservable<T> is the mathematical dual<\/a> of IEnumerable<T>.<\/p>\n

What can I do with the Rx Framework?<\/h5>\n

Consider this small sample (it is taken from Matthew Podwysocki’s blog<\/a>): We want to get notified whenever a user clicks on our form and moves the mouse within a special area (XPos and YPos smaller than 100px).<\/p>\n

First of all we define our observable by merging and filtering .NET events:<\/p>\n

\n

open<\/span> System.Windows.Forms<\/p>\n

 <\/p>\n

let<\/span> form = new<\/span> Form(Visible=true<\/span>, TopMost=true<\/span>)<\/p>\n

 <\/p>\n

\/\/\/ Creates two observables<\/span><\/p>\n

\/\/\/  – left is triggered when the left mouse button is down <\/span><\/p>\n

         and the mouse is in the area (x < 100 && y < 100)<\/span><\/p>\n

\/\/\/  – right is triggered when the right mouse button is down <\/span><\/p>\n

         and the mouse is in the area (x < 100 && y < 100)<\/span><\/p>\n

let<\/span> left,right = <\/p>\n

  form.MouseDown<\/p>\n

    |> Observable.merge form.MouseMove<\/p>\n

    |> Observable.filter (fun<\/span> args -><\/span> <\/p>\n

          args.Button = MouseButtons.Left ||<\/p>\n

          args.Button = MouseButtons.Right)<\/p>\n

    |> Observable.map (fun<\/span> args -><\/span> args.X, args.Y, args.Button)<\/p>\n

    |> Observable.filter (fun<\/span> (x,y,b) -><\/span> x < 100 && y < 100)<\/p>\n

    |> Observable.partition (fun<\/span> (_,_,button) -><\/span> button = MouseButtons.Left)<\/p>\n<\/p><\/div>\n

Now it\u2019s easy to subscribe a function to this observable:<\/p>\n

\n

let<\/span> leftSubscription =<\/p>\n

  left<\/p>\n

    |> Observable.subscribe<\/p>\n

         (fun<\/span> (x,y,_) -><\/span> printfn "Left (%d,%d)"<\/span> x y)<\/p>\n<\/p><\/div>\n

If we want to unsubscribe we only have to dispose the object:<\/p>\n

\n

\/\/ unsubscribe<\/span><\/p>\n

leftSubscription.Dispose()<\/p>\n<\/p><\/div>\n

We couldn\u2019t unsubscribe this way with \u201cclassic\u201d .NET events. Remember the \u2013= operator in C# doesn\u2019t work with lambda expressions.<\/p>\n

Exception handling<\/h5>\n

We have seen an easy way to subscribe and unsubscribe to complicated observables but what should we do if an error occurs? As far as I know this case is not implemented for F# at the moment, but we can easily add this functionality:<\/p>\n

\n

module<\/span> Observable<\/p>\n

 <\/p>\n

\/\/\/ Creates an observer with the given functions<\/span><\/p>\n

let<\/span> createObserver next error completed =<\/p>\n

    {new<\/span> System.IObserver<_> with<\/span><\/p>\n

        member<\/span> this.OnCompleted() = completed()<\/p>\n

        member<\/span> this.OnError(e) = error e<\/p>\n

        member<\/span> this.OnNext(args) = next args}<\/p>\n

 <\/p>\n

\/\/\/ Subscribes an observer with the given functions <\/span><\/p>\n

\/\/\/   param1: OnNext        (T -> unit)<\/span><\/p>\n

\/\/\/   param2: OnError       (Exception -> unit)<\/span><\/p>\n

\/\/\/   param3: OnCompleted   (unit -> unit)<\/span><\/p>\n

\/\/\/   param4: observable<\/span><\/p>\n

let<\/span> subscribeComplete next error completed (observable:System.IObservable<_>) =<\/p>\n

  createObserver next error completed<\/p>\n

    |> observable.Subscribe<\/p>\n<\/p><\/div>\n

Now we are able to create a complete IObserver<T> object and register the 3 functions:<\/p>\n

\n

let<\/span> rightSubscription =<\/p>\n

  right<\/p>\n

    |> Observable.subscribeComplete<\/p>\n

         (fun<\/span> (x,y,_) -><\/span> printfn "Right (%d,%d)"<\/span> x y)<\/p>\n

         (fun<\/span> error   -><\/span> printfn "Error: %s"<\/span> error.Message)<\/p>\n

         (fun<\/span> ()      -><\/span> printfn "Ready."<\/span>)<\/p>\n<\/p><\/div>\n","protected":false},"excerpt":{"rendered":"

One of the nice new features in .NET 4.0 beta 2 is the IObservable<T>\/IObserver<T> support from the Reactive Framework (\u201cRx Framework\u201d or sometimes \u201cLinqToEvents\u201d). It is a really powerful way to use reactive programming in .NET and especially in F# developed by Erik Meijer and his team. If you want to see some of the […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[448],"tags":[664,579,578],"_links":{"self":[{"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/posts\/878"}],"collection":[{"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/comments?post=878"}],"version-history":[{"count":3,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/posts\/878\/revisions"}],"predecessor-version":[{"id":881,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/posts\/878\/revisions\/881"}],"wp:attachment":[{"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/media?parent=878"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/categories?post=878"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.navision-blog.de\/blog\/wp-json\/wp\/v2\/tags?post=878"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}