task parallel library - Reactive Extensions subscribing to an observable (subject) -

i'm playing around reactive extensions first time in winforms application. mind have been doing web development past 4 years, , familiar observables , observable pattern in knockout, guessing contributing confusion here.

anyhow, question , code. have simple winforms experiment (see below) building illustrate question. subscribe below doesn't run until after thread in start new finished. can trace calls onnext, subscribe doesn't fire @ until 20-30 seconds later. can explain behavior me?

public partial class form1 : form {      private subject<int> progress;     private cancellationtoken cancellationtoken;     private ischeduler _scheduler;      public form1()     {         initializecomponent();          cancellationtokensource source = new cancellationtokensource();         cancellationtoken = source.token;          _scheduler = new synchronizationcontextscheduler(synchronizationcontext.current);       }      private void start_click(object sender, eventargs e)     {         progress             .observeon(_scheduler)             //.throttle(timespan.fromseconds(5))             .subscribe(                 (i) => {                     progressbar1.do<progressbar>(ctl =>                     {                         ctl.value = i;                     });                 },                 (ex) => { },             cancellationtoken         );          task countertask = task.factory.startnew(() =>         {             (var = 1; < 101; i++)             {                 thread.sleep(500);                 progress.onnext(i);             }         }, cancellationtoken,         taskcreationoptions.longrunning,         taskscheduler.fromcurrentsynchronizationcontext()       );     }      private void form1_load(object sender, eventargs e)     {        progress = new subject<int>();     }   }    public static class controlextensions {     public static void do<tcontrol>(this tcontrol control, action<tcontrol> action)       tcontrol : control     {         if (control.invokerequired)             control.invoke(action, control);         else             action(control);     } } 

your issue comes fact task running on ui thread, because you're using taskscheduler.fromcurrentsynchronizationcontext().

hence various sleep calls blocking ui thread, freezing ui (e.g. can't drag window) , preventing observable subscription execute (because observeon, it's supposed execute on ui thread scheduler).

replace taskscheduler.fromcurrentsynchronizationcontext() taskscheduler.default (background taskpool threads), , work expected.

note call do/invoke unnecessary, because you're on ui thread scheduler you've provided.