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Hot and Cold Observable

http://www.introtorx.com/content/v1.0.10621.0/14_HotAndColdObservables.html

public void SimpleColdSample() {
  var period = TimeSpan.FromSeconds(1);
  var observable = Observable.Interval(period);
  observable.Subscribe(i => Console.WriteLine("first subscription : {0}", i));
  Thread.Sleep(period);
  observable.Subscribe(i => Console.WriteLine("second subscription : {0}", i));
  Console.ReadKey();
  /* Output:
  first subscription : 0
  first subscription : 1
  second subscription : 0
  first subscription : 2
  second subscription : 1
  first subscription : 3
  second subscription : 2
  */
}

If we want to be able to share the actual data values and not just the observable instance, we can use the Publish which return IConnectableObservatle that extends Observable by adding a single Connect method.

var period = TimeSpan.FromSeconds(1);
var observable = Observable.Interval(period).Publish();
observable.Connect();
observable.Subscribe(i => Console.WriteLine("first subscription : {0}", i));
Thread.Sleep(period);
observable.Subscribe(i => Console.WriteLine("second subscription : {0}", i));

By calling Connect(), ConnectableObservable will subsctibe to the underlying Observable.Interval.

Connectable Observable Operators

def firstMillion  = Observable.range( 1, 1000000 ).sample(7, java.util.concurrent.TimeUnit.MILLISECONDS);

firstMillion.subscribe(
   { println("Subscriber #1:" + it); },       // onNext
   { println("Error: " + it.getMessage()); }, // onError
   { println("Sequence #1 complete"); }       // onCompleted
);

firstMillion.subscribe(
    { println("Subscriber #2:" + it); },       // onNext
    { println("Error: " + it.getMessage()); }, // onError
    { println("Sequence #2 complete"); }       // onCompleted
);

// output
Subscriber #1:211128
Subscriber #1:411633
Subscriber #1:629605
Subscriber #1:841903
Sequence #1 complete
Subscriber #2:244776
Subscriber #2:431416
Subscriber #2:621647
Subscriber #2:826996
Sequence #2 complete
def firstMillion  = Observable.range( 1, 1000000 ).sample(7, java.util.concurrent.TimeUnit.MILLISECONDS).publish();

firstMillion.subscribe(
   { println("Subscriber #1:" + it); },       // onNext
   { println("Error: " + it.getMessage()); }, // onError
   { println("Sequence #1 complete"); }       // onCompleted
);

firstMillion.subscribe(
   { println("Subscriber #2:" + it); },       // onNext
   { println("Error: " + it.getMessage()); }, // onError
   { println("Sequence #2 complete"); }       // onCompleted
);

firstMillion.connect();

// output
Subscriber #2:208683
Subscriber #1:208683
Subscriber #2:432509
Subscriber #1:432509
Subscriber #2:644270
Subscriber #1:644270
Subscriber #2:887885
Subscriber #1:887885
Sequence #2 complete
Sequence #1 complete

Mixing cold and got

http://nullzzz.blogspot.kr/2012/01/things-you-should-know-about-rx.html

var hotness = $(document).toObservable("keyup")
var temperature = Observable.FromArray("coldness").Concat(hotness.Select(always("hotness")))

You might expect to get an observable starting with "coldness" and producing "hotness" at each keyup. However, StartWith made your new observable a bit colder in the sense that any new subscriber will always get "coldness" first.

No matter how you combine coldness with hotness, you won't get a hot Observable back. It won't be cold as in "tree falling in the woods", but inconsistent anyway. Lukewarm? No, more like Groundhog Day.

Using StartWith won't save you. It's the same thing as concatenating with a cold stream of one event.

To Use Subject Or Not To Use Subject?

http://davesexton.com/blog/post/To-Use-Subject-Or-Not-To-Use-Subject.aspx