title: Using an Observable to detect a change in a variable
body: <p>I think I misunderstand how Observables are supposed to be used. I want to put a value in, and when the value changes it should emit the new value. I thought that was what they were for, but all the tutorials and docs don't seem to do this, but at the same time, I always see them being applied this way. For example, in angular when you subscribe to a "FirebaseListObservable", when the value in firebase changes it fires off a snapshot in the subscription. I want to make that for my own variable. Let's say I just have a string variable, and when it changes, it fires off any subscriptions.</p>