Using an Observable to detect a change in a variable

Question

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.

Answer 1

Normally I would have my observables in services that get subscribed to in components, but I bundled them all in one class for the convenience of this answer. I've listed comments explaining each step. I hope this helps. : )

import { Subject } from 'rxjs/Subject';

export class ClassName {
    // ------ Creating the observable ----------
   // Create a subject - The thing that will be watched by the observable
   public stringVar = new Subject<string>();

   // Create an observable to watch the subject and send out a stream of updates (You will subscribe to this to get the update stream)
   public stringVar$ = this.stringVar.asObservable() //Has a $ 

   // ------ Getting Your updates ----------
   // Subscribe to the observable you created.. data will be updated each time there is a change to Subject
   public subscription = this.stringVar$.subscribe(data => {
         // do stuff with data
         // e.g. this.property = data
   });

  // ------ How to update the subject ---------
   // Create a method that allows you to update the subject being watched by observable
   public updateStringSubject(newStringVar: string) {
     this.stringVar.next(newStringVar);
   }
   // Update it by calling the method..
   // updateStringSubject('some new string value')

   // ------- Be responsible and unsubscribe before you destory your component to save memory ------
   ngOnDestroy() {
     this.subscription.unsubscribe()
   }
}

Answer 2

Try this using ReplySubject . I used typescript, angularfire to explain in the below code example

export class MessageService {
  private filter$: ReplaySubject<any> = new ReplaySubject(1);

  getMessagesOfType():FirebaseListObservable<any>{
   return this.af.database.list(this.messagesPath, {
    query: {
      orderByChild: 'type',
      equalTo: this.filter$
    }
  });
  }
  getClosedMessage(): void {
    this.filter$.next('closed');
  }
  getOpenMessage(): void {
    this.filter$.next('open');
  }
}

// in some other class
// MessagesSubject is assigned to messageService 
this.messageService.getMessagesOfType().subscribe((listOfMessages)=>{
 // this list will be updated when the this.filter$ updated see below functions
 console.log(listOfMessages); 
});
// update this.filter$ like this to get 
this.messageService.getClosedMessage();
// to get open messges in 
this.messageService.getOpenMessage();