RxJS Observables-发出任何新值时调用函数

Question

I have following the observable which sucesfully obtains JSON data from a firebase DB:

this.http.get('https://testdb.firebaseio.com/.json').map(res => res.json()).subscribe(result => {
  // do something with result
});

I want to call a function, foo() , inside the observable when any new value arrives after the first pull from the database. To be clear, I want foo() only when a value arrives "later".

For instance:

this.http.get('https://testdb.firebaseio.com/.json').map(res => res.json()).subscribe(result => {

  // Only called when a new value arrives in the stream later, not on the first pull when the stream is first activated.
  foo();

});

By later, I mean whenever the subscribed database is updated. I also don't need the actual value that was updated, I just want to trigger a function (which takes no parameters) when any new value arrives.

Answer 1

I've no experience with firebase but if you want to skip the very first value, then yourObservable.skip(1) will do it. Where yourObservable could be your data source ie:

this.http.get('https://testdb.firebaseio.com/.json')
.map(res => res.json())
.skip(1)
.subscribe((result) => foo();)