2011-09-07

Zipping With Arrays in RxJs

Have you ever tried to zip a hot observable with a cold one? That's what happens below.
   var former = function(a, b) { return a }
   var latter = function(a, b) { return b }
   var logger = function(a) { console.log(a) }
   var numbers = Rx.Observable.FromArray([1, 2, 3])
   var timer = Rx.Observable.Interval(100)
   var zipped = numbers.Zip(timer, former)
   zipped.Subscribe(logger)
The idea is to print out the contents of an array to the console one-by-one, each 100 milliseconds. It doesn't work though. What I've heard is that it fails because of a bug in Rx. Instead of printing out numbers 1,2,3 to the console, the code does ... nothing.
A simple workaround (Thanks Jussi) seems to work, though: Just concat your array observable with Rx.Observable.Never!
   // ...
   numbers.Concat(Rx.Observable.Never()).Zip(timer, former).Subscribe(logger)

1
2
3

So, now it prints the stuff to the console as you'd expect.
To make zipping with arrays a bit easier, you could add a ZipWithArray method to the Observable prototype like this.
  Rx.Observable.prototype.ZipWithArray = function(array, selector) {·
      return this.Zip(Rx.Observable.FromArray(array).Concat(Rx.Observable.Never()), 
  }
I've put this extension, as well a bunch of others on Github, in my rxjs-extensions project.

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