Cascading calls

A cascading call means that based on what call is happening another call should take place and possibly another one based on that.

Dependant calls

A dependant call means the calls needs to happen in order. Call 2 must happen after Call 1 has returned. It might even be possible that Call2 needs be specified using data from Call 1.

Imagine you have the following scenario:

  • A user needs to login first
  • Then we can fetch their user details
  • Then we can fetch their orders

Promise approach

  1. login()
  2.      .then(getUserDetails)
  3.      .then(getOrdersByUser)

Rxjs approach

  1. let stream$ = Rx.Observable.of({ message : 'Logged in' })
  2.       .switchMap( result => {
  3.          return Rx.Observable.of({ id: 1, name : 'user' })
  4.       })
  5.       .switchMap((user) => {
  6.         return Rx.Observable.from(
  7.            [ { id: 114, userId : 1 },
  8.              { id: 117, userId : 1 }  ])
  9.       })
  11. stream$.subscribe((orders) => {
  12.   console.log('Orders', orders);
  13. })
  15. // Array of orders

I’ve simplied this one a bit in the Rxjs example but imagine instead of

  1. Rx.Observable.of()

it does the proper ajax() call like in Operators and Ajax

Semi dependant

  • We can fetch a users details
  • Then we can fetch Orders and Messages in parallell.

Promise approach

  1. getUser()
  2.    .then((user) => {
  3.       return Promise.all(
  4.         getOrders(),
  5.         getMessages()
  6.       )
  7.    })

Rxjs approach

  1. let stream$ = Rx.Observable.of({ id : 1, name : 'User' })
  2. stream.switchMap((user) => {
  3.   return Rx.Observable.forkJoin(
  4.      Rx.Observable.from([{ id : 114, user: 1}, { id : 115, user: 1}],
  5.      Rx.Observable.from([{ id : 200, user: 1}, { id : 201, user: 1}])
  6.   )
  7. })
  9. stream$.subscribe((result) => {
  10.   console.log('Orders', result[0]);
  11.   console.log('Messages', result[1]);
  13. })

GOTCHAS

We are doing switchMap() instead of flatMap() so we can abandon an ajax call if necessary, this will make more sense in Auto complete recipe