cascading-calls.md

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

login()
  .then(getUserDetails)
  .then(getOrdersByUser)

Rxjs approach

import { of, from } from 'rxjs';
import { switchMap } from 'rxjs/operators';

let stream$ = of({ message : 'Logged in' }).pipe(
  switchMap( result => {
    return of({ id: 1, name : 'user' })
  }),
  switchMap((user) => {
    return from(
     [{ id: 114, userId : 1 },
      { id: 117, userId : 1 }  ])
    })
);

stream$.subscribe((orders) => {
  console.log('Orders', orders);
})

// Array of orders

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

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

getUser()
  .then((user) => {
    return Promise.all(
      getOrders(),
      getMessages()
    )
   })

Rxjs approach

import { of } from 'rxjs';
import { switchMap, forkJoin } from 'rxjs/operators';

let stream$ = of({ id : 1, name : 'User' })
stream.pipe(
  switchMap((user) => {
    return forkJoin(
      from([{ id : 114, user: 1}, { id : 115, user: 1}],
      from([{ id : 200, user: 1}, { id : 201, user: 1}])
    )
  })
);

stream$.subscribe((result) => {
  console.log('Orders', result[0]);
  console.log('Messages', result[1]);
})

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