FlatMap¶

Think of it as an 2D array flat mapping into a 1D array

Traditional¶

Flat Map 2D => 1D Array of Int values

[ 
    1,2,3,
  4,5,6,
  7,8,9
]                    ====> [1,2,3,4,5,6,7,8,9] 

Reactive¶

// Imagine the variables as observable without the appropriate syntax.

let Observable1<[Int]> = [1,2,5]
let Observable2<[Int]> = [4,7,5]
let Observable3<[Int]> = [3,9,7]


[ 
    Observable1<[Int]>,
    Observable2<[Int]>,
    Observable3<[Int]>,
]

// Reaches into the observable of array of Int & unfurls itself
// Observable needs to be completed to unfurl. 

====> 
[ 1, 2, 5, // (Observable1.onComplete())
    4, 7, 5, // (Observable1.onComplete())
    3, 9, 7  // (Observable1.onComplete())
]

Inner observable -> completes of 1 out of 3 It will wait for all 3 observables to complete their observable chain. Flat maps are easier to discern if we one offs observable.of() or observable.just() which only processes one element and sends .onNext(value) => .onComplete(_) event to end its observable chain.

https://betterprogramming.pub/rxswift-deep-inside-flatmap-1ca150c41b18?gi=5416d91e4282

https://stackoverflow.com/questions/49361386/rxswift-wait-for-observable-to-complete

Error Observable < T.success, T.failure >¶

Error

 Declared closure result 'Observable<Result<User, Error>>' is incompatible with contextual type 'Result<T, Error>'
 Generic parameter 'T' could not be inferred

let userEmpty = authenticationSubject
    // This following code causes aforementioned compilation error
    .map { auth -> Observable<Result<User, Error>> in
        UserCoordinator().rx.currentUser(in: nil)
    }

This fixes it, don't fully understand it why but it could be related to unwrapping its internal observable sequence or else it would be Observable<Observable<Result<User, Error>>>

let userEmpty = authenticationSubject
   .flatMapLatest { auth -> Observable<Result<User, Error>> in
        UserCoordinator().rx.currentUser(in: nil)                 }