# combine
- 같은 타입만 가능


## startWith
- Prepends a sequence of values to an observable sequence.
- 초기 상태를 지정해줄 수 있다


## concat
- class method
- Concatenates all inner observable sequences, as long as the previous observable sequence terminated successfully.


## concat(_:)
- instance method
- Concatenates all observable sequences in the given sequence, as long as the previous observable sequence terminated successfully.


## concatMap
- Projects each element of an observable sequence to an observable sequence and concatenates the resulting observable sequences into one observable sequence.
- flatMap처럼 이전 시퀀스가 완료된 후 다음 시퀀스를 구독하므로 순서를 보장한다


## merge
- Merges elements from all observable sequences in the given enumerable sequence into a single observable sequence.
- emits the elements as soon as they arrive — there’s no predefined order.


## merge(maxConcurrent:)
- Merges elements from all inner observable sequences into a single observable sequence, limiting the number of concurrent subscriptions to inner sequences.


## combineLatest
- Merges the specified observable sequences into one observable sequence whenever **any of the observable sequences produces an element.**
- 다른 타입의 시퀀스를 결합할 수 있다!!!


## zip
- Merges the specified observable sequences into one observable sequence whenever all of the observable sequences have produced an element at a **corresponding index.**


## withLatestFrom(_:)
- Merges two observable sequences into one observable sequence by using latest element from the second sequence every time when self emitts an element
- you want the current (latest) value emitted from an observable, but only when a particular trigger occurs.
- 버튼 눌러서 최신값 얻고 싶을 때


## sample(_:)
- Samples the source observable sequence using a sampler observable sequence producing sampling ticks.
- **In case there were no new elements between sampler ticks, no element is sent to the resulting sequence.**
- 버튼 눌러서 최신값 얻을 수 있으나 최신값이 변하지 않았으면 버튼 눌러도 방출 안함




## amb
- Propagates the observable sequence that reacts first.
- 두 시퀀스를 구독해서 둘 중 늦게 방출하는 시퀀스를 구독해지한다
- 여러 서버중에 제일 먼저 응답하는 서버로 연결하기


## switchLatest()
- Transforms an observable sequence of observable sequences into an observable sequence producing values only from the most recent observable sequence
- Each time a new inner observable sequence is received, unsubscribe from the previous inner observable sequence.
- flatMapLatest 처럼 최근 구독만 유지


## reduce
- Applies an accumulator function over an observable sequence, returning the result of the aggregation as a single element in the result sequence. The specified seed value is used as the initial accumulator value.
- produces its summary (accumulated) value only when the source observable completes
- 완료시점에만 누적값을 방출


## scan
- Applies an accumulator function over an observable sequence and returns each intermediate result. The specified seed value is used as the initial accumulator value.
- 입력값이 추가될때마다 그때까지의 누적값을 방출
- local variable을 따로 만들지 않아도 상태변화를 추적할 수 있다