withLatestFrom note and playground example

junhojo · Dec 16, 2020 · 6717eab · 6717eab
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diff --git a/Rx.playground/Pages/Combining_Operators.xcplaygroundpage/Contents.swift b/Rx.playground/Pages/Combining_Operators.xcplaygroundpage/Contents.swift
@@ -155,6 +155,36 @@ example("switchLatest") {
 }
 /*:
  > In this example, adding ⚾️ onto `subject1` after adding `subject2` to `subjectsSubject` has no effect, because only the most recent inner `Observable` sequence (`subject2`) will emit elements.
+
+ ----
+ ## `withLatestFrom`
+ Merges two observable sequences into one observable sequence by combining each element from the first source with the latest element from the second source, if any.
+ */
+example("withLatestFrom") {
+ let disposeBag = DisposeBag()
+
+ let foodSubject = PublishSubject<String>()
+ let drinksSubject = PublishSubject<String>()
+
+ foodSubject.asObservable()
+ .withLatestFrom(drinksSubject) { "\($0) + \($1)" }
+ .subscribe(onNext: { print($0) })
+ .disposed(by: disposeBag)
+
+ foodSubject.onNext("🥗")
+
+ drinksSubject.onNext("☕️")
+ foodSubject.onNext("🥐")
+
+ drinksSubject.onNext("🍷")
+ foodSubject.onNext("🍔")
+
+ foodSubject.onNext("🍟")
+
+ drinksSubject.onNext("🍾")
+}
+/*:
+ > In this example 🥗 is not printed because `drinksSubject` did not emit any values before 🥗 was received. The last drink (🍾) will be printed whenever `foodSubject` will emit another event.
  */
 
 //: [Next](@next) - [Table of Contents](Table_of_Contents)
diff --git a/RxSwift/Observables/WithLatestFrom.swift b/RxSwift/Observables/WithLatestFrom.swift
@@ -12,6 +12,7 @@ extension ObservableType {
  Merges two observable sequences into one observable sequence by combining each element from self with the latest element from the second source, if any.
 
  - seealso: [combineLatest operator on reactivex.io](http://reactivex.io/documentation/operators/combinelatest.html)
+ - note: Elements emitted by self before the second source has emitted any values will be omitted.
 
  - parameter second: Second observable source.
  - parameter resultSelector: Function to invoke for each element from the self combined with the latest element from the second source, if any.
@@ -25,6 +26,7 @@ extension ObservableType {
  Merges two observable sequences into one observable sequence by using latest element from the second sequence every time when `self` emits an element.
 
  - seealso: [combineLatest operator on reactivex.io](http://reactivex.io/documentation/operators/combinelatest.html)
+ - note: Elements emitted by self before the second source has emitted any values will be omitted.
 
  - parameter second: Second observable source.
  - returns: An observable sequence containing the result of combining each element of the self with the latest element from the second source, if any, using the specified result selector function.