Ch4. Filtering Operators

filter(_:)

let numbers = (1...10).publisher
  
numbers
  .filter { $0.isMultiple(of: 3) }
  .sink(receiveValue: { n in
    print("\(n) is a multiple of 3!")
  })
  .store(in: &subscriptions)

3 is a multiple of 3!
6 is a multiple of 3!
9 is a multiple of 3!

 

removeDuplicates()

let words = "hey hey there! want to listen to mister mister ?"
	              .components(separatedBy: " ")
                  .publisher

words
  .removeDuplicates()
  .sink(receiveValue: { print($0) })
  .store(in: &subscriptions)

hey
there!
want
to
listen
to
mister
?

 

compactMap(_:)

let strings = ["a", "1.24", "3", "def", "45", "0.23"].publisher
  
strings
  .compactMap { Float($0) }
  .sink(receiveValue: {
    print($0)
  })
  .store(in: &subscriptions)

1.24
3.0
45.0
0.23

 

ignoreOutput()

let numbers = (1...10_000).publisher
  
numbers
  .ignoreOutput()
  .sink(receiveCompletion: { print("Completed with: \($0)") },
        receiveValue: { print($0) })
  .store(in: &subscriptions)

Completed with: finished

Finding values

first(where:_)

let numbers = (1...9).publisher
  
numbers
	.print("numbers")
  .first(where: { $0 % 2 == 0 })
  .sink(receiveCompletion: { print("Completed with: \($0)") },
        receiveValue: { print($0) })
  .store(in: &subscriptions)

numbers: receive subscription: (1...9)
numbers: request unlimited
numbers: receive value: (1)
numbers: receive value: (2)
numbers: receive cancel
2
Completed with: finished

로그에서 볼 수 있듯이 3부터는 이벤트도 전달되지 않았다. 2를 찾고 바로 시퀀스 종료

last(where:_)

let numbers = (1...9).publisher
  
numbers
  .last(where: { $0 % 2 == 0 })
  .sink(receiveCompletion: { print("Completed with: \($0)") },
        receiveValue: { print($0) })
  .store(in: &subscriptions)

8
Completed with: finished

last(where:)은 first(where:)과 반대로 시퀀스가 종료되어야 찾고자 하는 값을 찾을 수 있다.

 

Dropping values

RxSwift의 Skip과 동일한 기능

let numbers = (1...10).publisher
  
numbers
  .dropFirst(8)
  .sink(receiveValue: { print($0) })
  .store(in: &subscriptions)

9
10

 

drip(while:_)

let numbers = (1...10).publisher
  
numbers
  .drop(while: { $0 % 5 != 0 })
  .sink(receiveValue: { print($0) })
  .store(in: &subscriptions)

5
6
7
8
9
10

 

drop(untilOutputFrom:_)

et isReady = PassthroughSubject<Void, Never>()
let taps = PassthroughSubject<Int, Never>()
  
taps
	.drop(untilOutputFrom: isReady)
  .sink(receiveValue: { print($0) })
  .store(in: &subscriptions)

(1...5).forEach { n in
  taps.send(n)
  
  if n == 3 {
    isReady.send()
  }
}

4
5

Limiting values

let numbers = (1...10).publisher
  
numbers
  .prefix(2)
  .sink(receiveCompletion: { print("Completed with: \($0)") },
  		receiveValue: { print($0) })
  .store(in: &subscriptions)

1
2
Completed with: finished

prefix(while:_)

let numbers = (1...10).publisher
  
numbers
  .prefix(while: { $0 < 3 })
  .sink(receiveCompletion: { print("Completed with: \($0)") },
        receiveValue: { print($0) })
  .store(in: &subscriptions)

1
2
Completed with: finished

prefix(until:_)

let isReady = PassthroughSubject<Void, Never>()
let taps = PassthroughSubject<Int, Never>()
  
taps
  .prefix(untilOutputFrom: isReady)
  .sink(receiveCompletion: { print("Completed with: \($0)") },
        receiveValue: { print($0) })
  .store(in: &subscriptions)
  
(1...5).forEach { n in
  taps.send(n)
  
  if n == 2 {
    isReady.send()
  }
}

1
2
Completed with: finished