Ch3. Transforming Operators

collect()

["A", "B", "C", "D", "E"].publisher
	  .collect()
	  .sink(receiveCompletion: { print($0) },
        receiveValue: { print($0) })
	  .store(in: &subscriptions)

// collect에 parameter전달하여 메모리 과다사용 방지
["A", "B", "C", "D", "E"].publisher.collect(3)
    .sink(receiveCompletion: { print($0) },
          receiveValue: { print($0) })
    .store(in: &subscriptions)

["A", "B", "C", "D", "E"]
finished
["A", "B", "C"]
["D", "E"]
finished

map(_:)

let formatter = NumberFormatter()
formatter.numberStyle = .spellOut

[123, 4, 56].publisher
    .map {
        formatter.string(for: NSNumber(integerLiteral: $0)) ?? ""
    }
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)

one hundred twenty-three
four
fifty-six

Map key paths

map<T>(_*:)* map<T0, T1>(_:_:) **map<T0, T1, T2>(_:_:_:)

let publisher = PassthroughSubject<Coordinate, Never>()

publisher
    .map(\\.x, \\.y)
    .sink(receiveValue: { x, y in
        print(
            "The coordinate at (\\(x), \\(y)) is in quadrant",
            quadrantOf(x: x, y: y)
        )
    })
    .store(in: &subscriptions)

publisher.send(Coordinate(x: 10, y: -8))
publisher.send(Coordinate(x: 0, y: 5))

The coordinate at (10, -8) is in quadrant 4
The coordinate at (0, 5) is in quadrant boundary

tryMap(_:)

Just("Directory name that does not exist")
    .tryMap { try FileManager.default.contentsOfDirectory(atPath: $0) }
    .sink(receiveCompletion: { print($0) },
          receiveValue: { print($0) })
    .store(in: &subscriptions)

failure(Error Domain=NSCocoaErrorDomain Code=260 "The folder “Directory name that does not exist” doesn’t exist." UserInfo={NSUserStringVariant=(
    Folder
), NSFilePath=Directory name that does not exist, NSUnderlyingError=0x600003ab2cd0 {Error Domain=NSPOSIXErrorDomain Code=2 "No such file or directory"}})

Flatterning publishers

func decode(_ codes: [Int]) -> AnyPublisher<String, Never> {
    Just(
        codes
            .compactMap { code in
                guard (32...255).contains(code) else { return nil }
                return String(UnicodeScalar(code) ?? " ")
            }
            .joined()
    )
    .eraseToAnyPublisher()
}

[72, 101, 108, 108, 111, 44, 32, 87, 111, 114, 108, 100, 33]
    .publisher
    .collect()
    .flatMap(decode)
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)

Hello, World!

Source publisher가 너무 많으면 메모리 낭비 가능성이 있음

그래서 maxPublishers에 2를 넣었더니 P3은 전달이 안되는 것을 볼 수 있다.

replaceNil(with:)

["A", nil, "C"].publisher
    .eraseToAnyPublisher()
    .replaceNil(with: "-")
//		.replaceNil(with: "-" as String?) causes error. cannot return optional
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)

let tmp = ["A", nil, "C"].publisher
    .eraseToAnyPublisher()
    .replaceNil(with: "-")

A
-
C

replaceEmpty(with:)

let empty = Empty<Int, Never>()

// 2
empty
    .replaceEmpty(with: 1)
    .sink(receiveCompletion: { print($0) },
          receiveValue: { print($0) })
    .store(in: &subscriptions)

1
finished

scan(_:_:)

var dailyGainLoss: Int { .random(in: -10...10) }

// 2
let august2019 = (0..<22)
    .map { _ in dailyGainLoss }
    .publisher

// 3
august2019
    .scan(50) { latest, current in
        max(0, latest + current)
    }
    .sink(receiveValue: { _ in })
    .store(in: &subscriptions)

Key points

• Publisher의 output으로 특정 기능을 수행하는 메소드를 Operator라고 한다.
• Operator 또한 Publisher이다.
• Transforming Operator는 Upstream publisher에게 값을 받아 변환한 뒤, Downstream publisher에 전달한다.
• collect(), flatMap()과 같이 버퍼에 데이터가 쌓이는 operator는 신경 잘 써주셈
• Swift의 Transform메소드와 비슷한것들이 Combine에 있으니까 참고하자. Ex) map, flatMap
• 여러개의 Operator를 체이닝하여 사용할 수 있다.