Вы можете создать собственного подписчика, чтобы получать возвращающихся подписчиков. Demand.max (1). В этом случае подписчик будет запрашивать следующее значение только при его получении. Пример для Int.publisher, но некоторая случайная задержка в карте имитирует сетевой трафик c: -)
import PlaygroundSupport
import SwiftUI
import Combine
class MySubscriber: Subscriber {
typealias Input = String
typealias Failure = Never
func receive(subscription: Subscription) {
print("Received subscription", Thread.current.isMainThread)
subscription.request(.max(1))
}
func receive(_ input: Input) -> Subscribers.Demand {
print("Received input: \(input)", Thread.current.isMainThread)
return .max(1)
}
func receive(completion: Subscribers.Completion<Never>) {
DispatchQueue.main.async {
print("Received completion: \(completion)", Thread.current.isMainThread)
PlaygroundPage.current.finishExecution()
}
}
}
(110...120)
.publisher.receive(on: DispatchQueue.global())
.map {
print(Thread.current.isMainThread, Thread.current)
usleep(UInt32.random(in: 10000 ... 1000000))
return String(format: "%02x", $0)
}
.subscribe(on: DispatchQueue.main)
.subscribe(MySubscriber())
print("Hello")
PlaygroundPage.current.needsIndefiniteExecution = true
Печать игровой площадки ...
Hello
Received subscription true
false <NSThread: 0x600000064780>{number = 5, name = (null)}
Received input: 6e false
false <NSThread: 0x60000007cc80>{number = 9, name = (null)}
Received input: 6f false
false <NSThread: 0x60000007cc80>{number = 9, name = (null)}
Received input: 70 false
false <NSThread: 0x60000007cc80>{number = 9, name = (null)}
Received input: 71 false
false <NSThread: 0x60000007cc80>{number = 9, name = (null)}
Received input: 72 false
false <NSThread: 0x600000064780>{number = 5, name = (null)}
Received input: 73 false
false <NSThread: 0x600000064780>{number = 5, name = (null)}
Received input: 74 false
false <NSThread: 0x60000004dc80>{number = 8, name = (null)}
Received input: 75 false
false <NSThread: 0x60000004dc80>{number = 8, name = (null)}
Received input: 76 false
false <NSThread: 0x60000004dc80>{number = 8, name = (null)}
Received input: 77 false
false <NSThread: 0x600000053400>{number = 3, name = (null)}
Received input: 78 false
Received completion: finished true
ОБНОВЛЕНИЕ наконец я нашел .flatMap(maxPublishers: ), что заставляет меня обновить эту интересную топи c с немного другим подходом. Пожалуйста, обратите внимание, что я использую глобальную очередь для планирования, а не только некоторую случайную задержку, просто чтобы быть уверенным, что получение сериализованного потока не является «случайным» или «счастливым» поведением: -)
import PlaygroundSupport
import Combine
import Foundation
PlaygroundPage.current.needsIndefiniteExecution = true
let A = (1 ... 9)
.publisher
.flatMap(maxPublishers: .max(1)) { value in
[value].publisher
.flatMap { value in
Just(value)
.delay(for: .milliseconds(Int.random(in: 0 ... 100)), scheduler: DispatchQueue.global())
}
}
.sink { value in
print(value, "A")
}
let B = (1 ... 9)
.publisher
.flatMap { value in
[value].publisher
.flatMap { value in
Just(value)
.delay(for: .milliseconds(Int.random(in: 0 ... 100)), scheduler: RunLoop.main)
}
}
.sink { value in
print(" ",value, "B")
}
print
1 A
4 B
5 B
7 B
1 B
2 B
8 B
6 B
2 A
3 B
9 B
3 A
4 A
5 A
6 A
7 A
8 A
9 A
Исходя из написанного здесь
.serialize ()?
, определенного Клей Эллисом, принятый ответ можно заменить на
.publisher.flatMap (maxPublishers: .max (1)) {$ 0}
, в то время как в «несериализированной» версии необходимо использовать
.publisher.flatMap { $ 0}
"пример из реального мира"
import PlaygroundSupport
import Foundation
import Combine
let path = "postman-echo.com/get"
let urls: [URL] = "... which proves the downloads are happening serially .-)".map(String.init).compactMap { (parameter) in
var components = URLComponents()
components.scheme = "https"
components.path = path
components.queryItems = [URLQueryItem(name: parameter, value: nil)]
return components.url
}
//["https://postman-echo.com/get?]
struct Postman: Decodable {
var args: [String: String]
}
let collection = urls.compactMap { value in
URLSession.shared.dataTaskPublisher(for: value)
.tryMap { data, response -> Data in
return data
}
.decode(type: Postman.self, decoder: JSONDecoder())
.catch {_ in
Just(Postman(args: [:]))
}
}
extension Collection where Element: Publisher {
func serialize() -> AnyPublisher<Element.Output, Element.Failure>? {
guard let start = self.first else { return nil }
return self.dropFirst().reduce(start.eraseToAnyPublisher()) {
return $0.append($1).eraseToAnyPublisher()
}
}
}
var streamA = ""
let A = collection
.publisher.flatMap{$0}
.sink(receiveCompletion: { (c) in
print(streamA, " ", c, " .publisher.flatMap{$0}")
}, receiveValue: { (postman) in
print(postman.args.keys.joined(), terminator: "", to: &streamA)
})
var streamC = ""
let C = collection
.serialize()?
.sink(receiveCompletion: { (c) in
print(streamC, " ", c, " .serialize()?")
}, receiveValue: { (postman) in
print(postman.args.keys.joined(), terminator: "", to: &streamC)
})
var streamD = ""
let D = collection
.publisher.flatMap(maxPublishers: .max(1)){$0}
.sink(receiveCompletion: { (c) in
print(streamD, " ", c, " .publisher.flatMap(maxPublishers: .max(1)){$0}")
}, receiveValue: { (postman) in
print(postman.args.keys.joined(), terminator: "", to: &streamD)
})
PlaygroundPage.current.needsIndefiniteExecution = true
отпечатки
.w.h i.c hporves ht edownloadsa erh appeninsg eriall y.-) finished .publisher.flatMap{$0}
... which proves the downloads are happening serially .-) finished .publisher.flatMap(maxPublishers: .max(1)){$0}
... which proves the downloads are happening serially .-) finished .serialize()?
Мне также кажется очень полезным в других сценариях ios. Попробуйте использовать значение по умолчанию maxPublishers в следующем фрагменте и сравните результаты: -)
import Combine
let sequencePublisher = Publishers.Sequence<Range<Int>, Never>(sequence: 0..<Int.max)
let subject = PassthroughSubject<String, Never>()
let handle = subject
.zip(sequencePublisher.print())
//.publish
.flatMap(maxPublishers: .max(1), { (pair) in
Just(pair)
})
.print()
.sink { letters, digits in
print(letters, digits)
}
"Hello World!".map(String.init).forEach { (s) in
subject.send(s)
}
subject.send(completion: .finished)