从 iPlayground 2019 学到的简易版 Swift Combine 框架实现

combine_iplayground2019.swift

import Foundation

let url = URL(string: "https://www.bing.com")!

class Subscriber {
    let cancel: () -> Void
    
    init(cancel: @escaping () -> Void) {
        self.cancel = cancel
    }
    
    deinit {
        print("deinit")
        cancel()
    }
}

struct Publisher<Value> {
    let subscribe: (@escaping (Value) -> Void) -> Subscriber
    
    func map<NewValue>(_ transform: @escaping (Value) -> NewValue) -> Publisher<NewValue> {
        return Publisher<NewValue> { (newValueHandler) in
            return self.subscribe { (value) in
                let newValue = transform(value)
                newValueHandler(newValue)
            }
        }
    }
}

extension URLSession {
    func dataTaskPublisher(with url: URL) -> Publisher<Data> {
        return Publisher { (valueHandler) in
            let task = self.dataTask(with: url) { (data, response, error) in
                if let data = data {
                    valueHandler(data)
                }
            }
            task.resume()
            return Subscriber { task.cancel() }
        }
    }
}

let publisher = URLSession.shared.dataTaskPublisher(with: url)
.map({ $0.count })
.subscribe { print($0) }