SOE.swift

final class SharedOperation<T, E: Swift.Error> {
    typealias Completion = (Result<T, E>) -> Void

    private let queue = DispatchQueue(label: "SharedOperation.queue")
    private let execute: (@escaping Completion) -> Void

    private var completions: [Completion] = []

    init(_ execute: @escaping (@escaping Completion) -> Void) {
        self.execute = execute
    }

    func perform(_ completion: @escaping Completion) {
        self.queue.async {
            guard self.completions.isEmpty else {
                self.completions.append(completion)
                return
            }

            self.completions.append(completion)

            self.execute { result in
                self.queue.async {
                    self.completions.forEach {
                        $0(result)
                    }

                    self.completions = []
                }
            }
        }
    }
}