TaskQueue.swift

import Foundation
import PromiseKit


/// serialize async task execution
class TaskQueue {
    private let queue = DispatchQueue(label: "jp.digital-sensation.TaskQueue")
    private var tasks: [() -> Promise<Void>] = []
    private var running = false


    func schedule<Result>(task: @escaping () -> Promise<Result>) -> Promise<Result> {
        let (promise, joint) = Promise<Result>.joint()

        queue.async {
            self.tasks.append {
                let promise = task()
                promise.join(joint)
                return promise.asVoid()
            }
            self.next()
        }


        return promise
    }

    private func next() {
        // must be called on self.queue

        if running || tasks.count <= 0 {
            return
        }

        let task = tasks.removeFirst()

        running = true
        task().always(on: self.queue) {
            self.running = false
            self.next()
        }
    }
}