Possible Thread Queue

Queue.swift

public class Queue {
    private var strand: Strand! = nil
    private var queue: [Strand.Closure] = []
    private let lock = Lock()

    public init(_ operation: Strand.Closure? = nil) throws {
        if let operation = operation { queue.append(operation) }

        strand = try Strand { [weak self] in
            while let welf = self {
                welf.fire()
            }
        }
    }

    public func add(_ operation: () -> Void) {
        lock.locked {
            queue.append(operation)
        }
    }

    private func fire() {
        var next: Strand.Closure? = nil
        lock.locked {
            guard !queue.isEmpty else { return }
            next = queue.removeFirst()
        }

        // don't run operation in lock or `add` could block calling queue
        next?()
    }
}

TestQueue.swift

    func testQueue() throws {
        var collection = [0]

        let strand = try Queue {
            sleep(1)
            collection.append(1)
        }

        collection.append(2)
        sleep(2)
        XCTAssert(collection == [0, 2, 1])

        strand.add {
            sleep(1)
            collection.append(3)
        }
        collection.append(4)
        sleep(2)
        XCTAssert(collection == [0, 2, 1, 4, 3])
    }