Uploader.swift

class Uploader {
    [...]
    private func uploadImages() -> Future<Void, UploadError> {
        let promise = Promise<Void, UploadError>()
        var imageSequence: [Future<Void, UploadError>]! = [] // 1
        let queue = OperationQueue() // 2
        queue.maxConcurrentOperationCount = 3
        for case let image as Image in self.images {
            let promise = Promise<Void, UploadError>() // 3
            let operation = UploadOperation(image: image, promise: promise)
            imageSequence.append(promise.future) // 5
            queue.addOperation(operation) // 6
        }
        if imageSequence.count == 0 {
            promise.success()
        } else {
            imageSequence.sequence().onComplete(callback: { (_) in
                promise.success()
            }).onFailure(callback: { (error) in
                // throw error
            })
        }
        return promise.future
    }
}

See https://medium.com/@linesapp/use-brightfutures-alamofire-and-operations-to-queue-the-upload-of-multiple-images-23facd3b44f for a description