What is the proper way to work with AWSTask objects in Swift?

Question

Hello and thanks in advance for your time.

In my code I am making various requests to AWSSQS which all return AWSTask. I have found working with these AWSTask objects to be very difficult while also trying to keep all the logic specific to AWS in a single class so I can easily switch to a different cloud service if need be.

Ideally, what I would like to do is execute a series of AWS tasks asynchronously in a serial fashion. Normally I would just add tasks to a custom Serial Dispatch Queue but since The AWSTask objects are themselves asynchronous tasks, I can't do that.

Here is a simple example that illustrates the problem I am having. It doesn't have any real world purpose but it does a good job illustrating the problem. Below, I have code to create a SQS queue, send a message to a SQS queue, receive a message from an SQS queue, and delete a SQS queue. Let's say I want to do those four things in a serial, asynchronous fashion. In other words, I want to make sure the previous task succeeded before attempting the next task.

ViewController

DispatchQueue.global(qos: DispatchQoS.QoSClass.userInitiated).async {
        awsClass.runTest()
        DispatchQueue.main.async {
            print("Test Finished")
        }
    }

AwsClass

public func createQueue(){
    guard let createQueueRequest = AWSSQSCreateQueueRequest() else{fatalError()}

    createQueueRequest.queueName = "TestQueue"

    sqs.createQueue(createQueueRequest).continueWith(block: {(task) -> AnyObject? in
        if task.error != nil {
            print(task.error!)
        }
        else if task.result != nil {
            self.queueUrl = task.result!.queueUrl!
            print("created queue at: \(self.queueUrl!)")
        }
        return nil
    })
}

public func deleteQueue(){
    if queueUrl != nil {
        guard let deleteQueueRequest = AWSSQSDeleteQueueRequest() else{fatalError()}

        deleteQueueRequest.queueUrl = queueUrl

        sqs.deleteQueue(deleteQueueRequest).continueWith(block: {(task) -> AnyObject? in
            if task.error != nil {
                print(task.error!)
            }
            else if task.result != nil {
                print("queue sucessfully deleted from \(self.queueUrl!)")
                self.queueUrl = nil
            }
            return nil
        })
    }
    else{
        print("Queue has already been deleted")
    }
}

public func sendMessage(messageData: String, toConnectId: String) {
    guard let sendMessageRequest = AWSSQSSendMessageRequest() else{fatalError()}
    sendMessageRequest.queueUrl = toConnectId
    sendMessageRequest.delaySeconds = 0
    sendMessageRequest.messageBody = messageData
    sqs.sendMessage(sendMessageRequest).continueWith(block: {(task) -> AnyObject? in
        if task.error != nil {
            print(task.error!)
        }
        else if task.result != nil {
            print("successfully sent message to \(toConnectId)")
        }
        return nil
    })
}

public func receiveMessage(){
    guard let receiveMessageRequest = AWSSQSReceiveMessageRequest() else{fatalError()}
    receiveMessageRequest.queueUrl = self.queueUrl
    receiveMessageRequest.maxNumberOfMessages = 1

    sqs.receiveMessage(receiveMessageRequest).continueWith(block: {(task) -> AnyObject? in
        if task.error != nil {
            print(task.error!)
        }
        else if task.result != nil {
            let message = (task.result?.messages?.first)!
            print("successfully received message with body:  \(message.body ?? "failed")")
        }
        return nil
    })
}

public func runTest(){
    let mySerialQueue = DispatchQueue(label: "mySerialQueue")
    mySerialQueue.sync {
        self.createQueue()
    }
    mySerialQueue.sync {
        self.sendMessage(messageData: "test", toConnectId: "https://someUrl")
    }
    mySerialQueue.sync {
        self.receiveMessage()
    }
    mySerialQueue.sync {
        self.deleteQueue()
    }
}

Since the AWSTasks are asynchronous with completion functions, the code quickly makes all four calls and then the completion functions are called whenever those tasks finish. Instead I want the completion function of the first task to finish before the next task begins.

Answer 1

The AWSTask objects are meant to be "chained" together. Documentation can be found here: http://docs.aws.amazon.com/mobile/sdkforios/developerguide/awstask.html

A small example here:

sqs.createQueue(/* parameters */).continueWithSuccess(block: {(task) -> Void in
    // Success
    return sqs.sendMessage(/* parameters */)
}).continueWithSuccess(block: {(task) -> Void in
    // Success
    return sqs.receiveMessage(/* parameters */)
}).continueWithSuccess(block: {(task) -> Void in
    // Success
    return sqs.deleteQueue(/* parameters */)
})

Answer 2

Alright, so I found a solution to my question. It works exactly as desired but it does so in this nasty chain of completion functions. If anyone knows a more elegant solution, I am all ears!

ViewController

print("Starting Test")
    DispatchQueue.global(qos: DispatchQoS.QoSClass.userInitiated).async {
        atomConnector.runTest(completion: {
            print("test finshed")
        })
    }

AwsClass

public func createQueue(completion: @escaping () -> Void){
    guard let createQueueRequest = AWSSQSCreateQueueRequest() else{fatalError()}

    createQueueRequest.queueName = "TestQueue"

    sqs.createQueue(createQueueRequest).continueWith(block: {(task) -> Void in
        if task.error != nil {
            print(task.error!)
        }
        else if task.result != nil {
            self.queueUrl = task.result!.queueUrl!
            print("created queue at: \(self.queueUrl!)")
            completion()
        }
    })
}

public func deleteQueue(completion: @escaping () -> Void){
    if queueUrl != nil {
        guard let deleteQueueRequest = AWSSQSDeleteQueueRequest() else{fatalError()}

        deleteQueueRequest.queueUrl = queueUrl

        sqs.deleteQueue(deleteQueueRequest).continueWith(block: {(task) -> Void in
            if task.error != nil {
                print(task.error!)
            }
            else if task.result != nil {
                print("queue sucessfully deleted from \(self.queueUrl!)")
                self.queueUrl = nil
                completion()
            }
        })
    }
    else{
        print("Queue has already been deleted")
    }
}

public func sendMessage(messageData: String, toConnectId: String, completion: @escaping () -> Void) {
    guard let sendMessageRequest = AWSSQSSendMessageRequest() else{fatalError()}
    sendMessageRequest.queueUrl = toConnectId
    sendMessageRequest.delaySeconds = 0
    sendMessageRequest.messageBody = messageData
    sqs.sendMessage(sendMessageRequest).continueWith(block: {(task) -> Void in
        if task.error != nil {
            print(task.error!)
        }
        else if task.result != nil {
            print("successfully sent message to \(toConnectId)")
            completion()
        }
    })
}

public func receiveMessage(completion: @escaping () -> Void){
    guard let receiveMessageRequest = AWSSQSReceiveMessageRequest() else{fatalError()}
    receiveMessageRequest.queueUrl = self.queueUrl
    receiveMessageRequest.maxNumberOfMessages = 1

    sqs.receiveMessage(receiveMessageRequest).continueWith(block: {(task) -> Void in
        if task.error != nil {
            print(task.error!)
        }
        else if task.result != nil {
            let message = (task.result?.messages?.first)!
            print("successfully received message with body:  \(message.body ?? "failed")")
            self.deleteMessage(receiptHandle: message.receiptHandle, completion: completion)
        }
    })
}

public func deleteMessage(receiptHandle: String?, completion: @escaping () -> Void){
    guard let deleteMessageRequest = AWSSQSDeleteMessageRequest() else{fatalError()}
    deleteMessageRequest.queueUrl = self.queueUrl
    deleteMessageRequest.receiptHandle = receiptHandle

    sqs.deleteMessage(deleteMessageRequest).continueWith(block: {(task) -> Void in
        if task.error != nil {
            print(task.error!)
        }
        else if task.result != nil {
            print("successfully deleted message with receiptHandle:  \(receiptHandle)")
            completion()
        }
    })
}

public func runTest(completion: @escaping () -> Void){

    self.createQueue(completion: {
        self.sendMessage(messageData: "test", toConnectId: "https://someUrl", completion: {
            self.receiveMessage(completion: {
                self.deleteQueue(completion: {
                    completion()
                })
            })
        })
    })

}