A simple yet powerful queueing system for iOS (with persistence).

Overview

SYNQueue is a subclass of NSOperationQueue so you get:

• Serial or concurrent queues
• Task priority
• Multiple queues
• Dependencies
• KVC/KVO
• Thread safety

But it goes beyond NSOperationQueue and NSOperation to offer:

• Task persistence (via protocol)
• Queue specific logging (via protocol)
• Retries (exponential back-off)

Motivation

With a good queuing solution you can provide a much better user experience in areas such as:

• Web requests
• Saving/creating content (images, video, audio)
• Uploading data

Architecture

When we started building SYNQueue, persistence was the most important feature for us since we hadn’t seen a good generic implementation of it anywhere. With that in mind, we designed each SYNQueueTask (:NSOperation) to hold just metadata about the task rather than code itself.

The actual code to perform the task gets passed to the queue in the form of a taskHandler closure. Each SYNQueueTask must have a taskType key which corresponds to a specific taskHandler.

Example Code

For a thorough example see the demo project in the top level of the repository.

Create a queue

let queue = SYNQueue(queueName: "myQueue", maxConcurrency: 2, maxRetries: 3,
            logProvider: ConsoleLogger(), serializationProvider: NSUserDefaultsSerializer(),
            completionBlock: { [weak self] in self?.taskComplete($0, $1) })

The logProvider and serializationProvider must conform to the SYNQueueLogProvider and SYNQueueSerializationProvider protocols respectively.

See NSUserDefaultsSerializer.swift and ConsoleLogger.swift for example implementations.

The completionBlock is the block to run when a task in the queue completes (success or failure).

Create a task

let t1 = SYNQueueTask(queue: queue, taskID: "1234", taskType: "uploadPhoto", dependencyStrs: [], data: [:])

The queue is the queue you will add the task to. taskID is a unique ID for the task. taskType is the generic type of task to perform. Each taskType will have its own taskHandler. data is any metadata your task will need to perform its job.

Add dependencies

let t2 = SYNQueueTask(queue: queue, taskID: "5678", taskType: "submitForm", dependencyStrs: [], data: [:])
t2.addDependency(t1)

Add it to the queue

queue.addOperation(t2)
queue.addOperation(t1)

Notice that even though we add task t2 to the queue first, it will not execute until its dependency, t1 has finished executing.`

An important note on persistence

You may have realized that you are free to serialize tasks however you like through the SYNQueueSerializationProvider protocol. The one caveat is that all tasks must be idempotent. That is, even if called multiple times, the outcome of the task should be the same. For example: x = 1 is idempotent, x++ is not.

Hopefully this makes sense given that a serialized task may get interrupted before it finishes, and when we deserialize this task we will run it again. We only remove the serialized task after it has completed (success or failure).