Queuer

Queuer is a queue manager built on top of OperationQueue and Dispatch (aka GCD). It allows you to create asynchronous and synchronous tasks easily, all managed by a queue, with just a few lines.

Features

• Works on all Swift compatible platforms (Linux, Android, and Windows included)
• Easy to use
• Well documented (100% documented)
• Well tested (100% of code coverage)
• Create an operation block
• Create a single operation
• Create chained operations
• Manage a centralized queue
• Create unlimited queues
• Declare how many concurrent operations a queue can handle
• Create semaphores
• Create and handle schedules
• Automatically or manually retry an operation
• Throttling between each automatic operation retry
• Syntactic sugar to create and chain operations easier and faster
• Async/await operations with automatic retries on thrown errors
• Builds in the Swift 6 language mode with strict concurrency

Compatibility

[!NOTE] Some APIs require a newer OS than the minimum deployment target:

• AsyncConcurrentOperation, addChainedAsyncOperations(_:completionHandler:), addAsyncCompletionHandler(_:), addBarrier(_:), and barrier(_:) require macOS 10.15, iOS 13, tvOS 13, or watchOS 6.
• asyncWait(_:tolerance:clock:) requires macOS 13, iOS 16, tvOS 16, or watchOS 9.

On Linux, Android, and Windows every API is always available.

Installing

See Requirements section to check Swift, Queuer, and OS versions.

In your Package.swift Swift Package Manager manifest, add the following dependency to your dependencies argument:

.package(url: "https://github.com/FabrizioBrancati/Queuer.git", from: "4.0.0"),

Add the dependency to any targets you've declared in your manifest:

.target(
    name: "MyTarget", 
    dependencies: [
        .product(name: "Queuer", package: "Queuer"),
    ]
),

Usage

• Using the Shared Queuer
• Create a Custom Queue
• Create an Operation Block
• Use Chained Operations
• Use Group Operations
• Available Queue States
• Control Operation States
• Use a Synchronous Queue
• Create a Custom Operation
• Automatically Retry an Operation
• Throttle Automatic Retries
• Manually Retry an Operation
• Manually Finish an Operation
• Async Task in an Operation
• Create an Async Operation
• Set Up a Scheduler
• Use a Semaphore
• Syntactic Sugar

Using the Shared Queuer

Queuer offers a shared instance that you can use to add operations to a centralized queue:

Queuer.shared.addOperation(operation)

Create a Custom Queue

You can also create a custom queue:

let queue = Queuer(name: "MyCustomQueue")

You can even create a queue by defining the maxConcurrentOperationCount and the qualityOfService properties:

let queue = Queuer(name: "MyCustomQueue", maxConcurrentOperationCount: Int.max, qualityOfService: .default)

Create an Operation Block

You have three methods to add an Operation block.

1. Directly on the queue(or Queuer.shared):

   queue.addOperation {
       /// Your task here
   }
   

2. Creating a ConcurrentOperation with a block:

   let concurrentOperation = ConcurrentOperation { _ in
       /// Your task here
   }
   queue.addOperation(concurrentOperation)
   

[!NOTE] We will see how ConcurrentOperation works later.

Use Chained Operations

Chained Operations are Operations that add a dependency each other.

They follow the given array order, for example: [A, B, C] = A -> B -> C -> completionBlock.

let concurrentOperationA = ConcurrentOperation { _ in
    /// Your task A here
}
let concurrentOperationB = ConcurrentOperation { _ in
    /// Your task B here
}
queue.addChainedOperations([concurrentOperationA, concurrentOperationB]) {
    /// Your completion task here
}

You can also add a completionHandler after the queue creation with:

queue.addCompletionHandler {
    /// Your completion task here
}

Use Group Operations

Group Operations are Operations that handles a group of Operations with a completion handler.

Allows the execution of a block of Operations with a completion handler that will be called once all the operations are finished, for example: [A -> [[B & C & D] -> completionHandler] -> E] -> completionHandler. It should usually be used with a Chained Opetation.

let groupOperationA = GroupOperation(
    [
        ConcurrentOperation { _ in
            /// Your task A here
        },
        ConcurrentOperation { _ in
            /// Your task B here
        }
    ]
)

let concurrentOperationC = ConcurrentOperation { _ in
    /// Your task C here
}

queue.addChainedOperations([groupOperationA, concurrentOperationC]) {
    /// Your completion task here
}

In this case the output will be the following one: [[A & B -> completionHandler] -> C] -> completionHandler.

Available Queue States

There are a few method to handle the queue states.

1. Cancel all Operations in a queue:

   queue.cancelAll()
   

2. Pause a queue:

   queue.pause()
   

[!WARNING] By calling pause() you will not be sure that every Operation will be paused. If the Operation is already started it will not be on pause until it's a custom Operation that overrides pause() function.

3. Resume a queue:

   queue.resume()
   

[!WARNING] To have a complete pause and resume states you must create a custom Operation that overrides pause() and resume() function.

4. Wait until all Operations are finished:

   queue.waitUntilAllOperationsAreFinished()
   

[!IMPORTANT] This function means that the queue will blocks the current thread until all Operations are finished.

Control Operation States

You can control the ConcurrentOperation states by calling pause(), resume(), cancel().

[!NOTE] If you use a Queuer object with all ConcurrentOperation objects inside to manage your queue, you should avoid calling them directly on the ConcurrentOperation object, but you should call them on the Queuer one. The Queuer object will handle the ConcurrentOperation states automatically.

1. Pause a ConcurrentOperation:

   concurrentOperation.pause()
   

2. Resume a ConcurrentOperation:

   concurrentOperation.resume()
   

3. Cancel a ConcurrentOperation:

   concurrentOperation.cancel()
   

For convenience, you can set closures to the ConcurrentOperation object to handle the states with onPause, onResume, and onCancel properties. They will be called when the ConcurrentOperation is paused, resumed, or canceled.

Use a Synchronous Queue

Setting the maxConcurrentOperationCount property of a queue to 1 will make you sure that only one task at a time will be executed.

Create a Custom Operation

If you want to create your custom Operation, the class ConcurrentOperation is a class ready to be subclassed. It allows synchronous and asynchronous tasks, has a pause and resume states, can be easily added to a queue and can be created with a block.

You can create your custom ConcurrentOperation by subclassing it.

You must override execute() function and call the finish(success:) function, when the task has finished its job to notify the queue, regardless if it was successful or not.

Automatically Retry an Operation

An Operation is passed to every closure, with it you can set and handle the retry feature.

By default the retry feature is disabled, to enable it simply set the success property to false. With success to false the Operation will retry until reaches maximumRetries property value. To let the Operation know when everything is ok, you must set success to true.

With currentAttempt you can know at which attempt the Operation is.

let concurrentOperation = ConcurrentOperation { operation in
    /// Your task here
    if /* Successful */ {
      operation.success = true
    } else {
      operation.success = false
    }
}

Throttle Automatic Retries

You can throttle the automatic retries with the retryDelay property. Every retry waits the given time before executing, the first attempt is never delayed. Default is 0, retries happen immediately.

let concurrentOperation = ConcurrentOperation { operation in
    /// Your task here
    operation.success = false
}
concurrentOperation.retryDelay = 1

The same property is available on AsyncConcurrentOperation, where canceling the Operation also interrupts the delay right away:

let concurrentOperation = AsyncConcurrentOperation { operation in
    /// Your asynchronous task here
    operation.success = false
}
concurrentOperation.retryDelay = 1

You can also set it with the chainable syntactic sugar helper:

let concurrentOperation = ConcurrentOperation()
    .retryDelay(1)
    .executionBlock { operation in
        /// Your task here
        operation.success = false
    }

Manually Retry an Operation

You can manually retry an Operation when you think that the execution will be successful.

An Operation is passed to every closure, with it you can set and handle the retry feature.

By default the manual retry feature is disabled, to enable it simply set the manualRetry property to true, you must do this outside of the execution closure. You must also set success to true or false to let the Operation know when is everything ok, like the automatic retry feature.

To let the Operation retry your execution closure, you have to call the retry() function. Be sure to call it at least maximumRetries times, it is not a problem if you call retry() more times than is needed, your execution closure will not be executed more times than the maximumRetries value.

let concurrentOperation = ConcurrentOperation { operation in
    /// Your task here
    if /* Successful */ {
      operation.success = true
    } else {
      operation.success = false
    }
}
concurrentOperation.manualRetry = true

/// Later on your code
concurrentOperation.retry()

[!CAUTION] If the retry() function is not called, you may block the entire queue.

Manually Finish an Operation

By default the Operation will finish its job when the execution closure is completed.

This means, if you have an asynchronous task insiede the closure, the Operation will finish before the task is completed.

If you want to finish it manually, you can set the manualFinish property to true and call the finish(success:) function when the task is completed.

Call the finish(success:) function with success parameter to true or false to let the Operation know if the task was successful or not. If you don't explicitly set the success parameter, it will be set to true.

let concurrentOperation = ConcurrentOperation { operation in
    /// Your asynchonous task here
}
concurrentOperation.manualFinish = true

/// Later on your code in your asynchronous task
concurrentOperation.finish(success: true)

[!CAUTION] If you don't call the finish(success:) function, your queue will be blocked and it will never ends.

Async Task in an Operation

If you want to use async/await tasks, you need to set manualFinish to true in order to be fully in control of the Operation lifecycle.

[!NOTE] Read more about manually finish an Operation here.

let concurrentOperation = ConcurrentOperation { operation in
    Task {
        /// Your asynchonous task here
        operation.finish(success: true) // or false
    }
    /// Your asynchonous task here
}
concurrentOperation.manualFinish = true

[!CAUTION] If you don't set manualFinish to true, your Operation will finish before the async task is completed.

Create an Async Operation

AsyncConcurrentOperation is an Operation built for async/await, available on macOS 10.15, iOS 13, tvOS 13, and watchOS 6 or later. Its execution block is an async throwing function: a thrown error marks the attempt as failed and enables the retry feature, and canceling the Operation also cancels the Task running its block.

let concurrentOperation = AsyncConcurrentOperation { operation in
    /// Your asynchronous task here, it can throw
    try await yourAsyncTask()
}
concurrentOperation.addToQueue(queue)

You can chain them with an async completion handler:

queue.addChainedAsyncOperations([concurrentOperation1, concurrentOperation2]) {
    /// Your asynchronous completion here
    await yourAsyncCompletion()
}

Set Up a Scheduler

A Scheduler is a struct that uses the GDC's DispatchSourceTimer to create a timer that can execute functions with a specified interval and quality of service.

let schedule = Scheduler(deadline: .now(), repeating: .seconds(1)) {
    /// Your task here
}

You can even create a Scheduler without the handler and set it later:

var schedule = Scheduler(deadline: .now(), repeating: .seconds(1))
schedule.setHandler {
    /// Your task here
}

With timer property you can access to all DispatchSourceTimer properties and functions, like cancel():

schedule.timer.cancel()

Use a Semaphore

A Semaphore is a struct that uses the GCD's DispatchSemaphore to create a semaphore on the function and wait until it finish its job.

[!TIP] Is recommend to use a defer { semaphore.continue() } right after the Semaphore creation and wait() call.

let semaphore = Semaphore()
semaphore.wait()
defer { semaphore.continue() }
/// Your task here

You can even set a custom timeout, default is .distantFuture:

semaphore.wait(DispatchTime(uptimeNanoseconds: 1_000_000_000))

It's more useful if used inside an asynchronous task:

let concurrentOperation = ConcurrentOperation {
    /// Your task here
    semaphore.continue()
}
concurrentOperation.addToQueue(queue)
semaphore.wait()

Syntactic Sugar

Queuer offers a set of chainable helpers to create queues and operations in a more compact way. Every helper returns its instance, so the calls can be chained:

Queuer(name: "MyQueue")
    .maxConcurrentOperationCount(1)
    .qualityOfService(.background)
    .concurrent { _ in
        /// Your task here
    }
    .concurrent(retries: 2) { operation in
        /// Your retryable task here
        operation.success = false
    }
    .completion {
        /// Executed once the last operation currently in the queue is finished
    }
    .chained(operation1, operation2)
    .chained(
        { _ in
            /// First chained task here
        },
        { _ in
            /// Second chained task here
        }
    )
    .group(concurrentOperation1, concurrentOperation2)
    .completion {
        /// Executed at the end
    }

ConcurrentOperations can be configured with the same style:

let operation = ConcurrentOperation()
    .name("MyOperation")
    .queuePriority(.high)
    .qualityOfService(.utility)
    .manualFinish()
    .manualRetry()
    .maximumRetries(5)
    .executionBlock { operation in
        /// Your task here
        operation.finish()
    }
    .onPause { operation in
        /// Executed when the operation is paused
    }
    .onResume { operation in
        /// Executed when the operation is resumed
    }
    .onCancel { operation in
        /// Executed when the operation is canceled
    }

A barrier waits for all the operations currently in the queue and blocks every operation added after it, until the barrier is finished (requires macOS 10.15, iOS 13, tvOS 13 or watchOS 6):

queue
    .concurrent { _ in
        /// Your task here
    }
    .barrier {
        /// Executed once every operation added before the barrier is finished
    }
    .concurrent { _ in
        /// Executed after the barrier is finished
    }

You can also make the queue wait, either without blocking a thread (requires macOS 13, iOS 16, tvOS 16 or watchOS 9) or by blocking one of its threads:

queue
    .asyncWait(.seconds(1))
    .syncWait(1)
    .completion {
        /// Executed after both waits are finished
    }

AsyncConcurrentOperations have the same chainable helpers (requires macOS 10.15, iOS 13, tvOS 13 or watchOS 6):

queue
    .asyncConcurrent { _ in
        /// Your asynchronous task here, it can throw
        try await yourAsyncTask()
    }
    .asyncConcurrent(retries: 2) { operation in
        /// Your retryable asynchronous task here
        operation.success = false
    }
    .asyncCompletion {
        /// Your asynchronous completion here
        await yourAsyncCompletion()
    }

And they can be configured with the same fluent style:

let operation = AsyncConcurrentOperation()
    .name("MyAsyncOperation")
    .queuePriority(.high)
    .maximumRetries(5)
    .retryDelay(1)
    .executionBlock { operation in
        /// Your asynchronous task here, it can throw
        try await yourAsyncTask()
    }

Changelog

To see what has changed in recent versions of Queuer, see the CHANGELOG.md file.

Communication

• If you need help, open an issue.
• If you found a bug, open an issue.
• If you have a feature request, open an issue.
• If you want to contribute, see Contributing section.

Contributing

See CONTRIBUTING.md file.

License

Queuer is available under the MIT license. See the LICENSE file for more info.