NSOperation Subclass

Operation.swft

/// An NSOperation subclass with utilities for creating long-running operations or
/// operations that must pause and wait for some result, as well as a concise API
/// to make them easy to create, configure and add to background and main queues.
class Operation: NSOperation {
	
    /// Shared background queue
    static let backgroundQueue: NSOperationQueue = {
        var queue = NSOperationQueue()
        queue.maxConcurrentOperationCount = 5
        return queue
    }()
    
    private var _executing = false
    private var _finished = false
    
    /// Subclasses that do not implement `main()` and need to maintain excuting state call this to move into an excuting state.
    func beganExecuting () {
        executing = true
        finished = false
    }
    
    /// Subclasses that do not implement `main()` and need to maintain excuting state call this to move out of an executing state and are finished doing work.
    func finishedExecuting () {
        executing = false
        finished = true
    }
    
    var mainQueueCompletionBlock: (()->())? {
        didSet {
            if let completion = self.mainQueueCompletionBlock {
                self.completionBlock = {
                    dispatch_async( dispatch_get_main_queue(), completion )
                }
            }
        }
    }
    
    func queue( completion: (()->())? = nil ) {
        self.mainQueueCompletionBlock = completion
        dispatch_async( dispatch_get_main_queue() ) {
            NSOperationQueue.mainQueue().addOperation( self )
        }
    }
    
    func queueInBackground( completion: (()->())? = nil ) {
        self.mainQueueCompletionBlock = completion
        dispatch_async( dispatch_get_main_queue() ) {
            Operation.backgroundQueue.addOperation( self )
        }
    }
    
    // MARK: - KVO-able NSNotification State
    
    override var executing : Bool {
        get {return _executing }
        set {
            willChangeValueForKey("isExecuting")
            _executing = newValue
            didChangeValueForKey("isExecuting")
        }
    }
    
    override var finished : Bool {
        get {return _finished }
        set {
            willChangeValueForKey("isFinished")
            _finished = newValue
            didChangeValueForKey("isFinished")
        }
    }	
}

// MARK: - Usage exaples

// Main queue
MyOperation().queue() {
	print( "Main thread operation complete." )
}

// Background queue (with main queue completion closure)
MyOperation().queueInBackground() {
	print( "Background operation complete." )
}

// Using a custom queue as normal
var myQueue = NSOperationQueue()
let operation = MyOperation()
operation.mainQueueCompletionBlock = {
	print( "Background operation complete." )
}
myQueue.addOperation( operation )