Swift wrapper for Grand Central Dispatch (GCD)

dispatch.swift

//
//  dispatch.swift
//
//  Created by Adam Wróbel. Read more at:
//  http://adamwrobel.com/blog/2016/05/08/swift-gcd-wrapper/
//

import Foundation

internal extension dispatch_queue_t {
  /// Submits a block for asynchronous execution on this queue and returns
  /// immediately.
  final func async(block: dispatch_block_t) {
    dispatch_async(self, block)
  }

  /// Submits a block for asynchronous execution on this queue and associates
  /// the block with given group.
  final func async(group: dispatch_group_t, block: dispatch_block_t) {
    dispatch_group_async(group, self, block)
  }

  /// Submits a block object for execution on this queue and waits until that
  /// block completes.
  ///
  /// Calling this method on the current queue results in deadlock.
  final func sync(block: dispatch_block_t) {
    dispatch_sync(self, block)
  }

  /// Submits a block object for execution on this queue and waits until that
  /// block completes.
  ///
  /// Prevents deadlock by executing the block immediately if called on the
  /// current queue. Performing this safety check adds a little overhead when
  /// compared with `sync`.
  final func safeSync(block: dispatch_block_t) {
    isCurrent() ? block() : sync(block)
  }

  /// Enqueue a block for execution after specified number of seconds.
  final func after(seconds: Double, block: dispatch_block_t) {
    let when = dispatch_time(DISPATCH_TIME_NOW, Int64(seconds * 1_000_000_000))
    dispatch_after(when, self, block)
  }

  /// Asynchronously runs a block on this queue when the group's tasks are
  /// complete.
  final func after(group: dispatch_group_t, block: dispatch_block_t) {
    group.notify(self, block: block)
  }
}

internal extension dispatch_group_t {
  /// Waits until all of the group's tasks are complete.
  final func wait() {
    dispatch_group_wait(self, DISPATCH_TIME_FOREVER)
  }

  /// Asynchronously runs a block on the main queue when the group's tasks are
  /// complete.
  final func notify(block: dispatch_block_t) {
    dispatch_group_notify(self, dispatch.main, block)
  }

  /// Asynchronously runs a block on the given queue when the group's tasks are
  /// complete.
  final func notify(queue: dispatch_queue_t, block: dispatch_block_t) {
    dispatch_group_notify(self, queue, block)
  }
}

final internal class dispatch {
  /// Serial queue associated with the application’s main thread.
  class var main: dispatch_queue_t {
    return dispatch_get_main_queue()
  }

  /// Parallel queue used for work that is not user initiated or visible.
  class var bg: dispatch_queue_t {
    return dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0)
  }

  /// Parallel queue used for work which the user is unlikely to be immediately
  /// waiting for the results.
  class var utility: dispatch_queue_t {
    return dispatch_get_global_queue(QOS_CLASS_UTILITY, 0)
  }

  /// Parallel queue used for work that has been explicitly requested by the
  /// user, and for which results must be immediately presented in order to
  /// allow for further user interaction.
  class var user: dispatch_queue_t {
    return dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)
  }

  /// Parallel queue used for work directly involved in providing an
  /// interactive UI.
  class var interactive: dispatch_queue_t {
    return dispatch_get_global_queue(QOS_CLASS_USER_INTERACTIVE, 0)
  }

  /// Creates a new group with which block objects can be associated.
  class func createGroup() -> dispatch_group_t {
    return dispatch_group_create()
  }
}

// Private extensions used exclusively to support `safeSync`

private extension dispatch {
  static var identityKey = UnsafePointer<Int8>(bitPattern: 1234)

  class var currentIdentity: UnsafeMutablePointer<Void> {
    return dispatch_get_specific(identityKey)
  }
}

private extension dispatch_queue_t {
  /// Uses queue specific context to determine if we're executing on this queue.
  func isCurrent() -> Bool {
    var ourIdentity = identity

    if ourIdentity == nil {
      ourIdentity = UnsafeMutablePointer<Void>(label)
      dispatch_queue_set_specific(self, dispatch.identityKey, ourIdentity, nil)
    }

    return ourIdentity == dispatch.currentIdentity
  }

  var label: UnsafePointer<Int8> {
    return dispatch_queue_get_label(self)
  }

  var identity: UnsafeMutablePointer<Void> {
    return dispatch_queue_get_specific(self, dispatch.identityKey)
  }
}

gcd-example.swift

// this will wait for completion and will deadlock when called from main queue
dispatch.interactive.sync {
  print("hello from main queue")
}

// this will return immediately
dispatch.utility.async {
  print("working with the Utility QoS")
}

// a safe sync operation that can be sent from any queue to any queue
dispatch.main.safeSync {
  print("safety in mind")
}

// this schedules a delayed block
dispatch.main.after(2.5) {
  printWithTime("hello from main after 2.5 seconds")
}

gcd-groups.swift

let group = dispatch.createGroup()

// schedule some asynchronous tasks
dispatch.interactive.async(group) {
  print("part of the job")
}
dispatch.user.async(group) {
  print("another part of the job")
}

// schedule asynchronous finalizer
group.notify {
  print("this will be executed when all of the group tasks complete")
}

// or synchronously wait for the group to complete
group.wait()

print("group completed")