How to Dispatch_Sync, Dispatch_Async, Dispatch_After, etc in Swift 3, Swift 4, and Beyond

How do I dispatch_sync, dispatch_async, dispatch_after, etc in Swift 3, Swift 4, and beyond?

Since the beginning, Swift has provided some facilities for making ObjC and C more Swifty, adding more with each version. Now, in Swift 3, the new "import as member" feature lets frameworks with certain styles of C API -- where you have a data type that works sort of like a class, and a bunch of global functions to work with it -- act more like Swift-native APIs. The data types import as Swift classes, their related global functions import as methods and properties on those classes, and some related things like sets of constants can become subtypes where appropriate.

In Xcode 8 / Swift 3 beta, Apple has applied this feature (along with a few others) to make the Dispatch framework much more Swifty. (And Core Graphics, too.) If you've been following the Swift open-source efforts, this isn't news, but now is the first time it's part of Xcode.

Your first step on moving any project to Swift 3 should be to open it in Xcode 8 and choose Edit > Convert > To Current Swift Syntax... in the menu. This will apply (with your review and approval) all of the changes at once needed for all the renamed APIs and other changes. (Often, a line of code is affected by more than one of these changes at once, so responding to error fix-its individually might not handle everything right.)

The result is that the common pattern for bouncing work to the background and back now looks like this:

// Move to a background thread to do some long running work
DispatchQueue.global(qos: .userInitiated).async {
    let image = self.loadOrGenerateAnImage()
    // Bounce back to the main thread to update the UI
    DispatchQueue.main.async {
        self.imageView.image = image
    }
}

Note we're using .userInitiated instead of one of the old DISPATCH_QUEUE_PRIORITY constants. Quality of Service (QoS) specifiers were introduced in OS X 10.10 / iOS 8.0, providing a clearer way for the system to prioritize work and deprecating the old priority specifiers. See Apple's docs on background work and energy efficiency for details.

By the way, if you're keeping your own queues to organize work, the way to get one now looks like this (notice that DispatchQueueAttributes is an OptionSet, so you use collection-style literals to combine options):

class Foo { 
    let queue = DispatchQueue(label: "com.example.my-serial-queue",
                           attributes: [.serial, .qosUtility])
    func doStuff() {
        queue.async {
            print("Hello World")
        }
    }
}

Using dispatch_after to do work later? That's a method on queues, too, and it takes a DispatchTime, which has operators for various numeric types so you can just add whole or fractional seconds:

DispatchQueue.main.asyncAfter(deadline: .now() + 0.5) { // in half a second...
    print("Are we there yet?")
}

You can find your way around the new Dispatch API by opening its interface in Xcode 8 -- use Open Quickly to find the Dispatch module, or put a symbol (like DispatchQueue) in your Swift project/playground and command-click it, then brouse around the module from there. (You can find the Swift Dispatch API in Apple's spiffy new API Reference website and in-Xcode doc viewer, but it looks like the doc content from the C version hasn't moved into it just yet.)

See the Migration Guide for more tips.

Swift 2 to 3 Migration dispatch_get_global_queue

Try this it will work.

DispatchQueue.global(qos: .background).async {

    DispatchQueue.main.async {

    }
})

If you still need any help feel free to ask here.

dispatch_async in Swift

You can use this syntax:

dispatch_async(dispatch_get_main_queue(), {
    println("hello")
})

However, when the last argument is a block, Swift lets you put it outside the parentheses. This makes the function seem more like a control structure (like a for or if statement). Thus you can do this:

dispatch_async(dispatch_get_main_queue()) {
    println("hello")
}

translate dispatch_async in swift

The swift 3 syntax is

DispatchQueue.main.async {
      //your code
 }

iOS - Is possible to combine Swift 3 and Swift 4 code within one project?

If you're using CocoaPods to manage your 3rd party dependencies then yes you can. The Swift version can be set in the Build Settings for the pod target in the pod project.

So you can have an app running Swift 4 that uses libraries built for Swift 3.2 fine.

If you're adding the 3rd party libs to your main project then it might not be possible, although you could extract the libs into their own projects (or private cocoapods) and add to your workspace and set the Swift version on each individually

And Xcode should have a warning prompting you to convert to Swift 4 which you can click, which should convert your project to Swift 4 automatically but you may end up having to fix some warnings/errors manually

Queues and Swift 3 issue

In Swift 3 You can write like this

DispatchQueue.global(qos: .background).async {

}

It means what every the code written in between the bracket will perform in the background. and if you want to make any changes in this background thread you have to switch to the main thread. by writing the block below.

dispatch_async(dispatch_get_main_queue()) { 
     // Your code for UI Changes.
}

EDIT: Swift 3

DispatchQueue.main.async {
}

dispatch_async vs dispatch_sync in fetch data. Swift

GCD provides queues for executing tasks. Queues can be of two types – concurrent or serial. In serial queue tasks execute one at time (in FIFO order), in concurrent queue several tasks at time.

To prevent user fetch data while one fetch task is running, it is needed to not submit fetch task to queue at this moment. No matter what kind of queue is it – concurrent or serial.

var array = [Int]()
var isFethingData = false

func fetchData() {
    if !isFethingData {
        isFethingData = true
        dispatch_async(queueForFetchData) {
            …
            response(objects: [object], error: NSError?) {
                for object in objects {
                    array.append(object.number)
                }

                dispatch_async(dispatch_get_main_queue()) {
                    makeCollectionView()
                }
                isFethingData = false
            }
    }
}

dispatch_async and dispatch_sync is functions which submits tasks to queue. The difference is that dispatch_async return immediately after task has been submitted, but dispatch_sync wait until task complete. For example:

print("\(NSDate()) qq")
dispatch_sync(queue) {
    // … some code which runs for 10 minutes.
    print("\(NSDate()) ee")
}
print("\(NSDate()) ww")
// 2016-08-18 16:02:00 qq
// 2016-08-18 16:12:00 ee
// 2016-08-18 16:12:00 ww


print("\(NSDate()) qq")
dispatch_async(queue) {
    // … some code which runs for 10 minutes.
    print("\(NSDate()) ee")
}
print("\(NSDate()) ww")
// 2016-08-18 16:02:00 qq
// 2016-08-18 16:02:00 ww
// 2016-08-18 16:12:00 ee

Where do I need to use dispatch_async on main_queue?

1. dispatch_get_main_queue() function will return the main queue where your UI is running.
2. The dispatch_get_main_queue() function is very useful for updating the iOS app’s UI as UIKit methods are not thread safe (with a few exceptions) so any calls you make to update UI elements must always be done from the main queue.

for more see this link

https://www.hackingwithswift.com/read/9/4/back-to-the-main-thread-dispatch_get_main_queue

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