Swift Concurrency Continuations: Getting Began

Apple drastically improved learn how to write asynchronous code in Swift with the introduction of Swift Concurrency and the async/await API. Additionally they launched the Continuation API, and you need to use this instead of delegates and completion callbacks. You possibly can drastically streamline your code by mastering and utilizing this API.

You’ll be taught all concerning the Continuation API on this tutorial. Particularly, you’ll replace the tutorial app WhatsApp to make use of the Continuation API as an alternative of legacy patterns. You’ll be taught the next alongside the way in which:

• What the Continuation API is and the way it works
• wrap a delegate-based API part and supply an async interface for it
• present an async API by way of an extension for elements that use completion callbacks
• use the async API instead of legacy patterns

Getting Began

Obtain the starter undertaking by clicking the Obtain Supplies button on the high or backside of this tutorial.

Open WhatsThat from the starter folder, and construct and run.

WhatsThat is an image-classifier app. You choose a picture, and it supplies a picture description in return.

Right here above is Zohar, beloved Brittany Spaniel — in response to the classifier mannequin :]

The app makes use of one of many commonplace CoreML neural fashions to find out the picture’s foremost topic. Nevertheless, the mannequin’s dedication may very well be incorrect, so it additionally offers a detection accuracy share. The upper the proportion, the extra possible the mannequin believes its prediction is correct.

You possibly can both use the default pictures, or you may drag-and-drop your personal pictures into the simulator’s Photographs app. Both means, you’ll see the obtainable pictures in WhatsThat’s picture picker.

Check out the undertaking file hierarchy, and also you’ll discover these core information:

• AppMain.swift launches the SwiftUI interface.
• Display is a gaggle containing three SwiftUI views.
• ContentView.swift accommodates the primary app display.
• ImageView.swift defines the picture view utilized in the primary display.
• ImagePickerView.swift is a SwiftUI wrapper round a UIKit UIImagePickerController.

The Continuation API

As a quick refresher, Swift Concurrency lets you add async to a way signature and name await to deal with asynchronous code. For instance, you may write an asynchronous networking methodology like this:

// 1
non-public func fetchData(url: URL) async throws -> Information {

  // 2
  let (information, response) = attempt await URLSession.shared.information(from: url)

  // 3
  guard let response = response as? HTTPURLResponse, response.isOk else {
    throw URLError(.badServerResponse)
  }
  return information
}

Right here’s how this works:

1. You point out this methodology makes use of the async/await API by declaring async on its signature.
2. The await instruction is named a “suspension level.” Right here, you inform the system to droop the strategy when await is encountered and start downloading information on a unique thread.

Swift shops the state of the present operate in a heap, making a “continuation.” Right here, as soon as URLSession finishes downloading the information, the continuation is resumed, and the execution continues from the place it was stopped.

When working with async/await, the system robotically manages continuations for you. As a result of Swift, and UIKit particularly, closely use delegates and completion callbacks, Apple launched the Continuation API that can assist you transition present code utilizing an async interface. Let’s go over how this works intimately.

Suspending The Execution

SE-0300: Continuations for interfacing async duties with synchronous code defines 4 totally different features to droop the execution and create a continuation.

• withCheckedContinuation(_:)
• withCheckedThrowingContinuation(_:)
• withUnsafeContinuation(_:)
• withUnsafeThrowingContinuation(_:)

As you may see, the framework supplies two variants of APIs of the identical features.

• with*Continuation supplies a non-throwing context continuation
• with*ThrowingContinuation additionally permits throwing exceptions within the continuations

The distinction between Checked and Unsafe lies in how the API verifies correct use of the resume operate. You’ll study this later, so preserve studying… ;]

Resuming The Execution

To renew the execution, you’re purported to name the continuation supplied by the operate above as soon as, and solely as soon as, by utilizing one of many following continuation features:

• resume() resumes the execution with out returning a outcome, e.g. for an async operate returning Void.
• resume(returning:) resumes the execution returning the required argument.
• resume(throwing:) resumes the execution throwing an exception and is used for ThrowingContinuation solely.
• resume(with:) resumes the execution passing a End result object.

Okay, that’s sufficient for idea! Let’s leap proper into utilizing the Continuation API.

Changing Delegate-Primarily based APIs with Continuation

You’ll first wrap a delegate-based API and supply an async interface for it.

Have a look at the UIImagePickerController part from Apple. To deal with the asynchronicity of the interface, you set a delegate, current the picture picker after which look ahead to the person to select a picture or cancel. When the person selects a picture, the framework informs the app by way of its delegate callback.

Regardless that Apple now supplies the PhotosPickerUI SwiftUI part, offering an async interface to UIImagePickerController remains to be related. For instance, it’s possible you’ll must assist an older iOS or could have custom-made the stream with a selected picker design you wish to preserve.

The thought is so as to add a wrapper object that implements the UIImagePickerController delegate interface on one aspect and presents the async API to exterior callers.

Good day Picture Picker Service

Add a brand new file to the Providers group and title it ImagePickerService.swift.

Exchange the content material of ImagePickerService.swift with this:

import OSLog
import UIKit.UIImage

class ImagePickerService: NSObject {
  non-public var continuation: CheckedContinuation<UIImage?, By no means>?

  func pickImage() async -> UIImage? {
    // 1
    return await withCheckedContinuation { continuation in
      if self.continuation == nil {
        // 2
        self.continuation = continuation
      }
    }
  }
}

// MARK: - Picture Picker Delegate
extension ImagePickerService: UIImagePickerControllerDelegate, UINavigationControllerDelegate {
  func imagePickerController(
    _ picker: UIImagePickerController,
    didFinishPickingMediaWithInfo information: [UIImagePickerController.InfoKey: Any]
  ) {
    Logger.foremost.debug("Consumer picked picture")
    // 3
    continuation?.resume(returning: information[.originalImage] as? UIImage)
  }

  func imagePickerControllerDidCancel(_ picker: UIImagePickerController) {
    Logger.foremost.debug("Consumer canceled choosing up picture")
    // 4
    continuation?.resume(returning: UIImage())
  }
}

First, you’ll discover the pickImage() operate is async as a result of it wants to attend for customers to pick out a picture, and as soon as they do, return it.

Subsequent are these 4 factors of curiosity:

1. On hitting withCheckedContinuation the execution is suspended, and a continuation is created and handed to the completion handler. On this state of affairs, you employ the non-throwing variant as a result of the async operate pickImage() isn’t throwing.
2. The continuation is saved within the class so you may resume it later, as soon as the delegate returns.
3. Then, as soon as the person selects a picture, the resume is named, passing the picture as argument.
4. If the person cancels choosing a picture, you come an empty picture — at the very least for now.

As soon as the execution is resumed, the picture returned from the continuation is returned to the caller of the pickImage() operate.

Utilizing Picture Picker Service

Open ContentViewModel.swift, and modify it as follows:

1. Take away the inheritance from NSObject on the ContentViewModel declaration. This isn’t required now that ImagePickerService implements UIImagePickerControllerDelegate.
2. Delete the corresponding extension implementing UIImagePickerControllerDelegate and UINavigationControllerDelegate features, you’ll find it below // MARK: - Picture Picker Delegate. Once more, these aren't required anymore for a similar purpose.

Then, add a property for the brand new service named imagePickerService below your noImageCaption and imageClassifierService variables. You may find yourself with these three variables within the high of ContentViewModel:

non-public static let noImageCaption = "Choose a picture to categorise"
non-public lazy var imageClassifierService = attempt? ImageClassifierService()
lazy var imagePickerService = ImagePickerService()

Lastly, change the earlier implementation of pickImage() with this one:

@MainActor
func pickImage() {
  presentImagePicker = true

  Activity(precedence: .userInitiated) {
    let picture = await imagePickerService.pickImage()
    presentImagePicker = false

    if let picture {
      self.picture = picture
      classifyImage(picture)
    }
  }
}

As pickImage() is a synchronous operate, you have to use a Activity to wrap the asynchronous content material. Since you’re coping with UI right here, you create the duty with a userInitiated precedence.

The @MainActor attribute can be required since you’re updating the UI, self.picture right here.

After all of the adjustments, your ContentViewModel ought to appear to be this:

class ContentViewModel: ObservableObject {
  non-public static let noImageCaption = "Choose a picture to categorise"
  non-public lazy var imageClassifierService = attempt? ImageClassifierService()
  lazy var imagePickerService = ImagePickerService()

  @Printed var presentImagePicker = false
  @Printed non-public(set) var picture: UIImage?
  @Printed non-public(set) var caption = noImageCaption

  @MainActor
  func pickImage() {
    presentImagePicker = true

    Activity(precedence: .userInitiated) {
      let picture = await imagePickerService.pickImage()
      presentImagePicker = false

      if let picture {
        self.picture = picture
        classifyImage(picture)
      }
    }
  }

  non-public func classifyImage(_ picture: UIImage) {
    caption = "Classifying..."
    guard let imageClassifierService else {
      Logger.foremost.error("Picture classification service lacking!")
      caption = "Error initializing Neural Mannequin"
      return
    }

    DispatchQueue.international(qos: .userInteractive).async {
      imageClassifierService.classifyImage(picture) { end in
        let caption: String
        swap outcome {
        case .success(let classification):
          let description = classification.description
          Logger.foremost.debug("Picture classification outcome: (description)")
          caption = description
        case .failure(let error):
          Logger.foremost.error(
            "Picture classification failed with: (error.localizedDescription)"
          )
          caption = "Picture classification error"
        }

        DispatchQueue.foremost.async {
          self.caption = caption
        }
      }
    }
  }
}

Lastly, that you must change the UIImagePickerController‘s delegate in ContentView.swift to level to the brand new delegate.

To take action, change the .sheet with this:

.sheet(isPresented: $contentViewModel.presentImagePicker) {
  ImagePickerView(delegate: contentViewModel.imagePickerService)
}

Construct and run. It is best to see the picture picker working as earlier than, however it now makes use of a contemporary syntax that is simpler to learn.

Continuation Checks

Sadly, there’s an error within the code above!

Open the Xcode Debug pane window and run the app.

Now, choose a picture, and you need to see the corresponding classification. If you faucet Choose Picture once more to select one other picture, Xcode offers the next error:

Swift prints this error as a result of the app is reusing a continuation already used for the primary picture, and the usual explicitly forbids this! Bear in mind, you have to use a continuation as soon as, and solely as soon as.

When utilizing the Checked continuation, the compiler provides code to implement this rule. When utilizing the Unsafe APIs and also you name the resume greater than as soon as, nonetheless, the app will crash! For those who neglect to name it in any respect, the operate by no means resumes.

Though there should not be a noticeable overhead when utilizing the Checked API, it is definitely worth the value for the added security. As a default, desire to make use of the Checked API. If you wish to eliminate the runtime checks, use the Checked continuation throughout improvement after which swap to the Unsafe when transport the app.

Open ImagePickerService.swift, and you will see the pickImage now appears to be like like this:

func pickImage() async -> UIImage? {
  return await withCheckedContinuation { continuation in
    if self.continuation == nil {
      self.continuation = continuation
    }
  }
}

It’s essential make two adjustments to repair the error herein.

First, all the time assign the handed continuation, so that you must take away the if assertion, ensuing on this:

func pickImage() async -> UIImage? {
  await withCheckedContinuation { continuation in
    self.continuation = continuation
  }
}

Second, set the set the continuation to nil after utilizing it:

func imagePickerController(
  _ picker: UIImagePickerController,
  didFinishPickingMediaWithInfo information: [UIImagePickerController.InfoKey: Any]
) {
  Logger.foremost.debug("Consumer picked picture")
  continuation?.resume(returning: information[.originalImage] as? UIImage)
  // Reset continuation to nil
  continuation = nil
}

func imagePickerControllerDidCancel(_ picker: UIImagePickerController) {
  Logger.foremost.debug("Consumer canceled choosing up picture")
  continuation?.resume(returning: UIImage())
  // Reset continuation to nil
  continuation = nil
}

Construct and run and confirm which you could choose as many pictures as you want with out hitting any continuation-leak error.

Changing Callback-Primarily based APIs with Continuation

Time to maneuver on and modernize the remaining a part of ContentViewModel by changing the completion handler within the classifyImage(:) operate with a sleeker async name.

As you probably did for refactoring UIImagePickerController, you will create a wrapper part that wraps the ImageClassifierService and exposes an async API to ContentViewModel.

On this case, although, you may also prolong the ImageClassifier itself with an async extension.

Open ImageClassifierService.swift and add the next code on the finish:

// MARK: - Async/Await API
extension ImageClassifierService {
  func classifyImage(_ picture: UIImage) async throws -> ImageClassifierService.Classification {
    // 1
    return attempt await withCheckedThrowingContinuation { continuation in
      // 2
      classifyImage(picture) { end in
        // 3
        if case let .success(classification) = outcome {
          continuation.resume(returning: classification)
          return
        }
      }
    }
  }
}

Here is a rundown of the code:

1. As within the earlier case, the system blocks the execution on hitting the await withCheckedThrowingContinuation.
2. You needn’t retailer the continuation as within the earlier case since you’ll use it within the completion handler. Simply name the previous callback-based API and look ahead to the outcome.
3. As soon as the part invokes the completion callback, you name continuation.resume<(returning:) passing again the classification acquired.

Including an extension to the previous interface permits use of the 2 APIs concurrently. For instance, you can begin writing new code utilizing the async/await API with out having to rewrite present code that also makes use of the completion callback API.

You employ a Throwing continuation to mirror that the ImageClassifierService can throw an exception if one thing goes mistaken.

Utilizing Async ClassifyImage

Now that ImageClassifierService helps async/await, it is time to change the previous implementation and simplify the code. Open ContentViewModel.swift and alter the classifyImage(_:) operate to this:

@MainActor
non-public func classifyImage(_ picture: UIImage) async {
  guard let imageClassifierService else {
    Logger.foremost.error("Picture classification service lacking!")
    caption = "Error initializing Neural Mannequin"
    return
  }

  do {
    // 1
    let classification = attempt await imageClassifierService.classifyImage(picture)
    // 2
    let classificationDescription = classification.description
    Logger.foremost.debug(
      "Picture classification outcome: (classificationDescription)"
    )
    // 3
    caption = classificationDescription
  } catch let error {
    Logger.foremost.error(
      "Picture classification failed with: (error.localizedDescription)"
    )
    caption = "Picture classification error"
  }
}

Here is what is going on on:

1. You now name the ImageClassifierService.classifyImage(_:) operate asynchronously, which means the execution will pause till the mannequin has analyzed the picture.
2. As soon as that occurs, the operate will resume utilizing the continuation to the code beneath the await.
3. When you’ve gotten a classification, you need to use that to replace caption with the classification outcome.

Observe: In an actual app, you’d additionally wish to intercept any throwing exceptions at this degree and replace the picture caption with an error message if the classification fails.

There’s one last change earlier than you are prepared to check the brand new code. Since classifyImage(_:) is now an async operate, that you must name it utilizing await.

Nonetheless in ContentViewModel.swift, within the pickImage operate, add the await key phrase earlier than calling the classifyImage(_:) operate.

@MainActor
func pickImage() {
  presentImagePicker = true

  Activity(precedence: .userInitiated) {
    let picture = await imagePickerService.pickImage()
    presentImagePicker = false

    if let picture {
      self.picture = picture
      await classifyImage(picture)
    }
  }
}

Since you’re already in a Activity context, you may name the async operate instantly.

Now construct and run, attempt choosing a picture another time, and confirm that every little thing works as earlier than.

Dealing With Continuation Checks … Once more?

You are nearly there, however a number of issues stay to deal with. :]

Open the Xcode debug space to see the app’s logs, run and faucet Choose Picture; this time, nonetheless, faucet Cancel and see what occurs within the logs window.

Continuation checks? Once more? Did not you repair this already?

Properly, that was a unique state of affairs. Here is what’s occurring this time.

When you faucet Cancel, ImagePickerService returns an empty UIImage, which causes CoreML to throw an exception, not managed in ImageClassificationService.

Opposite to the earlier case, this continuation’s resume is rarely referred to as, and the code subsequently by no means returns.

To repair this, head again to the ImageClassifierService and modify the async wrapper to handle the case the place the mannequin throws an exception. To take action, you have to verify whether or not the outcomes returned within the completion handler are legitimate.

Open the ImageClassifierService.swift file and change the prevailing code of your async throwing classifyImage(_:) (the one within the extension) with this:

func classifyImage(_ picture: UIImage) async throws -> ImageClassifierService.Classification {
  return attempt await withCheckedThrowingContinuation { continuation in
    classifyImage(picture) { end in
      swap outcome {
      case .success(let classification):
        continuation.resume(returning: classification)
      case .failure(let error):
        continuation.resume(throwing: error)
      }
    }
  }
}

Right here you employ the extra continuation methodology resume(throwing:) that throws an exception within the calling methodology, passing the required error.

As a result of the case of returning a End result sort is widespread, Swift additionally supplies a devoted, extra compact instruction, resume(with:) permitting you to scale back what’s detailed above to this as an alternative:

func classifyImage(_ picture: UIImage) async throws -> ImageClassifierService.Classification {
  return attempt await withCheckedThrowingContinuation { continuation in
    classifyImage(picture) { end in
      continuation.resume(with: outcome)
    }
  }
}

Gotta adore it! Now, construct and run and retry the stream the place the person cancels choosing a picture. This time, no warnings can be within the console.

One Ultimate Repair

Though the warning about missing continuation is gone, some UI weirdness stays. Run the app, choose a picture, then attempt choosing one other one and faucet Cancel on this second picture.

As you see, the earlier picture is deleted, when you may desire to keep up it if the person already chosen one.

The ultimate repair consists of fixing the ImagePickerService imagePickerControllerDidCancel(:) delegate methodology to return nil as an alternative of an empty picture.

Open the file ImagePickerService.swift and make the next change.

func imagePickerControllerDidCancel(_ picker: UIImagePickerController) {
  Logger.foremost.debug("Consumer canceled choosing a picture")
  continuation?.resume(returning: nil)
  continuation = nil
}

With this final modification, if the person cancels choosing up a picture, the pickImage() operate of ImagePickerService returns nil, which means ContentViewModel will skip setting the picture and calling classifyImage(_:) in any respect.

Construct and run one final time and confirm the bug is gone.

The place to Go From Right here?

Properly finished! You streamlined your code and now have a constant code fashion in ContentViewModel.

You began with a ContentViewModel that contained totally different code kinds and needed to conform to NSObject because of delegate necessities. Little by little, you refactored this to have a contemporary and easier-to-follow implementation utilizing the async/await Continuation API.

Particularly, you:

• Changed the delegate-based part with an object that wraps the delegate and exposes an async operate.
• Made an async extension for completion handler-based part to permit a gradual rewrite of present elements of the app.
• Realized the variations between utilizing Checked and Unsafe continuations and learn how to deal with the corresponding verify errors.
• Had been launched to the forms of continuation features, together with async and async throwing.
• Lastly, you noticed learn how to resume the execution utilizing the resume directions and return a worth from a continuation context.

It was a enjoyable run, but as all the time, that is just the start of the journey. :]

To be taught extra concerning the Continuation API and the small print of the Swift Concurrency APIs, have a look at the Fashionable Concurrency in Swift e book.

You possibly can obtain the entire undertaking utilizing the Obtain Supplies button on the high or backside of this tutorial.

We hope you loved this tutorial. When you’ve got any questions, recommendations, feedback or suggestions, please be part of the discussion board dialogue beneath!