iOS Concurrency with GCD & Operations

Sep 12 2023 · Swift 5.8, macOS 13, iOS 16, Xcode 14.3

Part 3: Operations & OperationQueues

15. Operations

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Previous episode: 14. Challenge: Make Number Class Thread-Safe Next episode: 16. OperationQueues

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Operations are built on top of GCD, adding features like dependencies on other operations, the ability to cancel the running operation, and an object-oriented model to support more complex requirements.

BlockOperation

In the starter playground, here’s a variable to hold the result of adding two numbers:

var result: Int?

let sumOperation = BlockOperation(   // pause to see initializers

let sumOperation = BlockOperation {

}

result = 2 + 3

sumOperation.start()

result

<NSBlockOperation 0x7fc623406b30 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>
5

sleep(2)  // below result = 2 + 3

duration {
  sumOperation.start()  // existing
}

<NSBlockOperation 0x7fc623406b30 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>
5
0
2.005437970161438
()
5

let multiPrinter = BlockOperation()
multiPrinter.addExecutionBlock {  print("Hello"); sleep(2) }
multiPrinter.addExecutionBlock {  print("my"); sleep(2) }
multiPrinter.addExecutionBlock {  print("name"); sleep(2) }
multiPrinter.addExecutionBlock {  print("is"); sleep(2) }
multiPrinter.addExecutionBlock {  print("Audrey"); sleep(2) }

duration {
  multiPrinter.start()
}

2.00261402130127

is
name
Hello
my
Audrey

// right after let multiPrinter = BlockOperation()
multiPrinter.completionBlock = {
  print("Finished multiPrinting!")
}

name
is
my
Hello
Audrey
Finished multiPrinting!

Subclassing Operation

In Subclassing Operation, click the run button on the inputImage line, then tap the Show result button to see the input image.

let inputImage = UIImage(named: "dark_road_small.jpg")

w 500 h 574 (in side pane)

Create TiltShiftOperation

Double-click on the playground name to go back there. Here’s a TiltShiftOperation class — a subclass of Operation — with a CIContext property — you’ll need this to convert the filter’s output to a CGImage:

class TiltShiftOperation: Operation {
  private static let context = CIContext()
...
}

private let inputImage: UIImage?
var outputImage: UIImage?

init(image: UIImage?) {
  inputImage = image
  super.init()
}

guard let inputImage,
      let filter = TiltShiftFilter(image: inputImage),
      let output = filter.outputImage else {
  print("Failed to generate tilt shift image")
  return
}

private static let context = CIContext()  // scroll up to top of class

let fromRect = CGRect(origin: .zero, size: inputImage.size)

guard let cgImage =
        TiltShiftOperation.context.createCGImage(
          output,
          from: fromRect)
else {
  print("No image generated")
  return
}

outputImage = UIImage(cgImage: cgImage)

Run TiltShiftOperation

So now, you’ve encapsulated all the functionality the operation should do. To use your new operation subclass, instantiate one with your inputImage:

let tsOp = TiltShiftOperation(image: inputImage)

duration {
  tsOp.start()
}

tsOp.outputImage

0.3255159854888916
...
w 500 h 574