Skip to main content

Implementing the Operation Class

Overview​

When dealing with complex or long-running tasks in Swift, managing these tasks through closures on dispatch queues can be limiting. If you need to reuse tasks, track their state, or handle them in an object-oriented manner, Operation classes provide a robust solution. This guide will show you how to convert your tasks into Operation objects for better modularity, state management, and reusability.

Why Use the Operation Class?​

The Operation class allows you to wrap tasks in an object-oriented structure, offering benefits such as:

  • Reusability: Encapsulate task logic within a class, making it reusable across your application.
  • State Management: Track the execution state (e.g., isExecuting, isFinished) of operations.
  • Dependencies: Easily manage dependencies between different operations.

Example Implementation: Photo Book App​

Let's take a practical example of a photo book creation app. We’ll convert tasks like image resizing and PDF generation into operations:

Step 1: Define the Photo Resize Operation​

Create a new Swift file called PhotoResizeOperation.swift and subclass the Operation class:

import UIKit

class PhotoResizeOperation: Operation {
    let resizer: PhotoResizer
    let size: CGSize
    let photos: NSMutableArray
    let photoIndex: Int

    init(resizer: PhotoResizer, size: CGSize, photos: NSMutableArray, photoIndex: Int) {
        self.resizer = resizer
        self.size = size
        self.photos = photos
        self.photoIndex = photoIndex
    }

    override func main() {
        guard !isCancelled else { return }
        guard let photo = photos[photoIndex] as? UIImage else { return }
        
        var resizedPhoto = resizer.scaleWithAspectFill([photo], to: size)
        guard !isCancelled else { return }
        
        resizedPhoto = resizer.centerCrop(resizedPhoto, to: size)
        photos[photoIndex] = resizedPhoto[0]
    }
}

Step 2: Create the Photo Book Generation Operation​

Similarly, create a GeneratePhotoBookOperation.swift file:

import UIKit

class GeneratePhotoBookOperation: Operation {
    let builder: PhotoBookBuilder
    let photos: NSMutableArray
    var photobookURL: URL?

    init(builder: PhotoBookBuilder, photos: NSMutableArray) {
        self.builder = builder
        self.photos = photos
    }

    override func main() {
        guard !isCancelled else { return }
        guard let photosArray = photos as? [UIImage] else { return }
        
        photobookURL = builder.buildPhotobook(with: photosArray)
    }
}

Step 3: Integrate Operations in View Controller​

In your PhotoCollectionViewController.swift, modify the generatePhotoBook method to use these operations:

let processingQueue = OperationQueue()

func generatePhotoBook(with photos: [UIImage], completion: @escaping (URL) -> Void) {
    let resizer = PhotoResizer()
    let builder = PhotoBookBuilder()
    let size = resizer.smallestCommonSize(for: photos)
    let processedPhotos = NSMutableArray(array: photos)
    
    let generateBookOp = GeneratePhotoBookOperation(builder: builder, photos: processedPhotos)
    
    for index in 0..<processedPhotos.count {
        let resizeOp = PhotoResizeOperation(resizer: resizer, size: size, photos: processedPhotos, photoIndex: index)
        generateBookOp.addDependency(resizeOp)
        processingQueue.addOperation(resizeOp)
    }

    generateBookOp.completionBlock = {
        guard let photobookURL = generateBookOp.photobookURL else { return }
        OperationQueue.main.addOperation {
            completion(photobookURL)
        }
    }

    processingQueue.addOperation(generateBookOp)
}

How It Works​

  1. State Management: Operations manage their state with properties like isReady, isExecuting, and isFinished.
  2. Dependencies: You can set up dependencies between operations, ensuring they execute in the correct order.
  3. Cancellation: By checking isCancelled within the main() method, operations can exit early if needed.

Adding Cancellation​

To enable operation cancellation:

  1. Add guard isCancelled == false checks within the main() methods of your operations.
  2. Allow users to cancel operations through UI controls, calling processingQueue.cancelAllOperations() to cancel ongoing tasks.

References​