GetPreferredImageOperation.swift

final class GetPreferredImageOperation: PSGroupOperation {
  
  private let taskOperation: PSURLSessionTaskOperation
  private let downsampleOperation: DownsampleImageOperation
  private let adapterOperation: BlockOperation
  private let finishOperation: BlockOperation
  
  let identifier: UUID
  
  init(
    identifier: UUID,
    underlyingQueue: DispatchQueue? = nil,
    session: URLSession? = nil,
    at url: URL,
    to pointSize: CGSize,
    withScale scale: CGFloat,
    completionHandler: ((UIImage?) -> Void)? = nil
  ) {
    self.identifier = identifier
    
    // Task
    var request = URLRequest(url: url)
    request.networkServiceType = .responsiveData
    
    if let session = session {
      taskOperation = PSURLSessionTaskOperation(session: session, request: request)
    } else {
      taskOperation = PSURLSessionTaskOperation(request: request)
    }
    
    // Downsample
    downsampleOperation = DownsampleImageOperation(under: pointSize, withScale: scale)
    downsampleOperation.add(PSNoCancelledDependencies())
    
    // Adapter
    adapterOperation = BlockOperation { [unowned taskOperation, unowned downsampleOperation] in
      Thread.current.name = "Fetch"
      downsampleOperation.downsamplingImageData = taskOperation.taskData
    }
    
    // Finish
    finishOperation = BlockOperation { [unowned downsampleOperation] in
      completionHandler?(downsampleOperation.downsampledImage)
    }
    
    // Add dependencies
    adapterOperation.addDependency(taskOperation)
    downsampleOperation.addDependency(adapterOperation)
    finishOperation.addDependency(downsampleOperation)
    
    // Super
    if let underlyingQueue = underlyingQueue {
      super.init(
        underlyingQueue: underlyingQueue,
        operations: [taskOperation, adapterOperation, downsampleOperation, finishOperation]
      )
    } else {
      super.init(operations: [taskOperation, adapterOperation, downsampleOperation, finishOperation])
    }
  }
}