Operation and OperationQueue



As the docs note, there are four things to override for your asynchronous swift subclass:

  • -start()
  • isAsynchronous
  • isExecuting
  • isFinished

And that you must send KVO notifications for the 2 properties (usually isAsynchronous is hardcoded to be true, so sending KVO for that is a non-issue).

Sending KVO means sending -willChangeValue(forKey:), then changing the value, then sending -didChangeValue(forKey:), see the following sample implementation:

class MyAsyncOperation: Operation {
    override func start() {
        self.willChangeValue(forKey: "isExecuting")

        someAsyncWork {
            self.willChangeValue(forKey: "isExecuting")
            self._isExecuting = false
            self.didChangeValue(forKey: "isExecuting")

            self.willChangeValue(forKey: "isFinished")
            self._isFinished = true
            self.didChangeValue(forKey: "isFinished")

        self._isExecuting = true
        self.didChangeValue(forKey: "isExecuting")

    override var isAsynchronous: Bool { return true }

    private var _isExecuting: Bool = false
    override var isExecuting: Bool { return !self.isFinished && self._isExecuting }

    private var _isFinished: Bool = false
    override var isFinished: Bool { return self._isFinished }