iOS swift Operation Queue

Question

I am working on iOS app where I am using OperationQueue. I have created 2 operations. Operation2 is dependent on completion of Operation1.Operation2 needs to wait until Operation 1 get finished if its running. If operation 1 is not running then operation 2 should start immediately.

Its not working as expected so I am testing in playground

    class MyManager {

        var operationQueue: OperationQueue?

        var operation1: MyOperation? = nil
        var operation2: MyOperation? = nil

        typealias completion = (_ serverError: String?) -> Void

        func talkWithServer(completion: completion?) {

            completion?("competed!")
        }

        func doOperation1() {

            cancelProcess()
            setup()

            guard let operation1 = self.operation1 else { return }
            operation1.codeToRun = {
                print("operation1 started")
                self.talkWithServer(completion: { (completion) in
                    print("operation1 completed")
                    operation1.markAsFinished()
                })
            }
            operationQueue?.addOperation(operation1)

        }


        func doOperation2() {
           self.operation2 = MyOperation()

            guard let operation2 = self.operation2 else { return }



            operation2.codeToRun = {
                print("operation2 started")
                self.talkWithServer(completion: { (completion) in
                    print("operation2 completed")
                    operation2.markAsFinished()
                })
            }

if let operation1 = self.operation1 {
            if operation1.isExecuting {
                operation2.addDependency(operation1)
                operation1.completionBlock = {
                    print("operation1.completionBlock")
                    self.operationQueue?.addOperation(operation2)
                }
            }
            } else  {
                operationQueue?.addOperation(operation2)
            }

        }


        func cancelProcess() {
            print("cancelAllOperations")
            operationQueue?.cancelAllOperations()
        }

        func setup() {
            print("setup Called")
            operationQueue?.cancelAllOperations()
            operationQueue = OperationQueue()
            operation1 = MyOperation()
            operation2 = MyOperation()
        }
    }


    class MyOperation: Operation {
        var codeToRun: (()->Void)?

        var _executing = false
        var _finished = false

        override internal(set) var isExecuting: Bool {
            get {
                return _executing
            }
            set {
                _executing = newValue

            }
        }

        override internal(set) var isFinished: Bool {
            get {
                return _finished
            }
            set {
                _finished = newValue
            }
        }

        override var isAsynchronous: Bool {
            return true
        }

        override func start() {
            isExecuting = true
            isFinished = false
            if let closure = self.codeToRun {
                closure()
            }
        }

        func markAsFinished() {
            self.isExecuting = false
            self.isFinished = true
            completionBlock?()
        }
    }

    let manager = MyManager()

    manager.doOperation1()
    manager.doOperation2()

I am getting result

cancelAllOperations
setup Called
operation1 started
operation1 completed
operation1.completionBlock

Expected is

cancelAllOperations
setup Called
operation1 started
operation1 completed
operation1.completionBlock
operation2 started
operation2 completed

Am I missing here anything?

Not sure I understand your question. You have written in the 2nd line of your question that operation2 can start after operation1 is in progress or completed. Is this a typo because if you add dependency it will wait till operation1 completes. Why are you setting the `codeToRun` after adding it to the queue. It needs to be done before adding it to the queue. — user1046037, Feb 03 '18 at 04:42
Is this implementation of your operation copied from somewhere ? There are a number of issues with your code. Could you please the read the documentation of Operation. — user1046037, Feb 03 '18 at 04:48
Do you want the 2nd operation to start after the 1st operation completes ? If so could you edit your 2nd line of your question ? — user1046037, Feb 03 '18 at 05:03
Two ways to look into achieving this are to add operation1 as a dependency of operation2 but of course it you never have an operation1 for some reason that fails. The other method is to set the `maxConcurrentOperationCount` property of the operation queue to 1 so that only 1 operation can execute at any time. Then if operation1 starts first operation2 has to wait for it but if there is no operation1 operation 2 will just start. Doing this the operations will execute in the order they are added and each one waits for the previous one to finish. — Upholder Of Truth, Feb 03 '18 at 08:08

score 1 · Answer 1 · edited Apr 26 '19 at 12:39

I've been looking at your code. I found a few things:

First

manager.doOperation1()
manager.doOperation2()

this not means operation2 runs after operation1 finished, if you want to do this, you can add a completion closure for operation1.

Second

when you call

doOperation2()

in this function seems like the code never executed after:

guard let operation2 = self.operation2 else { return }

After all

It seems like you want to create your own wheels. I suggest you to learn something about GCD,you can find resources here:

Grand Central Dispatch Crash Course for Swift 3

Grand Central Dispatch Tutorial for Swift 3: Part 1/2

Thanks for documentation. I would like to use OperationQueue instead of GCD. I am trying to figure out why dependency is not working — Kapil, Feb 03 '18 at 03:43

score 0 · Answer 2 · answered Feb 03 '18 at 05:55

There are a couple of things:

Implementation:

Implement KVO for isExecuting and isFinished
Fix doOperation2
Once you set the dependencies the operation2 wouldn't start even if it is added to the queue until operation1 completes.
Check for isCancelled inside MyOperation

Below is not the ideal way to implement doOperation2, but removes some of the clutter from your code. I will leave it to you to implement the whole thing based on the Design section mentioned below.

func doOperation2() {

    self.operation2 = MyOperation()

    guard let operation2 = self.operation2 else {
        return
    }

    operation2.codeToRun = {
        print("operation2 started")

        self.talkWithServer(completion: { (completion) in
            print("operation2 completed")
        })
    }

    operationQueue?.addOperation(operation2)
}

Design:

In your implementation, MyOperation seems to be generic and you seem to be doing most of the real work at the place where you call them
Modify MyOperation to do the real work.
Call site should be simple
Example FetchData() is an operation, ParseData() is another operation.
So an operation contains business logic.
At the call site you can just add dependencies.

score 0 · Answer 3 · edited Dec 08 '19 at 19:08

All you need to do is just add dependency on the dependent operation.

let queue = OperationQueue()

let operation1 = BlockOperation(block: { [weak self] in
    self?.doOperation1()
})

let operation2 = BlockOperation(block: { [weak self] in
    self?.doOperation2()
})

operation1.addDependency(operation2) // THIS IS THE KEY CODE IN YOUR CASE

queue.addOperation(operation1)
queue.addOperation(operation2)

Hope this may help you solve your dependency problem.