Understanding the dependency flow in Swift

Question

Referring to https://developer.apple.com/reference/foundation/operation , I have Playground settings as -

class myOperation1 : Operation {
    override func main() {
        print("op1 working....")
    }
}

class myOperation2 : Operation {
    override func main() {
        print("op2 working....")
    }
}

let op1 = myOperation1()
let op2 = myOperation2()

op1.completionBlock = {
    print("op1 finished")
}

op2.completionBlock = {
    print("op2 finished")
}

op2.addDependency(op1)

let opsQue = OperationQueue()
opsQue.addOperation(op1)
opsQue.addOperation(op2)

And the console magazine -

op1 working....
op2 working....
op1 finished
op2 finished

Shouldn't we expect an exit as a result of addiction? -

op1 working....
op1 finished
op2 working....
op2 finished

Same result using - opsQue.addOperations([op1, op2], waitUntilFinished: true)

op1 working....
op2 working....
op1 finished
op2 finished

Answer 1

Actually, I can’t determine what exactly is the secret of why your code is not working properly, but I decided that you will find 3 workarounds to achieve what you are trying:

If you expect the output should always be:

op1 working....
op1 finished
op2 working....
op2 finished

then

1 - , , :

class myOperation1 : Operation {
    override func main() {
        print("op1 working....")
    }
}

class myOperation2 : Operation {
    override func main() {
        print("op2 working....")
    }
}

let opsQue = OperationQueue()

let op1 = myOperation1()

op1.completionBlock = {
    print("op1 finished")

    opsQue.addOperation(op2)
}

let op2 = myOperation2()

op2.completionBlock = {
    print("op2 finished")
}

opsQue.addOperation(op1)

2 - maxConcurrentOperationCount 1, :

class myOperation1 : Operation {
    override func main() {
        print("op1 working....")
    }
}

class myOperation2 : Operation {
    override func main() {
        print("op2 working....")
    }
}

let op1 = myOperation1()
let op2 = myOperation2()

op1.completionBlock = {
    print("op1 finished")
}

op2.completionBlock = {
    print("op2 finished")
}

op2.addDependency(op1)

let opsQue = OperationQueue()
// setting maxConcurrentOperationCount to 1
opsQue.maxConcurrentOperationCount = 1
opsQue.addOperation(op1)
opsQue.addOperation(op2)

3 - waitUntilAllOperationsAreFinished() :

let opsQue = OperationQueue()
opsQue.addOperation(op1)
opsQue.waitUntilAllOperationsAreFinished()
opsQue.addOperation(op2)

---

btw, GCD.

, .

Answer 2

( ),

, . , - , . https://developer.apple.com/documentation/foundation/operation/1408085-completionblock

, , , , - :

class myOperation1 : Operation {
    override func main() {
        print("op1 working....")
        //Do things
        print("op1 finished")
    }
}

class myOperation2 : Operation {
    override func main() {
        print("op2 working....")
        //Do things
        print("op2 finished")
    }
}

Answer 3

, , .

@Xoronis:

, . , - , .

https://developer.apple.com/documentation/foundation/operation/1408085-completionblock

:

class myOperation1 : Operation {
    override func main() {
        print("op1 working....")
        for i in 1...10 {
            print("\(i)")
        }
    }
}

class myOperation2 : Operation {
    override func main() {
        print("op2 working....")
    }
}

let op1 = myOperation1()
let op2 = myOperation2()

op1.completionBlock = {
    print("op1 completed")
}

op2.completionBlock = {
    print("op2 completed")
}

op2.addDependency(op1)

let opsQue = OperationQueue()
opsQue.addOperations([op1, op2], waitUntilFinished: true)

op1 working....
1
2
3
4
5
6
7
8
9
10
op2 working....
op1 completed
op2 completed

, , .

Answer 4

maxConcurrentOperationCount 1 , , .

     let operationQueue = OperationQueue()
     operationqueue?.maxConcurrentOperationCount = 1

    let operation1 = Operation()
    let operation2 = Operation()

    operation1.completionBlock = {
        print("operation1 finished")
    }

    operation2.completionBlock = {
        print("operation2 finished")
    }

    operation2.addDependency(operation1)

    operationQueue.addOperation(operation1)
    operationQueue.addOperation(operation2)

Answer 5

, , op2 op1, , op2 op1.

Answer 6

, , .

let queue = OperationQueue()
let downloadOp = Operation()
let resizeOp = Operation()
downloadOp.dependency(resizeOp)
queue.isSuspended = true
queue.addOperation(downloadOp)
queue.addOperation(resizeOp)
queue.isSuspended = false

Answer 7

, isSuspended. .

class OperationChain: Operation {
    var mainQ: OperationQueue?
    var text: String?
    
    init(with name: String, by mainqueue:OperationQueue){
        self.text = name
        self.mainQ = mainqueue
    }
    
    override func main() {
        self.mainQ!.isSuspended = true
        print(text!)
        sleep(5)
        self.mainQ!.isSuspended = false
    }
}

let oq = OperationQueue()
oq.maxConcurrentOperationCount = 1

let q1 = OperationChain(with: "Operation.main.q1", by: oq)
print("q1")

q1.completionBlock = {
    //sleep(5)
    q1.mainQ!.isSuspended = true
    var i = 0
    repeat {
        i = i + 1
    } while i < 100
    print("q1.completionBlock") 
    q1.mainQ!.isSuspended = false
}

oq.addOperations([q1], waitUntilFinished: true)

Run code

Answer 8

You can use the adapter to verify the order of execution:

op1 working....
op1 finished
op2 working....
op2 finished

Here is the modified code:

import Foundation

class myOperation1 : Operation {
    override func main() {
        print("op1 working....")
    }
}

class myOperation2 : Operation {
    override func main() {
        print("op2 working....")
    }
}

let op1 = myOperation1()
let op2 = myOperation2()

op1.completionBlock = {
    print("op1 finished")
}

op2.completionBlock = {
    print("op2 finished")
}

let adapter = BlockOperation(block: {})

adapter.addDependency(op1)
op2.addDependency(adapter)

let opsQue = OperationQueue()
opsQue.addOperation(op1)
opsQue.addOperation(op2)
opsQue.addOperation(adapter)