理解 Swift 中具有依赖关系的操作序列

Question

参考https://developer.apple.com/reference/foundation/operation，我将 Playground 设置为 -

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)

控制台日志是 -

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

我们不应该期望输出是依赖的结果吗？ -

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

使用相同的结果 - opsQue.addOperations([op1, op2], waitUntilFinished: true)

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

Answer 1

事实上，我无法确定为什么您的代码无法正常工作的原因究竟是什么，但我想出了 3 个解决方法来实现您的目标：

如果您期望输出应该总是是：

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

然后：

1-您可能希望将第二个操作添加到第一个的完成块中的队列中，如下所示：

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- 设置maxConcurrentOperationCount操作队列为1，如下：

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-将第一个操作加入队列后调用waitUntilAllOperationsAreFinished()，如下：

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

---

顺便说一句，对于不复杂的任务，我更喜欢使用 GCD。

希望这会有所帮助。

Answer 2

完成块是在依赖操作开始后调用的，但这并不意味着第一个操作没有结束。

正如@Xoronis 的回答中所引用的：

无法保证完成块的确切执行上下文，但通常是辅助线程。因此，您不应使用此块来执行任何需要非常特定的执行上下文的工作。

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

看看这个例子：

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)

会导致

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

第一个操作确实在开始其依赖之前结束，但是在依赖已经开始之后调用完成块。

Answer 3

根据完成块的文档（重点是我的），

您的完成块的确切执行上下文不能保证，但通常是辅助线程。因此，您不应使用此块来执行任何需要非常特定的执行上下文的工作。 https://developer.apple.com/documentation/foundation/operation/1408085-completionblock

因此，从更现实的意义上说，当您想确切地知道操作何时完成时，您应该执行以下操作：

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 4

您需要将操作队列的 maxConcurrentOperationCount 设置为 1，才能按预期工作。

     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

通过指定依赖，保证op2在op1完成后被调度，但不一定op2在op1的完成处理程序完成后被调度。

Answer 6

初始化 operationQueue 挂起会给你你想要的。

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

我测试过使用 isSuspended。每次操作完成后才可以操作主队列。

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)

Answer 8

您可以使用适配器来确定执行的顺序：

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

这里是修改后的代码：

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)