等待队列中的所有操作完成后再执行任务

Question

我有一个操作子类和 maxConcurrentOperationCount = 1 的操作队列。

这会按我添加它们的顺序执行我的操作，这很好，但现在我需要等到所有操作完成后再运行另一个进程。

我试图使用通知组，但由于这是在 for 循环中运行，一旦操作被添加到队列中，通知组就会触发。.我如何等待所有操作离开队列，然后再运行另一个进程？

for (index, _) in  self.packArray.enumerated() {

    myGroup.enter()
    let myArrayOperation = ArrayOperation(collection: self.outerCollectionView, id: self.packArray[index].id, count: index)
    myArrayOperation.name = self.packArray[index].id
    downloadQueue.addOperation(myArrayOperation)
    myGroup.leave()

}

myGroup.notify(queue: .main) {
 // do stuff here
}

Answer 1

您可以使用操作依赖关系在一系列其他操作完成后启动某些操作：

let queue = OperationQueue()

let completionOperation = BlockOperation {
    // all done
}

for object in objects {
    let operation = ...
    completionOperation.addDependency(operation)
    queue.addOperation(operation)
}

OperationQueue.main.addOperation(completionOperation)  // or, if you don't need it on main queue, just `queue.addOperation(completionOperation)`

或者，在 iOS 13 及更高版本中，您可以使用屏障：

let queue = OperationQueue()

for object in objects {
    queue.addOperation(...)
}

queue.addBarrierBlock {
    DispatchQueue.main.async {
        // all done
    }
}

Answer 2

一个合适的解决方案是 KVO

首先在循环之前添加观察者（假设

queue

是

OperationQueue

实例）

queue.addObserver(self, forKeyPath:"operations", options:.new, context:nil)

然后实施

override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if object as? OperationQueue == queue && keyPath == "operations" {
        if queue.operations.isEmpty {
            // Do something here when your queue has completed
            self.queue.removeObserver(self, forKeyPath:"operations")
        }
    } else {
        super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
    }
}

编辑：

在 Swift 4 中更容易

声明属性：

var observation : NSKeyValueObservation?

并创建观察者

observation = queue.observe(\.operationCount, options: [.new]) { [unowned self] (queue, change) in
    if change.newValue! == 0 {
        // Do something here when your queue has completed
        self.observation = nil
    }
}

自 iOS13 和 macOS15

operationCount

起已弃用。替代的是观察

progress.completedUnitCount

。

另一种现代方法是使用

Combine

的 KVO 发布者

var cancellable: AnyCancellable?

cancellable = queue.publisher(for: \.progress.completedUnitCount)
    .filter{$0 == queue.progress.totalUnitCount}
    .sink() { _ in 
       print("queue finished") 
       self.cancellable = nil           
    }

Answer 3

我使用下一个解决方案：

private let queue = OperationQueue()

private func addOperations(_ operations: [Operation], completionHandler: @escaping () -> ()) {
    DispatchQueue.global().async { [unowned self] in
        self.queue.addOperations(operations, waitUntilFinished: true)
        DispatchQueue.main.async(execute: completionHandler)
    }
}

Answer 4

设置最大并发操作数为1

operationQueue.maxConcurrentOperationCount = 1

然后每个操作将按顺序执行（就好像每个操作都依赖于前一个操作），并且您的完成操作将在最后执行。

Answer 5

代码位于队列末尾 请参考此链接

NSOperation 和 NSOperationQueue 是用于异步任务的伟大且有用的基础框架工具。但有一件事让我困惑：在所有队列操作完成后如何运行代码？简单的答案是：使用队列中操作之间的依赖关系（NSOperation 的独特功能）。只需 5 行代码即可解决。

NSOperation 依赖技巧 使用 Swift 很容易实现，如下所示：

extension Array where Element: NSOperation {
/// Execute block after all operations from the array.
func onFinish(block: () -> Void) {
    let doneOperation = NSBlockOperation(block: block)
    self.forEach { [unowned doneOperation] in doneOperation.addDependency($0) }
    NSOperationQueue().addOperation(doneOperation)
}}

Answer 6

我的解决方案类似于https://stackoverflow.com/a/42496559/452115，但我没有将

completionOperation

添加到主操作队列中，而是添加到队列本身中。这对我有用：

var a = [Int](repeating: 0, count: 10)

let queue = OperationQueue()

let completionOperation = BlockOperation {
    print(a)
}

queue.maxConcurrentOperationCount = 2
for i in 0...9 {
    let operation = BlockOperation {
        a[i] = 1
    }
    completionOperation.addDependency(operation)
    queue.addOperation(operation)
}

queue.addOperation(completionOperation)

print("Done 🎉")

Answer 7

我也遇到这个问题了。好吧，GCD 已经提供了 api，而不是 NSOperation：

dispatch_group

，例如：

dispatch_group_t myGroup = dispatch_group_create();

// task1
dispatch_group_enter(myGroup);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // excute task1
    // ...

    // task1 completed，leave group
    dispatch_group_leave(myGroup);
});

// task2
dispatch_group_enter(myGroup);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // excute task2
    // ...

    // task2 completed，leave group
    dispatch_group_leave(myGroup);
});

dispatch_group_notify(myGroup, dispatch_get_main_queue(), ^{
    // call back on all tasks c finished
    NSLog(@"all tasks are finished");
});

// continue other tasks.