具有自定义 maxConcurrentOperationCount 的 OperationQueue 在完成第一个操作后不会拾取/执行队列中的所有操作

Question

我确定我的逻辑有问题，只是想不通是什么。

有一个“服务”class，它有一个操作队列：

class Service {

    let queue: OperationQueue = {

        var queue = OperationQueue()
        queue.name = "my.operationQueue"
        queue.maxConcurrentOperationCount = 1
        return queue
    }()

    func add(operation: Operation) {
        queue.addOperation(operation)
    }
}

该操作是异步的，因此它会覆盖状态，start 函数：

class MyOp: Operation {

    private var state: State = .ready
    private var id: Int

    init(id: Int) {
        self.id = id
    }

    override var isAsynchronous: Bool {
        return true
    }

    override var isReady: Bool {
        return state == .ready
    }

    override var isExecuting: Bool {
        return state == .started
    }

    /// See: `Operation`
    override var isFinished: Bool {
        return state == .finished || state == .cancelled
    }

    /// See: `Operation`
    override var isCancelled: Bool {
        return state == .cancelled
    }

    override func start() {

        guard state == .ready else {
            return
        }

        state = .started
        print("\(Date()) started \(id)")

        DispatchQueue.global().asyncAfter(deadline: .now() + 2) {
            self.state = .finished
            print("\(Date()) finished \(self.id)")
        }
    }
}

private extension MyOp {

    enum State {

        case ready
        case started
        case cancelled
        case finished
    }
}

我正在向队列中添加多个操作（使用 concurrentPerform 进行测试，实际上是不同的）：

let iterations = 20
let service = Service()

DispatchQueue.concurrentPerform(iterations: iterations) { iteration in

    let operation = MyOp(id: iteration)
    service.add(operation: operation)
}
DispatchQueue.global().asyncAfter(deadline: .now() + 40) {
    print("\(Date()) after run \(String(describing: service.queue.operations))")
}

我期待什么

20 个操作被添加到队列中（因为 let iterations = 20）
1 操作立即开始运行，其他人在队列中等待（因为queue.maxConcurrentOperationCount = 1）
第一个操作完成后，第二个开始，依此类推。
打印队列内容的最后一个块不应包含任何项目，或最多 1-2 个剩余项目。

实际发生了什么

操作已按预期添加到队列中。

我看到只有 1 个操作开始和完成，其余操作从未开始。最后一个块，在添加所有操作后 40 秒打印队列内容（大致足以完成所有或几乎所有操作的时间），显示剩余操作仍在队列中，而不是运行ning。这是一个例子：

<NSOperationQueue: 0x7fd477f09460>{name = 'my.operationQueue'}
2022-03-23 21:05:51 +0000 started 11
2022-03-23 21:05:53 +0000 finished 11
2022-03-23 21:06:31 +0000 after run [
  <__lldb_expr_25.MyOp 0x7fd479406660 isFinished=YES isReady=NO isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd477f04080 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd479206a70 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd460904190 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd479004080 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd479406550 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd460804080 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd470904480 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd460904080 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd460804190 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd460a04080 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd4793068c0 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd460b04080 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd477f0a160 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd460a04190 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd479406770 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd4608042a0 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd4792092f0 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd47910a360 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>, 
  <__lldb_expr_25.MyOp 0x7fd4609042a0 isFinished=NO isReady=YES isCancelled=NO isExecuting=NO>
 ]

那我做错了什么？

注：

这不是 print 错误的问题，因为在实际代码中我没有使用它
同样在实际代码中没有 DispatchQueue.global().asyncAfter(deadline: .now() + 2) - 这只是为了模拟一个运行ning 异步操作。

更新：我将问题提炼为maxConcurrentOperationCount：如果我删除行queue.maxConcurrentOperationCount = 1，队列将按预期工作。将其设置为任何其他值，都会产生类似的问题。

还是不明白为什么不对。

Answer 1

问题是这些方法不 KVC/KVO 兼容。正如 Operation documentation 所说：

The NSOperation class is key-value coding (KVC) and key-value observing (KVO) compliant for several of its properties.

…

If you provide custom implementations for any of the preceding properties, your implementations must maintain KVC and KVO compliance.

并发程度的约束（例如，两者都 maxConcurrentOperationCount and addDependency(_:)）依赖于 KVO 来了解先前的操作何时完成。如果您未能执行所需的 KVO 通知，队列将不知道后续操作何时可以进行。

有关示例实现，请参阅的后半部分。

FWIW，这是一个异步操作实现：

public class AsynchronousOperation: Operation {

    @Atomic @objc private dynamic var state: OperationState = .ready

    // MARK: - Various `Operation` properties

    open         override var isReady:        Bool { state == .ready && super.isReady }
    public final override var isExecuting:    Bool { state == .executing }
    public final override var isFinished:     Bool { state == .finished }
    public final override var isAsynchronous: Bool { true }

    // KVO for dependent properties

    open override class func keyPathsForValuesAffectingValue(forKey key: String) -> Set<String> {
        if [#keyPath(isReady), #keyPath(isFinished), #keyPath(isExecuting)].contains(key) {
            return [#keyPath(state)]
        }

        return super.keyPathsForValuesAffectingValue(forKey: key)
    }

    // Start

    public final override func start() {
        if isCancelled {
            state = .finished
            return
        }

        state = .executing

        main()
    }

    /// Subclasses must implement this to perform their work and they must not call `super`. The default implementation of this function throws an exception.

    open override func main() {
        fatalError("Subclasses must implement `main`.")
    }

    /// Call this function to finish an operation that is currently executing

    public final func finish() {
        if !isFinished { state = .finished }
    }
}

private extension AsynchronousOperation {
    /// State for this operation.

    @objc enum OperationState: Int {
        case ready
        case executing
        case finished
    }
}

具有以下内容：

@propertyWrapper
public class Atomic<T> {
    private var _wrappedValue: T
    private var lock = NSLock()

    public var wrappedValue: T {
        get { lock.synchronized { _wrappedValue } }
        set { lock.synchronized { _wrappedValue = newValue } }
    }

    public init(wrappedValue: T) {
        _wrappedValue = wrappedValue
    }
}

extension NSLocking {
    func synchronized<T>(block: () throws -> T) rethrows -> T {
        lock()
        defer { unlock() }
        return try block()
    }
}

通过上面的内容，我将异步 Operation 代码抽象为可以子类化并继承异步行为的代码。例如，这是一个与您的示例执行相同 asyncAfter 的操作（但有一些额外的 OSLog 路标，因此我可以直观地看到 Instruments 中的操作）：

import os.log

private let log = OSLog(subsystem: "Op", category: .pointsOfInterest)

class MyOperation: AsynchronousOperation {
    var value: Int

    init(value: Int) {
        self.value = value
        super.init()
    }

    override func main() {
        let id = OSSignpostID(log: log)
        os_signpost(.begin, log: log, name: "Operation", signpostID: id, "%d", value)

        DispatchQueue.main.asyncAfter(deadline: .now() + 1) { [self] in
            finish()
            os_signpost(.end, log: log, name: "Operation", signpostID: id, "%d", value)
        }
    }
}

然后...

let queue = OperationQueue()
queue.maxConcurrentOperationCount = 1

for i in 0..<5 {
    queue.addOperation(MyOperation(value: i))
}

... 生成如下操作的时间表：

具有自定义 maxConcurrentOperationCount 的 OperationQueue 在完成第一个操作后不会拾取/执行队列中的所有操作

OperationQueue with custom `maxConcurrentOperationCount` does not pick up / execute all operations in the queue, after finishing first operation

concurrency

swift

operationqueue