终止多个操作而无死锁

Question

我有多个后台线程，这些线程会不时更新UI。我需要从UI线程中正确终止它们，并确保没有死锁/争用条件。

下面是创建100个操作然后终止它们的示例：

class CancellationToken {
    private let updateLock = DispatchSemaphore(value: 1)    
    private var canceled = false

    var isCanceled: Bool {
        updateLock.wait()
        defer { updateLock.signal() }
        return canceled
    }

    func cancel() {
        updateLock.wait()
        defer { updateLock.signal() }
        canceled = true
    }
}

// UI thread
let processingQueue = OperationQueue()

let cancelationToken = CancelationToken()

for _ in 0..100 {
    processingQueue.addOperation {
        // do some stuff on background thread
        ...

        # 2
        guard !cancelationToken.isCanceled else { return }
        # 3
        DispatchQueue.main.async {
            # 4
            guard !cancelationToken.isCanceled else { return }
            // update UI
        }
    }
}

// we are still on UI thread
cancelationToken.cancel()
for operation in processingQueue.operations {
    operation.waitUntilFinished() #1
}

#2 -> #1 -> #3 -> deadlock可能在那吗？