Question

有人可以用简短的例子回答：

如何使用条件正确锁定代码部分：如果此部分被某个线程锁定，则不保留其他线程只是跳过其他线程的这部分继续运行。

Answer 1

好的，这是工作示例（功劳归于@KenThomases ...）

import Dispatch

let semaphore = DispatchSemaphore(value: 1)
let printQueue = DispatchQueue(label: "print queue")
let group = DispatchGroup()

func longRuningTask(i: Int) {

    printQueue.async(group: group) {
        print(i,"GREEN semaphore")
    }
    usleep(1000)               // cca 1 milisecond
    printQueue.async(group: group) {
        print(i,"job done")
    }
}

func shortRuningTask(i: Int) {
    group.enter()
    guard semaphore.wait(timeout: .now() + 0.001) == .success else { // wait for cca 1 milisecond from now
        printQueue.async(group: group) {
            print(i,"RED semaphore, job not done")
        }
        group.leave()
        return
    }
    longRuningTask(i: i)
    semaphore.signal()
    group.leave()
}

printQueue.async(group: group) {
    print("running")
}

DispatchQueue.concurrentPerform(iterations: 10, execute: shortRuningTask )
group.wait()
print("all done")

及其打印输出

running
0 GREEN semaphore
2 RED semaphore, job not done
1 RED semaphore, job not done
3 RED semaphore, job not done
0 job done
4 GREEN semaphore
5 RED semaphore, job not done
6 RED semaphore, job not done
7 RED semaphore, job not done
4 job done
8 GREEN semaphore
9 RED semaphore, job not done
8 job done
all done
Program ended with exit code: 0

GCD - 关键部分/互斥体

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