GCD临界区/互斥

Question

有人可以用简单的例子回答：

如何正确锁码部分与条件：如果这部分是由一些线程不持有其他线程锁定只是通过其他跳过这部分线程并继续前进。

Answer 1

好吧，这里是工作示例（归功于@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

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