信号量

信号量，使当前线程只执行一件事

dispatch_semaphore_t frameRenderingSemaphore;

create      frameRenderingSemaphore = dispatch_semaphore_create(1);

wait          dispatch_semaphore_wait(frameRenderingSemaphore, DISPATCH_TIME_NOW) != 0

singal       dispatch_semaphore_signal(frameRenderingSemaphore)

release     dispatch_release(frameRenderingSemaphore)

frameRenderingSemaphore = DispatchSemaphore.init(value: 1)

frameRenderingSemaphore?.wait(timeout: DispatchTime.now()) == .timedOut

frameRenderingSemaphore?.signal()