ios - iOS 中的操作同步挑战

Question

我有 3 个操作:A、B、C。

• A,B可以并发处理
• 如果 C 运行，A 和 B 应该等待
• 如果 A 或 B 运行 C 应该等待

我会用一个调度组和一个信号量来解决它:

public var dgLoadMain = dispatch_group_create()
public var semaLoadMain = dispatch_semaphore_create(1)

A, B 看起来像这样:

dispatch_group_enter(dgLoadMain)
dispatch_semaphore_wait(semaLoadMain, DISPATCH_TIME_FOREVER)
dispatch_semaphore_signal(semaLoadMain) //maybe odd, but right after wait, it signals, it just check wether C is in critical section, if not, release semaphore, and let other B or A continue too
//..
dispatch_group_leave(dgLoadMain)

C 看起来像这样:

dispatch_group_wait(dgLoadMain, DISPATCH_TIME_FOREVER)
dispatch_semaphore_wait(semaLoadMain, DISPATCH_TIME_FOREVER)
//..
dispatch_semaphore_signal(semaLoadMain)

你觉得可以吗？

Answer 1

dispatch_barrier_async 更复杂。看看这段代码。它是 Objective-C，但同样的概念绝对适用于 Swift。

#import <Foundation/Foundation.h>

void A()
{
    NSLog(@"A begin");
    sleep(1);
    NSLog(@"A end");
}

void B()
{
    NSLog(@"B begin");
    sleep(1);
    NSLog(@"B end");
}

void C()
{
    NSLog(@"C begin");
    sleep(1);
    NSLog(@"C end");
}

int main()
{
    dispatch_queue_t q = dispatch_queue_create("ABC", DISPATCH_QUEUE_CONCURRENT);

    dispatch_queue_t qA = dispatch_queue_create("A", DISPATCH_QUEUE_SERIAL);
    dispatch_set_target_queue(qA, q);

    dispatch_queue_t qB = dispatch_queue_create("B", DISPATCH_QUEUE_SERIAL);
    dispatch_set_target_queue(qB, q);

    dispatch_barrier_async(q, ^{C();});
    dispatch_async(qA, ^{A();});
    dispatch_async(qA, ^{A();});
    dispatch_async(qB, ^{B();});
    dispatch_barrier_async(q, ^{C();});
    dispatch_async(qB, ^{B();});
    dispatch_barrier_async(q, ^{C();});
    dispatch_async(qA, ^{A();});

    dispatch_main();
    return 0;
}

结果。

C begin
C end
A begin
B begin
A end
B end
B begin
A begin
B end
A end
A begin
A end
C begin
C end
C begin
C end