iOS GCD(六)线程加锁

作者: 奔跑吧小蚂蚁 | 来源:发表于2018-04-27 16:01 被阅读233次

iOS GCD (一) 任务+队列基础组合
 iOS GCD (二 ) dispatch_group 队列组
 iOS GCD(三) dispatch_barrier_async 栅栏方法
 iOS GCD (四) dispatch_semaphore 信号量
 iOS GCD(五) 死锁案例分析
 iOS GCD(六)线程加锁

推荐链接
https://bestswifter.com/ios-lock/

线程枷锁分为九种方式可以参考以上链接，我们只详细的讲解其中三种 semaphore， NSLock，@synchronized

需要对线程加锁的原因很多接下来我们只讲其中一种情况:
当多条线程同时编辑一个任务时所造成的数据错乱
举例说明：两个火车票销售窗口共同销售车站总共的50张车票。看代码你最明白。

错误案例：

/**
 * 非线程安全：不使用 semaphore
 * 初始化火车票数量、卖票窗口(非线程安全)、并开始卖票
 */
- (void)initTicketStatusNotSave {
    NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"semaphore---begin");
    
    self.ticketSurplusCount = 50;
    
    // queue1 代表北京火车票售卖窗口
    dispatch_queue_t queue1 = dispatch_queue_create("net.bujige.testQueue1", DISPATCH_QUEUE_SERIAL);
    // queue2 代表上海火车票售卖窗口
    dispatch_queue_t queue2 = dispatch_queue_create("net.bujige.testQueue2", DISPATCH_QUEUE_SERIAL);
    
    __weak typeof(self) weakSelf = self;
    dispatch_async(queue1, ^{
        [weakSelf saleTicketNotSafe];
    });
    
    dispatch_async(queue2, ^{
        [weakSelf saleTicketNotSafe];
    });
}

/**
 * 售卖火车票(非线程安全)
 */
- (void)saleTicketNotSafe {
    while (1) {
        
        if (self.ticketSurplusCount > 0) {  //如果还有票，继续售卖
            self.ticketSurplusCount--;
            NSLog(@"%@", [NSString stringWithFormat:@"剩余票数：%d 窗口：%@", self.ticketSurplusCount, [NSThread currentThread]]);
            [NSThread sleepForTimeInterval:0.2];
        } else { //如果已卖完，关闭售票窗口
            NSLog(@"所有火车票均已售完");
            break;
        }
        
    }
}

输出结果（部分）：
2018-02-23 22:25:35.789072+0800 YSC-GCD-demo[20712:5258914] currentThread---<NSThread: 0x604000068880>{number = 1, name = main}
2018-02-23 22:25:35.789260+0800 YSC-GCD-demo[20712:5258914] semaphore---begin
2018-02-23 22:25:35.789641+0800 YSC-GCD-demo[20712:5259176] 剩余票数：48 窗口：<NSThread: 0x60000027db80>{number = 3, name = (null)}
2018-02-23 22:25:35.789646+0800 YSC-GCD-demo[20712:5259175] 剩余票数：49 窗口：<NSThread: 0x60000027e740>{number = 4, name = (null)}
2018-02-23 22:25:35.994113+0800 YSC-GCD-demo[20712:5259175] 剩余票数：47 窗口：<NSThread: 0x60000027e740>{number = 4, name = (null)}
2018-02-23 22:25:35.994129+0800 YSC-GCD-demo[20712:5259176] 剩余票数：46 窗口：<NSThread: 0x60000027db80>{number = 3, name = (null)}
2018-02-23 22:25:36.198993+0800 YSC-GCD-demo[20712:5259176] 剩余票数：45 窗口：<NSThread: 0x60000027db80>{number = 3, name = (null)}

可以看到在不考虑线程安全，不使用 semaphore 的情况下，得到票数是错乱的，这样显然不符合我们的需求，所以我们需要考虑线程安全问题。防止两条线程同时对此任务进行编辑，每次只能有一条线程执行此任务。所以就用到了线程加锁

加锁方式一 dispatch_semaphore 自旋锁

@interface ViewController (){
    NSInteger _ticketSurplusCount;
    dispatch_semaphore_t _semaphoreLock;
    NSLock *_lock;
  }
@end
/**
 * 线程安全：使用 semaphore 加锁
 * 初始化火车票数量、卖票窗口(线程安全)、并开始卖票
 */
- (void)initTicketStatusSave {
    NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"semaphore---begin");
    
    semaphoreLock = dispatch_semaphore_create(1);
    
    self.ticketSurplusCount = 50;
    
    // queue1 代表北京火车票售卖窗口
    dispatch_queue_t queue1 = dispatch_queue_create("net.bujige.testQueue1", DISPATCH_QUEUE_SERIAL);
    // queue2 代表上海火车票售卖窗口
    dispatch_queue_t queue2 = dispatch_queue_create("net.bujige.testQueue2", DISPATCH_QUEUE_SERIAL);
    
    __weak typeof(self) weakSelf = self;
    dispatch_async(queue1, ^{
        [weakSelf saleTicketSafe];
    });
    
    dispatch_async(queue2, ^{
        [weakSelf saleTicketSafe];
    });
}

/**
 * 售卖火车票(线程安全)
 */
- (void)saleTicketSafe {
    while (1) {
        // 相当于加锁
        dispatch_semaphore_wait(semaphoreLock, DISPATCH_TIME_FOREVER);
        
        if (self.ticketSurplusCount > 0) {  //如果还有票，继续售卖
            self.ticketSurplusCount--;
            NSLog(@"%@", [NSString stringWithFormat:@"剩余票数：%d 窗口：%@", self.ticketSurplusCount, [NSThread currentThread]]);
            [NSThread sleepForTimeInterval:0.2];
        } else { //如果已卖完，关闭售票窗口
            NSLog(@"所有火车票均已售完");
            
            // 相当于解锁
            dispatch_semaphore_signal(semaphoreLock);
            break;
        }
        
        // 相当于解锁
        dispatch_semaphore_signal(semaphoreLock);
    }
}

输出结果为：
2018-02-23 22:32:19.814232+0800 YSC-GCD-demo[20862:5290531] currentThread---<NSThread: 0x6000000783c0>{number = 1, name = main}
2018-02-23 22:32:19.814412+0800 YSC-GCD-demo[20862:5290531] semaphore---begin
2018-02-23 22:32:19.814837+0800 YSC-GCD-demo[20862:5290687] 剩余票数：49 窗口：<NSThread: 0x6040002709c0>{number = 3, name = (null)}
2018-02-23 22:32:20.017745+0800 YSC-GCD-demo[20862:5290689] 剩余票数：48 窗口：<NSThread: 0x60000046c640>{number = 4, name = (null)}
2018-02-23 22:32:20.222039+0800 YSC-GCD-demo[20862:5290687] 剩余票数：47 窗口：<NSThread: 0x6040002709c0>{number = 3, name = (null)}
......
2018-02-23 22:32:29.024817+0800 YSC-GCD-demo[20862:5290689] 剩余票数：4 窗口：<NSThread: 0x60000046c640>{number = 4, name = (null)}
2018-02-23 22:32:29.230110+0800 YSC-GCD-demo[20862:5290687] 剩余票数：3 窗口：<NSThread: 0x6040002709c0>{number = 3, name = (null)}
2018-02-23 22:32:29.433615+0800 YSC-GCD-demo[20862:5290689] 剩余票数：2 窗口：<NSThread: 0x60000046c640>{number = 4, name = (null)}
2018-02-23 22:32:29.637572+0800 YSC-GCD-demo[20862:5290687] 剩余票数：1 窗口：<NSThread: 0x6040002709c0>{number = 3, name = (null)}
2018-02-23 22:32:29.840234+0800 YSC-GCD-demo[20862:5290689] 剩余票数：0 窗口：<NSThread: 0x60000046c640>{number = 4, name = (null)}
2018-02-23 22:32:30.044960+0800 YSC-GCD-demo[20862:5290687] 所有火车票均已售完
2018-02-23 22:32:30.045260+0800 YSC-GCD-demo[20862:5290689] 所有火车票均已售完

加锁方式二 @synchronized 互斥锁

/**
 * 线程安全：使用 semaphore 加锁
 * 初始化火车票数量、卖票窗口(线程安全)、并开始卖票
 */
- (void)initTicketStatusSave {
    NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"semaphore---begin");
    
    _semaphoreLock = dispatch_semaphore_create(1);
    
    _ticketSurplusCount = 50;
    
    // queue1 代表北京火车票售卖窗口
    dispatch_queue_t queue1 = dispatch_queue_create("net.bujige.testQueue1", DISPATCH_QUEUE_SERIAL);
    // queue2 代表上海火车票售卖窗口
    dispatch_queue_t queue2 = dispatch_queue_create("net.bujige.testQueue2", DISPATCH_QUEUE_SERIAL);
    
    __weak typeof(self) weakSelf = self;
    dispatch_async(queue1, ^{
        [weakSelf saleTicketSafe];
    });
    
    dispatch_async(queue2, ^{
        [weakSelf saleTicketSafe];
    });
}

/**
 * 售卖火车票(线程安全)
 */
- (void)saleTicketSafe {
    while (1) {
        // 相当于加锁
        @synchronized(self) {
            if (self.ticketSurplusCount > 0) {  //如果还有票，继续售卖
                self.ticketSurplusCount--;
                NSLog(@"%@", [NSString stringWithFormat:@"剩余票数：%d 窗口：%@", _ticketSurplusCount, [NSThread currentThread]]);
                [NSThread sleepForTimeInterval:0.2];
            } else { //如果已卖完，关闭售票窗口
                NSLog(@"所有火车票均已售完");
                break;
            }
        }
    }
}

输出结果为:
2018-04-27 14:58:49.255255+0700 GCD[86633:4440145] 剩余票数：8 窗口：<NSThread: 0x60000026fec0>{number = 3, name = (null)}
2018-04-27 14:58:49.459695+0700 GCD[86633:4440148] 剩余票数：6 窗口：<NSThread: 0x600000270480>{number = 4, name = (null)}
2018-04-27 14:58:49.459695+0700 GCD[86633:4440145] 剩余票数：7 窗口：<NSThread: 0x60000026fec0>{number = 3, name = (null)}
2018-04-27 14:58:49.663997+0700 GCD[86633:4440145] 剩余票数：5 窗口：<NSThread: 0x60000026fec0>{number = 3, name = (null)}
2018-04-27 14:58:49.664065+0700 GCD[86633:4440148] 剩余票数：4 窗口：<NSThread: 0x600000270480>{number = 4, name = (null)}
2018-04-27 14:58:49.868848+0700 GCD[86633:4440145] 剩余票数：3 窗口：<NSThread: 0x60000026fec0>{number = 3, name = (null)}
2018-04-27 14:58:49.869419+0700 GCD[86633:4440148] 剩余票数：2 窗口：<NSThread: 0x600000270480>{number = 4, name = (null)}
2018-04-27 14:58:50.072308+0700 GCD[86633:4440145] 剩余票数：1 窗口：<NSThread: 0x60000026fec0>{number = 3, name = (null)}
2018-04-27 14:58:50.072308+0700 GCD[86633:4440148] 剩余票数：0 窗口：<NSThread: 0x600000270480>{number = 4, name = (null)}
2018-04-27 14:58:50.276839+0700 GCD[86633:4440148] 所有火车票均已售完
2018-04-27 14:58:50.276839+0700 GCD[86633:4440145] 所有火车票均已售完

加锁方式三 NSLock

/**
 * 线程安全：使用 semaphore 加锁
 * 初始化火车票数量、卖票窗口(线程安全)、并开始卖票
 */
- (void)initTicketStatusSave {
    NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"semaphore---begin");
    
    _semaphoreLock = dispatch_semaphore_create(1);
    
    _ticketSurplusCount = 50;
    
    // queue1 代表北京火车票售卖窗口
    dispatch_queue_t queue1 = dispatch_queue_create("net.bujige.testQueue1", DISPATCH_QUEUE_SERIAL);
    // queue2 代表上海火车票售卖窗口
    dispatch_queue_t queue2 = dispatch_queue_create("net.bujige.testQueue2", DISPATCH_QUEUE_SERIAL);
    
    __weak typeof(self) weakSelf = self;
    dispatch_async(queue1, ^{
        [weakSelf saleTicketSafe];
    });
    
    dispatch_async(queue2, ^{
        [weakSelf saleTicketSafe];
    });
}

/**
 * 售卖火车票(线程安全)
 */
- (void)saleTicketSafe {
    while (1) {
        // 相当于加锁
         [_lock lock];
        if (self.ticketSurplusCount > 0) {  //如果还有票，继续售卖
            self.ticketSurplusCount--;
            NSLog(@"%@", [NSString stringWithFormat:@"剩余票数：%d 窗口：%@", _ticketSurplusCount, [NSThread currentThread]]);
            [NSThread sleepForTimeInterval:0.2];
        } else { //如果已卖完，关闭售票窗口
            NSLog(@"所有火车票均已售完");
            break;
        }
        [_lock unlock];
    }
}

输出结果为:
2018-04-27 14:58:49.255255+0700 GCD[86633:4440145] 剩余票数：8 窗口：<NSThread: 0x60000026fec0>{number = 3, name = (null)}
2018-04-27 14:58:49.459695+0700 GCD[86633:4440148] 剩余票数：6 窗口：<NSThread: 0x600000270480>{number = 4, name = (null)}
2018-04-27 14:58:49.459695+0700 GCD[86633:4440145] 剩余票数：7 窗口：<NSThread: 0x60000026fec0>{number = 3, name = (null)}
2018-04-27 14:58:49.663997+0700 GCD[86633:4440145] 剩余票数：5 窗口：<NSThread: 0x60000026fec0>{number = 3, name = (null)}
2018-04-27 14:58:49.664065+0700 GCD[86633:4440148] 剩余票数：4 窗口：<NSThread: 0x600000270480>{number = 4, name = (null)}
2018-04-27 14:58:49.868848+0700 GCD[86633:4440145] 剩余票数：3 窗口：<NSThread: 0x60000026fec0>{number = 3, name = (null)}
2018-04-27 14:58:49.869419+0700 GCD[86633:4440148] 剩余票数：2 窗口：<NSThread: 0x600000270480>{number = 4, name = (null)}
2018-04-27 14:58:50.072308+0700 GCD[86633:4440145] 剩余票数：1 窗口：<NSThread: 0x60000026fec0>{number = 3, name = (null)}
2018-04-27 14:58:50.072308+0700 GCD[86633:4440148] 剩余票数：0 窗口：<NSThread: 0x600000270480>{number = 4, name = (null)}
2018-04-27 14:58:50.276839+0700 GCD[86633:4440148] 所有火车票均已售完
2018-04-27 14:58:50.276839+0700 GCD[86633:4440145] 所有火车票均已售完

此处总结完毕，后续更精彩。

iOS GCD(六)线程加锁

加锁方式一 dispatch_semaphore 自旋锁

加锁方式二 @synchronized 互斥锁

加锁方式三 NSLock

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