iOS 开发之 GCD 不同场景使用

时间:2023-03-08 16:08:40
iOS 开发之 GCD 不同场景使用

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iOS 开发之 GCD 不同场景使用

本文在iOS 开发值 GCD 基础 的基础上,继续总结了 GCD 的一些API 和在不同场景下的使用。

GCD 栅栏方法:dispatch_barrier_async

我们有时需要异步执行两组操作,而且第一组操作执行完之后,才能开始执行第二组操作。这样我们就需要一个相当于 栅栏 一样的一个方法将两组异步执行的操作组给分割起来,当然这里的操作组里可以包含一个或多个任务。这就需要用到dispatch_barrier_async方法在两个操作组间形成栅栏。

dispatch_barrier_async函数会等待前边追加到并发队列中的任务全部执行完毕之后,再将指定的任务追加到该异步队列中。然后在dispatch_barrier_async函数追加的任务执行完毕之后,异步队列才恢复为一般动作,接着追加任务到该异步队列并开始执行.

示意如图:iOS 开发之 GCD 不同场景使用

/**
 * 栅栏方法 dispatch_barrier_async
 */ - (void)barrier { // 1. 创建并发队列队列
    dispatch_queue_t queue = dispatch_queue_create("com.xiaoyouPrince", DISPATCH_QUEUE_CONCURRENT); // 2. 添加异步任务
    dispatch_async(queue, ^{         // 追加任务1
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"1---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });     dispatch_async(queue, ^{
        // 追加任务2
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"2---%@",[NSThread currentThread]);      // 打印当前线程
        }
    }); // 栅栏任务
    dispatch_barrier_async(queue, ^{
        // 追加任务 barrier
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"barrier---%@",[NSThread currentThread]);// 打印当前线程
        }
    });     dispatch_async(queue, ^{
        // 追加任务3
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"3---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });     dispatch_async(queue, ^{
        // 追加任务4
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"4---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });
} ------------------打印结果-----------------
1---{number = 4, name = (null)}
2---{number = 3, name = (null)}
1---{number = 4, name = (null)}
2---{number = 3, name = (null)}
barrier---{number = 4, name = (null)}
barrier---{number = 4, name = (null)}
4---{number = 3, name = (null)}
3---{number = 4, name = (null)}
4---{number = 3, name = (null)}
3---{number = 4, name = (null)}
------------------打印结果-----------------

在dispatch_barrier_async相关代码执行结果中可以看出:

在执行完栅栏前面的操作之后,才执行栅栏操作,最后再执行栅栏后边的操作

GCD 延时执行方法:dispatch_after

我们经常会遇到这样的需求:在指定时间(例如3秒)之后执行某个任务。可以用 GCD 的dispatch_after函数来实现。

需要注意的是:dispatch_after函数并不是在指定时间之后才开始执行处理,而是在指定时间之后将任务追加到主队列中。严格来说,这个时间并不是绝对准确的,但想要大致延迟执行任务,dispatch_after函数是很有效的。

/**
 * 延时执行方法 dispatch_after
 */ - (void)after {     NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"after---begin");     dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0* NSEC_PER_SEC)), dispatch_get_main_queue(), ^{         // 2.0秒后异步追加任务代码到主队列,并开始执行
        NSLog(@"after---%@",[NSThread currentThread]);  // 打印当前线程
    }); } 输出结果: 2018-08-29 17:53:08.713784+0800 GCD-demo[20282:5080295] currentThread---{number = 1, name = main} 2018-08-29 17:53:08.713962+0800 GCD-demo[20282:5080295] after---begin 2018-08-29 17:53:10.714283+0800 GCD-demo[20282:5080295] after---{number = 1, name = main}

在dispatch_after相关代码执行结果中可以看出:在打印 after---begin 之后大约 2.0 秒的时间,打印了 after---{number = 1, name = main}

GCD 一次性代码(只执行一次):dispatch_once

我们在创建单例、或者有整个程序运行过程中只执行一次的代码时,我们就用到了 GCD 的 dispatch_once 函数。使用dispatch_once 函数能保证某段代码在程序运行过程中只被执行1次,并且即使在多线程的环境下,dispatch_once也可以保证线程安全。


/**
 * 一次性代码(只执行一次)dispatch_once
 */ - (void)once { static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
// 只执行1次的代码(这里面默认是线程安全的)
});
}

GCD 快速迭代方法:dispatch_apply

通常我们会用 for 循环遍历,但是 GCD 给我们提供了快速迭代的函数dispatch_apply。dispatch_apply按照指定的次数将指定的任务追加到指定的队列中,并等待全部队列执行结束。

我们可以利用异步队列同时遍历。比如说遍历 0~5 这6个数字,for 循环的做法是每次取出一个元素,逐个遍历。dispatch_apply可以同时遍历多个数字。

/**
 * 快速迭代方法 dispatch_apply
 */ - (void)apply {     dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);     NSLog(@"apply---begin");     dispatch_apply(6, queue, ^(size_t index) {
        NSLog(@"%zd---%@",index, [NSThread currentThread]);
    });
    
    NSLog(@"apply---end");
} --------------------输出结果:------------------- apply---begin 1---{number = 3, name = (null)}
0---{number = 1, name = main}
2---{number = 4, name = (null)}
3---{number = 5, name = (null)}
4---{number = 3, name = (null)}
5---{number = 1, name = main} apply---end --------------------输出结果:-------------------

从 dispatch_apply 相关代码执行结果中可以看出:

0~5 打印顺序不定,最后打印了 apply---end。

因为是在并发队列中异步队执行任务,所以各个任务的执行时间长短不定,最后结束顺序也不定。但是apply---end一定在最后执行。这是因为 dispatch_apply 函数会等待全部任务执行完毕。

GCD 的队列组:dispatch_group

有时候我们会有这样的需求:分别异步执行2个耗时任务,然后当2个耗时任务都执行完毕后再回到主线程执行任务。这时候我们可以用到 GCD 的队列组。

调用队列组的 dispatch_group_async 先把任务放到队列中,然后将队列放入队列组中。或者使用队列组的 dispatch_group_enter、dispatch_group_leave 组合 来实现dispatch_group_async。

调用队列组的 dispatch_group_notify 回到指定线程执行任务。或者使用 dispatch_group_wait回到当前线程继续向下执行(会阻塞当前线程)。

dispatch_group_notify

监听 group 中任务的完成状态,当所有的任务都执行完成后,追加任务到 group 中,并执行任务。

/**
 * 队列组 dispatch_group_notify
 */ - (void)groupNotify {     NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"group---begin"); // 1. 创建 group
    dispatch_group_t group =  dispatch_group_create(); // 2. 异步执行组内的全局队列任务
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{         // 追加任务1
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"1---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });     dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{         // 追加任务2
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"2---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });     dispatch_group_notify(group, dispatch_get_main_queue(), ^{         // 等前面的异步任务1、任务2都执行完毕后,回到主线程执行下边任务
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"3---%@",[NSThread currentThread]);      // 打印当前线程
        }
        NSLog(@"group---end");
    });
} -------------------输出结果:-----------------
currentThread---{number = 1, name = main}
group---begin 1---{number = 4, name = (null)}
2---{number = 3, name = (null)}
2---{number = 3, name = (null)}
1---{number = 4, name = (null)}
3---{number = 1, name = main}
3---{number = 1, name = main} group---end

从dispatch_group_notify相关代码运行输出结果可以看出:
当所有任务都执行完成之后,才执行 dispatch_group_notify block 中的任务。

dispatch_group_wait

暂停当前线程(阻塞当前线程),等待指定的 group 中的任务执行完成后,才会往下继续执行。

/**
 * 队列组 dispatch_group_wait
 */
- (void)groupWait {     NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"group---begin"); // 1. 创建group
    dispatch_group_t group =  dispatch_group_create(); // 2. 异步执行组中全局队列任务
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{         // 追加任务1
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"1---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });     dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 追加任务2
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"2---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });     // 等待上面的任务全部完成后,会往下继续执行(会阻塞当前线程)
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    
    NSLog(@"group---end");
} ------------------输出结果:------------------ currentThread---{number = 1, name = main}
group---begin
2---{number = 4, name = (null)}
1---{number = 3, name = (null)}
2---{number = 4, name = (null)}
1---{number = 3, name = (null)}
group---end
------------------输出结果:------------------

从dispatch_group_wait相关代码运行输出结果可以看出:

当所有任务执行完成之后,才执行 dispatch_group_wait 之后的操作。但是,使用dispatch_group_wait 会阻塞当前线程。

dispatch_group_enter、dispatch_group_leave

dispatch_group_enter 标志着一个任务追加到 group,执行一次,相当于 group 中未执行完毕任务数+1

dispatch_group_leave 标志着一个任务离开了 group,执行一次,相当于 group 中未执行完毕任务数-1。

当 group 中未执行完毕任务数为0的时候,才会使dispatch_group_wait解除阻塞,以及执行追加到dispatch_group_notify中的任务。

/**
 * 队列组 dispatch_group_enter、dispatch_group_leave
 */
- (void)groupEnterAndLeave
{     NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"group---begin"); // 1.创建 group
    dispatch_group_t group = dispatch_group_create(); // 2. 获取全局并发队列
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); // 3. 进入 group 中执行
    dispatch_group_enter(group); // 3.1. 向队列中异步执行任务
    dispatch_async(queue, ^{         // 追加任务1
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"1---%@",[NSThread currentThread]);      // 打印当前线程
        }
        
        // 3.2. 任务执行之后,离开组【leave 和 enter 必须成对使用】
        dispatch_group_leave(group);
    }); // 4. 再次进入组执行新的任务
    dispatch_group_enter(group);
// 4.1. 向队列中异步执行任务
    dispatch_async(queue, ^{         // 追加任务2
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"2---%@",[NSThread currentThread]);      // 打印当前线程
        }
        
// 4.2. 任务执行之后,离开组【leave 和 enter 必须成对使用】
        dispatch_group_leave(group);
    }); // group 内部的任务都执行完之后通知执行下面代码
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{         // 等前面的异步操作都执行完毕后,回到主线程.
        for(inti = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"3---%@",[NSThread currentThread]);      // 打印当前线程
        }
        NSLog(@"group---end");
    });
} -------------------输出结果------------------
currentThread---{number = 1, name = main}
group---begin
1---{number = 4, name = (null)}
2---{number = 3, name = (null)}
1---{number = 4, name = (null)}
2---{number = 3, name = (null)}
3---{number = 1, name = main}
3---{number = 1, name = main}
group---end
-------------------输出结果------------------

从dispatch_group_enter、dispatch_group_leave相关代码运行结果中可以看出:当所有任务执行完成之后,才执行 dispatch_group_notify 中的任务。这里的dispatch_group_enter、dispatch_group_leave组合,其实等同于dispatch_group_async。

GCD 信号量:dispatch_semaphore

GCD 中的信号量是指 Dispatch Semaphore,是持有计数的信号。类似于过高速路收费站的栏杆。可以通过时,打开栏杆,不可以通过时,关闭栏杆。在 Dispatch Semaphore 中,使用计数来完成这个功能,计数为0时等待,不可通过。计数为1或大于1时,计数减1且不等待,可通过。Dispatch Semaphore 提供了三个函数。

dispatch_semaphore_create:创建一个Semaphore并初始化信号的总量

dispatch_semaphore_signal:发送一个信号,让信号总量加1

dispatch_semaphore_wait:可以使总信号量减1,当信号总量为0时就会一直等待(阻塞所在线程),否则就可以正常执行。

注意:信号量的使用前提是:想清楚你需要处理哪个线程等待(阻塞),又要哪个线程继续执行,然后使用信号量。

Dispatch Semaphore 在实际开发中主要用于:

保持线程同步,将异步执行任务转换为同步执行任务

保证线程安全,为线程加锁

Dispatch Semaphore 线程同步

我们在开发中,会遇到这样的需求:异步执行耗时任务,并使用异步执行的结果进行一些额外的操作。换句话说,相当于,将将异步执行任务转换为同步执行任务。比如说:AFNetworking 中 AFURLSessionManager.m 里面的 tasksForKeyPath: 方法。通过引入信号量的方式,等待异步执行任务结果,获取到 tasks,然后再返回该 tasks。

- (NSArray *)tasksForKeyPath:(NSString *)keyPath {

    __block NSArray *tasks = nil;

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

    [self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {

        if([keyPath isEqualToString:NSStringFromSelector(@selector(dataTasks))]) {

            tasks = dataTasks;

        } else if([keyPath isEqualToString:NSStringFromSelector(@selector(uploadTasks))]) {

            tasks = uploadTasks;

        } else if([keyPath isEqualToString:NSStringFromSelector(@selector(downloadTasks))]) {

            tasks = downloadTasks;

        } else if([keyPath isEqualToString:NSStringFromSelector(@selector(tasks))]) {

            tasks = [@[dataTasks, uploadTasks, downloadTasks] valueForKeyPath:@"@unionOfArrays.self"];

        }
        
        dispatch_semaphore_signal(semaphore);
    }];     dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    
    return tasks;
}

下面,我们来利用 Dispatch Semaphore 实现线程同步,将异步执行任务转换为同步执行任务。

/**
 * semaphore 线程同步
 */ - (void)semaphoreSync {     NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程     NSLog(@"semaphore---begin");     dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);     dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);     __block int number = 0;     dispatch_async(queue, ^{         // 追加任务1
        [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
        NSLog(@"1---%@",[NSThread currentThread]);      // 打印当前线程
        
        number = 100;         dispatch_semaphore_signal(semaphore);
    });     dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);     NSLog(@"semaphore---end,number = %zd",number); } ----------------------输出结果-----------------
currentThread---{number = 1, name = main}
semaphore---begin
1---{number = 3, name = (null)}
semaphore---end, number = 100

从 Dispatch Semaphore 实现线程同步的代码可以看到:

semaphore---end 是在执行完number = 100; 之后才打印的。而且输出结果 number 为 100。这是因为异步执行不会做任何等待,可以继续执行任务。异步执行将任务1追加到队列之后,不做等待,接着执行dispatch_semaphore_wait方法。此时 semaphore == 0,当前线程进入等待状态。然后,异步任务1开始执行。任务1执行到dispatch_semaphore_signal之后,总信号量加1,此时 semaphore == 1,dispatch_semaphore_wait方法检测到总信号量为1,正在被阻塞的线程(主线程)恢复继续执行。最后打印semaphore---end,number = 100。这样就实现了线程同步,将异步执行任务转换为同步执行任务。

Dispatch Semaphore 线程安全和线程同步(为线程加锁)

线程安全:如果你的代码所在的进程中有多个线程在同时运行,而这些线程可能会同时运行这段代码。如果每次运行结果和单线程运行的结果是一样的,而且其他的变量的值也和预期的是一样的,就是线程安全的。

若每个线程中对全局变量、静态变量只有读操作,而无写操作,一般来说,这个全局变量是线程安全的;若有多个线程同时执行写操作(更改变量),一般都需要考虑线程同步,否则的话就可能影响线程安全。

线程同步:可理解为线程 A 和 线程 B 一块配合,A 执行到一定程度时要依靠线程 B 的某个结果,于是停下来,示意 B 运行;B 依言执行,再将结果给 A;A 再继续操作。

举个简单例子就是:两个人在一起聊天。两个人不能同时说话,避免听不清(操作冲突)。等一个人说完(一个线程结束操作),另一个再说(另一个线程再开始操作)。

下面,我们模拟火车票售卖的方式,实现 NSThread 线程安全和解决线程同步问题。

场景:总共有50张火车票,有两个售卖火车票的窗口,一个是北京火车票售卖窗口,另一个是上海火车票售卖窗口。两个窗口同时售卖火车票,卖完为止。

非线程安全(不使用 semaphore)

先来看看不考虑线程安全的代码:

/**
 * 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; // 初始化车票 50 张     // queue1 代表北京火车票售卖窗口
    dispatch_queue_t queue1 = dispatch_queue_create("com.xiaoyouPrince1", DISPATCH_QUEUE_SERIAL);     // queue2 代表上海火车票售卖窗口
    dispatch_queue_t queue2 = dispatch_queue_create("com.xiaoyouPrince2", DISPATCH_QUEUE_SERIAL); // 北京窗口异步执行卖票操作
    __weak typeof(self) weakSelf = self;
    dispatch_async(queue1, ^{
        [weakSelf saleTicketNotSafe];
    }); // 上海窗口异步执行卖票操作
    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(@"票已经售完");
            break;
        }
    }
} ----------------- 输出结果(部分打印) -------------------
currentThread <NSThread: 0x608000079280>{number = 1, name = main}
non_semaphore_begin
剩余票数 48,窗口:{number = 4, name = (null)}
剩余票数 49,窗口:{number = 3, name = (null)}
剩余票数 47,窗口:{number = 4, name = (null)}
剩余票数 46,窗口:{number = 3, name = (null)}
剩余票数 45,窗口:{number = 4, name = (null)}
剩余票数 44,窗口:{number = 3, name = (null)}
剩余票数 43,窗口:{number = 4, name = (null)}
剩余票数 42,窗口:{number = 3, name = (null)}
剩余票数 40,窗口:{number = 4, name = (null)}
剩余票数 41,窗口:{number = 3, name = (null)}
剩余票数 39,窗口:{number = 4, name = (null)}
剩余票数 0,窗口:{number = 3, name = (null)}
...\
票已经售完
票已经售完
----------------- 输出结果(部分打印) -------------------

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。

线程安全(使用 semaphore 加锁)

考虑线程安全的代码:

/**

 * 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");
    
    // 有成员变量 dispatch_semaphore_t semaphreLock
    semaphreLock = 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("com.xiaoyouPrince1", DISPATCH_QUEUE_SERIAL);     // queue2 代表上海火车票售卖窗口
    dispatch_queue_t queue2 = dispatch_queue_create("com.xiaoyouPrince2", 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(@"车票已经售完");
            
            // 相当于解锁
            dispatch_semaphore_signal(semaphoreLock);
            break;
        }
        
        // 相当于解锁
        dispatch_semaphore_signal(semaphoreLock);
    }
} ----------------- 输出结果(部分打印) -------------------
currentThread <NSThread: 0x608000079280>{number = 1, name = main}
non_semaphore_begin
剩余票数 49,窗口:{number = 4, name = (null)}
剩余票数 48,窗口:{number = 3, name = (null)}
剩余票数 47,窗口:{number = 4, name = (null)}
剩余票数 46,窗口:{number = 3, name = (null)}
剩余票数 45,窗口:{number = 4, name = (null)}
....\
剩余票数 4,窗口:{number = 3, name = (null)}
剩余票数 3,窗口:{number = 4, name = (null)}
剩余票数 2,窗口:{number = 3, name = (null)}
剩余票数 1,窗口:{number = 4, name = (null)}
剩余票数 0,窗口:{number = 3, name = (null)}
票已经售完
----------------- 输出结果(部分打印) -------------------

可以看出,在考虑了线程安全的情况下,使用dispatch_semaphore 机制之后,得到的票数是正确的,没有出现混乱的情况。我们也就解决了多个线程同步的问题。

创建一个 semaphoreLock 对象。后续可能有多个线程同时访问被车票余量,当有线程访问到票余量时进行 dispatch_semaphore_wait 操作。使得 semaphoreLock 总信号量 减 1 (等于0 ,相当于加锁) ,并继续操作票余量,进行一次 self.ticketSurplusCount-- 操作。当票数量减少之后执行 dispatch_semaphore_signal 使得 semaphoreLock 总信号量 + 1 (等于1 ,相当于解锁)

此过程中如果有其他并发线程要访问 票余量。同样会先来到 dispatch_semaphore_wait 操作。此时 semaphoreLock 的总信号量为 0 ,直接阻塞线程。当上一个访问余量的线程操作完成之后执行 dispatch_semaphore_signal 操作解锁之后,就会继续本线程的访问。从而确保了票余量的线程安全。

资料:
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