header{font-size:1em;padding-top:1.5em;padding-bottom:1.5em}
.markdown-body{overflow:hidden}
.markdown-body>div,.markdown-body>article{width:100%}
aside.sidebar{float:none;padding:0 18px 1px;background-color:#f7f7f7;border-top:1px solid #e0e0e0}
.flex-content,article img,article video,article .flash-video,aside.sidebar img{max-width:100%;height:auto}
.basic-alignment.left,article img.left,article video.left,article .left.flash-video,aside.sidebar img.left{float:left;margin-right:1.5em}
.basic-alignment.right,article img.right,article video.right,article .right.flash-video,aside.sidebar img.right{float:right;margin-left:1.5em}
.basic-alignment.center,article img.center,article video.center,article .center.flash-video,aside.sidebar img.center{display:block;margin:0 auto 1.5em}
.basic-alignment.left,article img.left,article video.left,article .left.flash-video,aside.sidebar img.left,.basic-alignment.right,article img.right,article video.right,article .right.flash-video,aside.sidebar img.right{margin-bottom:.8em}
.toggle-sidebar,.no-sidebar .toggle-sidebar{display:none}
.markdown-body img,.markdown-body video,.markdown-body .flash-video{ -webkit-border-radius:0.3em;-moz-border-radius:0.3em;-ms-border-radius:0.3em;-o-border-radius:0.3em;border-radius:0.3em;-webkit-box-shadow:rgba(0,0,0,0.15) 0 1px 4px;-moz-box-shadow:rgba(0,0,0,0.15) 0 1px 4px;box-shadow:rgba(0,0,0,0.15) 0 1px 4px;-webkit-box-sizing:border-box;-moz-box-sizing:border-box;box-sizing:border-box;border:#fff 0.5em solid;}
.markdown-body img,.markdown-body video{max-width: 100%;}
.markdown-body video,.markdown-body .flash-video{margin:0 auto 1.5em}
.markdown-body video{display:block;width:100%}
.markdown-body .flash-video>div{position:relative;display:block;padding-bottom:56.25%;padding-top:1px;height:0;overflow:hidden}
.markdown-body .flash-video>div iframe,.markdown-body .flash-video>div object,.markdown-body .flash-video>div embed{position:absolute;top:0;left:0;width:100%;height:100%}
.markdown-body>footer{padding-bottom:2.5em;margin-top:2em}
.markdown-body>footer p.meta{margin-bottom:.8em;font-size:.85em;clear:both;overflow:hidden}
body,pre{ background:#fdf6e3 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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：
/**
* 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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