一次性代码

整个程序运行过程中只会执行一次。本身是线程安全的，可以用于实现单例模式。

内部实现原理：判断onceToken的值来决定是否执行block块的代码。

- (void)once {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        // 一次性操作
    });
}

一次性代码实现单例模式

+ (instancetype)sharedManager {
    static DPLManager *_instance;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _instance = [[DPLManager alloc] init];
    });
    return _instance;
}

延迟执行

可以设置需要延迟执行的任务在哪个队列中执行。
延迟执行的原理：先等待，再将任务添加到队列中。

- (void)delay {
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        // 需要延迟执行的任务
    });
}

定时器

开启定时器
定时器类型为dispatch_source_t，通常需要设置属性或全局变量保证对象不被释放。

- (void)tiemr {
    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_global_queue(0, 0));
    dispatch_source_set_timer(self.timer, DISPATCH_TIME_NOW, 2.0 * NSEC_PER_SEC, 0 * NSEC_PER_SEC);
    dispatch_source_set_event_handler(self.timer, ^{
        NSLog(@"currentThread:%@", [NSThread currentThread]);
    });
    dispatch_resume(self.timer);
}

关闭定时器

- (void)stopTimer {
    dispatch_cancel(self.timer);
}

快速迭代

开启多条子线程，和主线程一起并发执行任务。

- (void)apply {
    dispatch_apply(10, dispatch_get_global_queue(0, 0), ^(size_t index) {
        NSLog(@"index:%zu, cuurentThread:%@", index, [NSThread currentThread]);
    });
}

栅栏函数

执行顺序:
栅栏前面的任务并发执行 -> 执行栅栏函数 -> 栅栏后面的任务并发执行

注意：不能使用全局并发队列

- (void)barrier {
    dispatch_queue_t queue = dispatch_queue_create("com.dpl.queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"1---currentThread:%@", [NSThread currentThread]);
    });
    
    dispatch_async(queue, ^{
        NSLog(@"2---currentThread:%@", [NSThread currentThread]);
    });
    
    dispatch_barrier_async(queue, ^{
        NSLog(@"栅栏---currentThread:%@", [NSThread currentThread]);
    });
    
    dispatch_async(queue, ^{
        NSLog(@"3---currentThread:%@", [NSThread currentThread]);
    });
    
    dispatch_async(queue, ^{
        NSLog(@"4---currentThread:%@", [NSThread currentThread]);
    });
}

队列组

队列组的实现方式：

• dispatch_group_enter(group)
• dispatch_group_leave(group)

- (void)gourp {
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("com.dpl.queue", DISPATCH_QUEUE_CONCURRENT);
    
    dispatch_group_async(group, queue, ^{
        NSLog(@"1---currentThread:%@", [NSThread currentThread]);
    });
    
    dispatch_group_async(group, queue, ^{
        NSLog(@"2---currentThread:%@", [NSThread currentThread]);
    });
    
    dispatch_group_async(group, queue, ^{
        NSLog(@"3---currentThread:%@", [NSThread currentThread]);
    });
    
    dispatch_group_notify(group, queue, ^{
        NSLog(@"队列组执行完成---currentThread:%@", [NSThread currentThread]);
    });
}