GCD定时器

• NSTimer依赖于RunLoop，如果RunLoop的任务过于繁重，可能会导致NSTimer不准时
• 而GCD的定时器直接操作系统内核，不依赖于RunLoop，会更加准时
  定时器使用示例

@interface ViewController ()
@property (strong, nonatomic) dispatch_source_t timer;
@end

@implementation ViewController
  - (void)viewDidLoad {
    [super viewDidLoad];
    
    // 队列
    //任务在主线程执行
    // dispatch_queue_t queue = dispatch_get_main_queue();
    
    //任务在子线程执行
    dispatch_queue_t queue = dispatch_queue_create("timer", DISPATCH_QUEUE_SERIAL);
    
    // 创建定时器
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    
    // 设置时间
    uint64_t start = 2.0; // 2秒后开始执行
    uint64_t interval = 1.0; // 每隔1秒执行
    dispatch_source_set_timer(timer,
                              dispatch_time(DISPATCH_TIME_NOW, start * NSEC_PER_SEC),
                              interval * NSEC_PER_SEC, 0);
    
    // 设置回调
    //    dispatch_source_set_event_handler(timer, ^{
    //        NSLog(@"1111");
    //    });
    dispatch_source_set_event_handler_f(timer, timerFire);  

    // 启动定时器
    dispatch_resume(timer);
    self.timer = timer;
}
void timerFire(void *param)
{
    NSLog(@"2222 - %@", [NSThread currentThread]);
}