使用GCD创建定时器

NSTimer基于runloop，如果runloop的任务过于繁重，可能会导致NSTimer不准时。

使用GCD创建的定时器基于系统内核，不依赖runloop，相较于NSTimer更加准时。

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

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue());
    
    dispatch_source_set_timer(self.timer,
                              dispatch_time(DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC),
                              1.0 * NSEC_PER_SEC,
                              0);
    
    dispatch_source_set_event_handler(self.timer, ^{
        NSLog(@"%s",__func__);
    });
    
    dispatch_resume(self.timer);
}

dispatch_source_create方法的第四个参数为定时器执行回调方法所在的队列

API_AVAILABLE(macos(10.6), ios(4.0))
DISPATCH_EXPORT DISPATCH_MALLOC DISPATCH_RETURNS_RETAINED DISPATCH_WARN_RESULT
DISPATCH_NOTHROW
dispatch_source_t
dispatch_source_create(dispatch_source_type_t type,
    uintptr_t handle,
    uintptr_t mask,
    dispatch_queue_t _Nullable queue);

dispatch_source_set_timer方法的第二个参数为定时器开始时间，第三个参数为执行间隔，第四个参数为误差，单位为纳秒。

API_AVAILABLE(macos(10.6), ios(4.0))
DISPATCH_EXPORT DISPATCH_NONNULL_ALL DISPATCH_NOTHROW
void
dispatch_source_set_timer(dispatch_source_t source,
    dispatch_time_t start,
    uint64_t interval,
    uint64_t leeway);

回调方式除了block以外，还可以使用函数进行回调

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue());
    
    dispatch_source_set_timer(self.timer,
                              dispatch_time(DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC),
                              1.0 * NSEC_PER_SEC,
                              0);
    
    dispatch_source_set_event_handler_f(self.timer, func);
    
    dispatch_resume(self.timer);
}

void func (void *p)
{
    NSLog(@"%s",__func__);
}