前面我们介绍了NSTimer和CADisplayLink两种定时器，而且也介绍了如何去解决这些定时器引起的循环引用的问题，这次来介绍一下GCD定时器，这个也是我们很常用的定时器。

简单的使用

// 创建串行队列
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 = 1.0; // 1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 - %@", [NSThread currentThread]);
});

// 启动定时器
dispatch_resume(timer);

封装工具

@implementation DCTimer

static NSMutableDictionary *timers_;
dispatch_semaphore_t semaphore_;
// 单例
+ (void)initialize {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        timers_ = [NSMutableDictionary dictionary];
        semaphore_ = dispatch_semaphore_create(1);
    });
}

+ (NSString *)execTask:(void (^)(void))task start:(NSTimeInterval)start interval:(NSTimeInterval)interval repeats:(BOOL)repeats async:(BOOL)async {
    if (!task || start < 0 || (interval <= 0 && repeats)) return nil;

    // 创建队列
    dispatch_queue_t queue = async ? dispatch_get_global_queue(0, 0) : dispatch_get_main_queue();

    // 创建定时器
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);

    // 设置时间
    dispatch_source_set_timer(timer,
                              dispatch_time(DISPATCH_TIME_NOW, start * NSEC_PER_SEC),
                              interval * NSEC_PER_SEC, 0);

    /// 信号量控制(类似于加锁)
    dispatch_semaphore_wait(semaphore_, DISPATCH_TIME_FOREVER);
    // 定时器的唯一标识
    NSString *name = [NSString stringWithFormat:@"%zd", timers_.count];
    // 存放到字典中
    timers_[name] = timer;
    /// 信号量控制
    dispatch_semaphore_signal(semaphore_);

    // 设置回调
    dispatch_source_set_event_handler(timer, ^{
        task();
        if (!repeats) { // 不重复的任务
            [self cancelTask:name];
        }
    });

    // 启动定时器
    dispatch_resume(timer);

    return name;
}

+ (NSString *)execTask:(id)target selector:(SEL)selector start:(NSTimeInterval)start interval:(NSTimeInterval)interval repeats:(BOOL)repeats async:(BOOL)async {
    if (!target || !selector) return nil;
    return [self execTask:^{
        if ([target respondsToSelector:selector]) {
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
            [target performSelector:selector];
#pragma clang diagnostic pop
        }
    } start:start interval:interval repeats:repeats async:async];
}

/// 取消任务
+ (void)cancelTask:(NSString *)name {
    if (name.length == 0) return;
    dispatch_semaphore_wait(semaphore_, DISPATCH_TIME_FOREVER);
    dispatch_source_t timer = timers_[name];
    if (timer) {
        dispatch_source_cancel(timer);
        [timers_ removeObjectForKey:name];
    }
    dispatch_semaphore_signal(semaphore_);
}

@end