iOS 倒计时 读秒定时器

// 开启倒计时效果
-(void)openCountdown{
    __block NSInteger time = 59; //倒计时时间
    
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_source_t _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    
    dispatch_source_set_timer(_timer,dispatch_walltime(NULL, 0),1.0*NSEC_PER_SEC, 0); //每秒执行
    
    dispatch_source_set_event_handler(_timer, ^{
        
        if(time <= 0){ //倒计时结束，关闭
            
            dispatch_source_cancel(_timer);
            dispatch_async(dispatch_get_main_queue(), ^{
                //设置按钮的样式
                [self.getCodeBtn setTitle:@"重新发送" forState:UIControlStateNormal];
                [self.getCodeBtn setTitleColor:COLOR_INPUTING forState:UIControlStateNormal];
                self.getCodeBtn.userInteractionEnabled = YES;
            });
        }else{
            
            int seconds = time % 60;
            dispatch_async(dispatch_get_main_queue(), ^{
                //设置按钮显示读秒效果
                [self.getCodeBtn setTitle:[NSString stringWithFormat:@"%.2d秒后可重发", seconds] forState:UIControlStateNormal];
                [self.getCodeBtn setTitleColor:COLOR(204) forState:UIControlStateNormal];
                self.getCodeBtn.userInteractionEnabled = NO;
            });
            time--;
        }
    });
    dispatch_resume(_timer);
}