倒计时功能的实现

@implementation SYViewController
{
int totalSeconds;
NSTimer *downTimer;
UILabel *labelText;
}

- (void)viewDidLoad {

[super viewDidLoad];

totalSeconds = 60;

labelText = [[UILabel alloc] init];
labelText.frame = CGRectMake(100, 100, 300, 30);
[self.view addSubview:labelText];
downTimer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:@selector(timerChange) userInfo:nil repeats:YES];
labelText.text = [NSString stringWithFormat:@"%d",totalSeconds];

}

- (void)timerChange
{
totalSeconds--;
labelText.text = [NSString stringWithFormat:@"%d",totalSeconds];

if (totalSeconds==0) {
[downTimer invalidate];
downTimer = nil;
}
}

}

__block int timeout=60; //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(timeout<=0){ //倒计时结束，关闭
dispatch_source_cancel(_timer);
dispatch_async(dispatch_get_main_queue(), ^{
//设置界面的按钮显示 根据自己需求设置
labelText.text = @"倒计时结束";
});
}else{
int minutes = timeout / 60;
int seconds = timeout % 60;
NSString *strTime = [NSString stringWithFormat:@"%d分%.2d秒后重新获取验证码",minutes, seconds];
dispatch_async(dispatch_get_main_queue(), ^{
//设置界面的按钮显示 根据自己需求设置
labelText.text = [NSString stringWithFormat:@"%@",strTime];

});
timeout--;

}
});
dispatch_resume(_timer);