刚刚有朋友让我写个倒计时器,我就简单写了一个,比较简陋,大家可以在这个基础上写点更复杂的.好了,代码里的注释很详细,上代码.

#import "ViewController.h"

@interface ViewController ()
@property(nonatomic, strong)NSTimer *timer;//定时器
@property(nonatomic, assign)CGFloat timeValue;//时间值
@property(nonatomic, strong)UILabel *label;//用于显示时间
@property(nonatomic, strong)UIButton *button;//用于选择暂停或是继续倒计时
@end
static NSInteger flag = 0;
@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    _label = [[UILabel alloc]initWithFrame:CGRectMake(100, 100, 100, 100)];
    _label.font = [UIFont systemFontOfSize:50];
    [self.view addSubview:_label];
    
    _timeValue = 5.0;//初始化时间是5秒
    _label.text = [NSString stringWithFormat:@"%.1f",_timeValue];//将_timeValue的值显示再label上面
    _timer = [NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(timeChanged) userInfo:nil repeats:YES];//这个定时器没隔0.1秒执行一次,并刷新_label的值
    
    /*添加一个button用于控制暂停或是继续倒计时*/
    _button = [UIButton buttonWithType:UIButtonTypeSystem];
    _button.frame = CGRectMake(200, 100, 200, 100);
    [_button setTitle:@"暂停" forState:UIControlStateNormal];
    [_button addTarget:self action:@selector(buttonAction) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:_button];
    
    
}

-(void)buttonAction
{
    if (flag == 0) {
        [_timer setFireDate:[NSDate distantFuture]];//暂停NSTimer
        [_button setTitle:@"继续" forState:UIControlStateNormal];
        flag = 1;
    }
    else
    {
        [_timer setFireDate:[NSDate distantPast]];//继续NSTimer
        [_button setTitle:@"暂停" forState:UIControlStateNormal];
        flag = 0;
    }
}

-(void)timeChanged
{
    _timeValue -= 0.1;//让_timeValue值减1
    _label.text = [NSString stringWithFormat:@"%.1f",_timeValue];
    if (_timeValue <= 0.0) {
        //当时间到了0,让label的值为0.0,并且弹出一个UIAlertController让用户选择确定还是重来
        _label.text = @"0.0";
        [_timer setFireDate:[NSDate distantFuture]];//暂停NSTimer
        UIAlertController *alertControl = [UIAlertController alertControllerWithTitle:@"时间到" message:nil preferredStyle:UIAlertControllerStyleAlert];
        UIAlertAction *stop = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleCancel handler:nil];
         UIAlertAction *goon = [UIAlertAction actionWithTitle:@"重来" style:UIAlertActionStyleDefault handler:^(UIAlertAction * _Nonnull action) {
             //点击重来时执行的操作,重置时间,并让NSTimer恢复
            _timeValue = 5.0;
            [_timer setFireDate:[NSDate distantPast]];
        }];
        [alertControl addAction:stop];
        [alertControl addAction:goon];
        [self presentViewController:alertControl animated:YES completion:nil];//让alertControl显示出来
    }
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

效果图是这样的

Simulator Screen Shot 2015年12月3日 下午10.09.14.png