做的一个倒计时的例子，代码如下：
现在的问题是 _timer = [NSTimer timerWithTimeInterval:1 target:self selector:@selector(tickAction) userInfo:nil repeats:YES]; 这段代码不起作用。。

import "ViewController.h"

@interface ViewController (){
UIDatePicker * _myPicker;
NSTimeInterval _sec;
UILabel * _label;
UIButton * _btn;
NSTimer * _timer;
}

@end

@implementation ViewController

• (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view, typically from a nib.
  [self creatPicker];
  [self creatButton];
  [self creatLabel];
  }

• (void)creatPicker{
  _myPicker = [[UIDatePicker alloc] initWithFrame:CGRectMake(20, 20, 0, 0)];
  _myPicker.datePickerMode = UIDatePickerModeCountDownTimer;
  [self.view addSubview:_myPicker];
  }

• (void)creatButton{
  _btn = [[UIButton alloc] initWithFrame:CGRectMake(50, 270, 100, 50)];
  [_btn setTitle:@"开始" forState:UIControlStateNormal];
  [_btn setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
  [_btn addTarget:self action:@selector(btnAction:) forControlEvents:UIControlEventTouchUpInside];
  [self.view addSubview:_btn];
  }

• (void)btnAction:(UIButton*)btn{
  _sec = _myPicker.countDownDuration;
  [self creatAlert];
  }

• (void)creatAlert{
  NSString * str = [NSString stringWithFormat:@"开始倒计时，您还剩[%.f]秒",_sec];
  UIAlertController * alert = [UIAlertController alertControllerWithTitle:@"倒计时" message:str preferredStyle:UIAlertControllerStyleAlert];
  UIAlertAction * okAction = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction * action) {
  [self alertAction];
  }];
  UIAlertAction * cancelAction = [UIAlertAction actionWithTitle:@"取消" style:UIAlertActionStyleDefault handler:nil];
  [alert addAction:okAction];
  [alert addAction:cancelAction];
  [self presentViewController:alert animated:YES completion:nil];
  }

• (void)alertAction{
  _label.text = [NSString stringWithFormat:@"%.f",_sec];
  _btn.enabled = NO;
  _myPicker.enabled = NO;
  _timer = [NSTimer timerWithTimeInterval:1 target:self selector:@selector(tickAction) userInfo:nil repeats:YES];
  }

• (void)tickAction{
  float letSec = _sec--;
  _label.text = [NSString stringWithFormat:@"%.f",letSec];
  NSLog(@"%.f",_sec);
  if (letSec <= 0) {
  [_timer invalidate];
  _btn.enabled = YES;
  _myPicker.enabled = YES;
  }
  }

• (void)creatLabel{
  _label = [[UILabel alloc] initWithFrame:CGRectMake(200, 270, 100, 50)];
  _label.text = @"0";
  _label.textColor = [UIColor redColor];
  [self.view addSubview:_label];
  }

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

@end