验证码倒计时

#import "ViewController.h"
@interface ViewController ()
@property (nonatomic, strong) UIButton * timeButton;
@property (nonatomic, strong) NSTimer * timer;
@property (nonatomic, strong)UIButton * btn;
@end
@implementation ViewController
{
    NSInteger _time;
}
- (void)viewDidLoad {
    [super viewDidLoad];
    _time = 60;
    self.btn = [UIButton buttonWithType:UIButtonTypeCustom]; 
      _btn.backgroundColor = [UIColor orangeColor];
     [_btn setTitle:@"获取验证码" forState:UIControlStateNormal]; 
    _btn.titleLabel.font = [UIFont systemFontOfSize:15];
    [_timeButton setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
    [_btn addTarget:self action:@selector(btnAction:) forControlEvents:UIControlEventTouchUpInside];
    [self refreshButtonWidth];
    [self.view addSubview:self.btn];
}
- (void)refreshButtonWidth{
    CGFloat width = 0;
    if (_btn.enabled){
        width = 100;
    } else {
        width = 100;
    }
    _btn.center = CGPointMake(self.view.frame.size.width/2, 200);
    _btn.bounds = CGRectMake(0, 0, width, 40);
    //每次刷新，保证区域正确
    [_btn setBackgroundImage:[self imageWithColor:[UIColor orangeColor] andSize:_btn.frame.size] forState:UIControlStateNormal];
    [_btn setBackgroundImage:[self imageWithColor:[UIColor lightGrayColor] andSize:_btn.frame.size] forState:UIControlStateDisabled];
}
- (UIImage *)imageWithColor:(UIColor *)color andSize:(CGSize)aSize{
    CGRect rect = CGRectMake(0.0f, 0.0f, aSize.width, aSize.height); UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext(); CGContextSetFillColorWithColor(context, [color CGColor]); CGContextFillRect(context, rect);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext();
    return image;
}
- (void)btnAction:(UIButton *)sender{
    sender.enabled = NO;
    [self refreshButtonWidth];
    [sender setTitle:[NSString stringWithFormat:@"%zi", _time] forState:UIControlStateNormal];
    _timer = [NSTimer scheduledTimerWithTimeInterval:0.2 target:self selector:@selector(timeDown) userInfo:nil repeats:YES];
}
- (void)timeDown{
    _time --;
    if (_time == 0) {
        [_btn setTitle:@"重新获取" forState:UIControlStateNormal];
       _btn.enabled = YES;
        [self refreshButtonWidth];
        [_timer invalidate];
        _timer = nil; 
        _time = 60 ;
        return; 
    } 
    [_btn setTitle:[NSString stringWithFormat:@"%zi", _time] forState:UIControlStateNormal];
}