088实现自动倒计时功能

效果如下：

ViewController.h

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
@property (strong, nonatomic) UIDatePicker *datePChoice;
@property (strong, nonatomic) NSTimer *timer;

@end

ViewController.m

#import "ViewController.h"

@interface ViewController ()
- (void)layoutUI;
- (void)timerDidRun:(NSTimer *)sender;
@end

@implementation ViewController
#define timeInterval 60.0

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self layoutUI];
}

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

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    //画面隐藏时，取消计数器
    if ([_timer isValid]) {
        [_timer invalidate];
    }
}

- (void)layoutUI {
    _datePChoice = [[UIDatePicker alloc] initWithFrame:CGRectMake(0, 0, 0, 0)];
    CGPoint newPoint = self.view.center;
    _datePChoice.center = newPoint;
    _datePChoice.datePickerMode = UIDatePickerModeCountDownTimer;
    //设置时间显示为1小时5分钟；即65分钟 ＝ 65 * 60（s）
    _datePChoice.countDownDuration = 65 * 60;
    [self.view addSubview:_datePChoice];
    
    //使用计数器；设置每隔timeInterval（60秒）循环调用一次
    _timer = [NSTimer timerWithTimeInterval:8
                                     target:self
                                   selector:@selector(timerDidRun:)
                                   userInfo:nil
                                    repeats:YES]; //这里为了测试；更改timerWithTimeInterval:timeInterval为timerWithTimeInterval:8，即每隔8秒循环调用一次
    NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
    [runLoop addTimer:_timer forMode:NSDefaultRunLoopMode];
}

- (void)timerDidRun:(NSTimer *)sender {
    if (_datePChoice.countDownDuration > 0.0) {
        _datePChoice.countDownDuration -= timeInterval;
    } else {
        //日期选择器时间倒计时为0时，取消计数器
        if ([_timer isValid]) {
            [_timer invalidate];
        }
    }
}

@end