1.最终效果图

2.实现思路

1. 在ios中默认是绕着中心点旋转的，因为锚点默认在图层的中点，要想绕着下边中心点转，需要改变图层锚点的位置。
2. 根据锚点，设置position坐标，为时钟的中点。
3. 思考秒针旋转的角度，怎么知道当前秒针旋转到哪，当前秒针旋转的角度 = 当前秒数 * 每秒转多少°。
   1> 计算一秒转多少° 360 * 60 = 6
   2> 获取当前秒数，通过日历对象，获取日期组成成分 NSCalendar -> NSDateComponents -> 获取当前秒数
4. 每隔一秒，获取最新秒数，更新时钟。
   
   • 分钟一样的做法
   • 时钟也一样
5. 每一分钟，时钟也需要旋转，60分钟 -> 1小时 - > 30° ==》 每分钟 30 / 60.0 一分钟时针转0.5°
6. 把时针和秒针头尾变尖，设置圆角半径

2.完整代码

#import "ViewController.h"
//获得当前的年月日 时分秒
#define  CURRENTSEC [[NSCalendar currentCalendar] component:NSCalendarUnitSecond fromDate:[NSDate date]]
#define  CURRENTMIN [[NSCalendar currentCalendar] component:NSCalendarUnitMinute fromDate:[NSDate date]]
#define  CURRENTHOUR [[NSCalendar currentCalendar] component:NSCalendarUnitHour fromDate:[NSDate date]]
#define  CURRENTDAY  [[NSCalendar currentCalendar] component:NSCalendarUnitDay fromDate:[NSDate date]]
#define  CURRENTMONTH [[NSCalendar currentCalendar] component:NSCalendarUnitMonth fromDate:[NSDate date]]
#define  CURRENTYEAR [[NSCalendar currentCalendar] component:NSCalendarUnitYear fromDate:[NSDate date]]

//角度转换成弧度
#define  ANGEL(x) x/180.0 * M_PI

#define kPerSecondA     ANGEL(6)
#define kPerMinuteA     ANGEL(6)
#define kPerHourA       ANGEL(30)
#define kPerHourMinuteA ANGEL(0.5)
@interface ViewController ()


@property (nonatomic,strong) UIImageView *imageClock;

@property (nonatomic,strong) CALayer *layerSec;
@property (nonatomic,strong) CALayer *layerMin;
@property (nonatomic,strong) CALayer *layerHour;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];


    [self.view addSubview:self.imageClock];
    [self.imageClock.layer addSublayer:self.layerSec];
    [self.imageClock.layer addSublayer:self.layerMin];
    [self.imageClock.layer addSublayer:self.layerHour];

    [self timeChange];
    [NSTimer scheduledTimerWithTimeInterval:1 target:self selector:@selector(timeChange) userInfo:nil repeats:YES];
    // Do any additional setup after loading the view, typically from a nib.
}

- (void)timeChange
{
    self.layerSec.transform = CATransform3DMakeRotation(CURRENTSEC * kPerSecondA, 0, 0, 1);
    self.layerMin.transform = CATransform3DMakeRotation(CURRENTMIN * kPerMinuteA, 0, 0, 1);

    self.layerHour.transform = CATransform3DMakeRotation(CURRENTHOUR * kPerHourA, 0, 0, 1);
    self.layerHour.transform = CATransform3DMakeRotation(CURRENTMIN * kPerHourMinuteA + CURRENTHOUR*kPerHourA, 0, 0, 1);
}

- (UIImageView *)imageClock
{
    if (_imageClock == nil) {
        _imageClock = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"钟表"]];
        _imageClock.frame = CGRectMake(100, 100, 200, 200);
    }

    return _imageClock;
}

- (CALayer *)layerSec
{
    if (_layerSec == nil) {
        _layerSec = [CALayer layer];
        _layerSec.bounds = CGRectMake(0, 0, 2, 80);
        _layerSec.backgroundColor = [UIColor redColor].CGColor;
        _layerSec.cornerRadius = 5;
        _layerSec.anchorPoint = CGPointMake(0.5, 1);
        _layerSec.position = CGPointMake(self.imageClock.bounds.size.width/2, self.imageClock.bounds.size.height/2);
    }
    return _layerSec;
}

- (CALayer *)layerMin
{
    if (_layerMin == nil) {
        _layerMin = [CALayer layer];
        _layerMin.bounds = CGRectMake(0, 0, 4, 60);
        _layerMin.backgroundColor = [UIColor blackColor].CGColor;
        _layerMin.cornerRadius = 5;
        _layerMin.anchorPoint = CGPointMake(0.5, 1);
        _layerMin.position = CGPointMake(self.imageClock.bounds.size.width/2, self.imageClock.bounds.size.height/2);
    }
    return _layerMin;
}

- (CALayer *)layerHour
{
    if (_layerHour == nil) {
        _layerHour = [CALayer layer];
        _layerHour.bounds = CGRectMake(0, 0, 6, 40);
        _layerHour.backgroundColor = [UIColor blackColor].CGColor;
        _layerHour.cornerRadius = 5;
        _layerHour.anchorPoint = CGPointMake(0.5, 1);
        _layerHour.position = CGPointMake(self.imageClock.bounds.size.width/2, self.imageClock.bounds.size.height/2);
    }
    return _layerHour;
}

3.Demo程序

https://github.com/Esdeath/clock