一个时钟控件，用CoreAnimation实现
@property (strong, nonatomic, nullable) UIImage *clockBackgroundImage;
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- (void)startClockAtNow;
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- (void)startClockAtHour:(NSUInteger)hour minute:(NSUInteger)minute second:(NSUInteger)second
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- (void)startClockAtDate:(NSDate *)date;
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- (void)pauseClock;
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- (void)resumeClock;
