ViewController.h

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
@property (nonatomic, strong) CABasicAnimation *animationCopy;
@end

ViewController.m

#import "ViewController.h"

@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIView *square;

@end

@implementation ViewController

/**
 * 注册进出前后台观察者
 */
///////////////////////////////////////////////////////////////////////////////////////////////////
- (void)viewDidLoad
{
  [super viewDidLoad];
	// Do any additional setup after loading the view, typically from a nib.
  [[NSNotificationCenter defaultCenter] addObserver:self
                                           selector:@selector(appWillResignActive:)
                                               name:UIApplicationWillResignActiveNotification
                                             object:nil];
  [[NSNotificationCenter defaultCenter] addObserver:self
                                           selector:@selector(appWillEnterForeground:)
                                               name:UIApplicationWillEnterForegroundNotification
                                             object:nil];
}


/**
 * 待入后台之时备份当前动画，并暂停图层动画
 */
///////////////////////////////////////////////////////////////////////////////////////////////////
-(void)appWillResignActive:(NSNotification*)note {
  self.animationCopy = [[_square.layer animationForKey:@"movingAnimation"] copy];
  [self pauseLayer:_square.layer];
}


/**
 * 取出备份动画，加入图层继续运行
 */
///////////////////////////////////////////////////////////////////////////////////////////////////
-(void)appWillEnterForeground:(NSNotification*)note {
  if (self.animationCopy != nil) {
    [_square.layer addAnimation:self.animationCopy forKey:@"movingAnimation"]; // re-add the core animation to the view
    self.animationCopy = nil;
  }
  
  [self resumeLayer:_square.layer];
}

/**
 * 将正方形视图从上至下移动
 */
///////////////////////////////////////////////////////////////////////////////////////////////////
- (IBAction)startButtonPressed:(id)sender {
  CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"transform.translation.y"];
  animation.toValue = [NSNumber numberWithInt:350];
  animation.duration = 10;
  
  [self.square.layer addAnimation:animation forKey:@"movingAnimation"];
}

/**
 * from: https://developer.apple.com/library/ios/qa/qa1673/_index.html#//apple_ref/doc/uid/DTS40010053
 */
///////////////////////////////////////////////////////////////////////////////////////////////////
-(void)pauseLayer:(CALayer*)layer {
  CFTimeInterval pausedTime = [layer convertTime:CACurrentMediaTime() fromLayer:nil];
  layer.speed = 0.0;
  layer.timeOffset = pausedTime;
}


///////////////////////////////////////////////////////////////////////////////////////////////////
-(void)resumeLayer:(CALayer*)layer {
  CFTimeInterval pausedTime = [layer timeOffset];
  layer.speed = 1.0;
  layer.timeOffset = 0.0;
  layer.beginTime = 0.0;
  CFTimeInterval timeSincePause = [layer convertTime:CACurrentMediaTime() fromLayer:nil] - pausedTime;
  layer.beginTime = timeSincePause;
}


///////////////////////////////////////////////////////////////////////////////////////////////////
- (void)dealloc {
  [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationWillResignActiveNotification object:nil];
  [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationWillTerminateNotification object:nil];
}

@end