手动动画

timeOffset一个很有用的功能在于你可以它可以让你手动控制动画进程，通过设置speed为0，可以禁用动画的自动播放，然后来使用timeOffset来来回显示动画序列。这可以使得运用手势来手动控制动画变得很简单。

举个简单的例子：还是之前关门的动画，修改代码来用手势控制动画。我们给视图添加一个UIPanGestureRecognizer，然后用timeOffset左右摇晃。

因为在动画添加到图层之后不能再做修改了，我们来通过调整layer的timeOffset达到同样的效果（清单9.4）。

清单9.4 通过触摸手势手动控制动画

1. @interface ViewController ()

3. @property (nonatomic, weak) UIView *containerView;
4. @property (nonatomic, strong) CALayer *doorLayer;

6. @end

8. @implementation ViewController

10. - (void)viewDidLoad
11. {
12. [super viewDidLoad];
13. //add the door
14. self.doorLayer = [CALayer layer];
15. self.doorLayer.frame = CGRectMake(0, 0, 128, 256);
16. self.doorLayer.position = CGPointMake(150 - 64, 150);
17. self.doorLayer.anchorPoint = CGPointMake(0, 0.5);
18. self.doorLayer.contents = (__bridge id)[UIImage imageNamed:@"Door.png"].CGImage;
19. [self.containerView.layer addSublayer:self.doorLayer];
20. //apply perspective transform
21. CATransform3D perspective = CATransform3DIdentity;
22. perspective.m34 = -1.0 / 500.0;
23. self.containerView.layer.sublayerTransform = perspective;
24. //add pan gesture recognizer to handle swipes
25. UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc] init];
26. [pan addTarget:self action:@selector(pan:)];
27. [self.view addGestureRecognizer:pan];
28. //pause all layer animations
29. self.doorLayer.speed = 0.0;
30. //apply swinging animation (which won't play because layer is paused)
31. CABasicAnimation *animation = [CABasicAnimation animation];
32. animation.keyPath = @"transform.rotation.y";
33. animation.toValue = @(-M_PI_2);
34. animation.duration = 1.0;
35. [self.doorLayer addAnimation:animation forKey:nil];
36. }

38. - (void)pan:(UIPanGestureRecognizer *)pan
39. {
40. //get horizontal component of pan gesture
41. CGFloat x = [pan translationInView:self.view].x;
42. //convert from points to animation duration //using a reasonable scale factor
43. x /= 200.0f;
44. //update timeOffset and clamp result
45. CFTimeInterval timeOffset = self.doorLayer.timeOffset;
46. timeOffset = MIN(0.999, MAX(0.0, timeOffset - x));
47. self.doorLayer.timeOffset = timeOffset;
48. //reset pan gesture
49. [pan setTranslation:CGPointZero inView:self.view];
50. }

52. @end

这其实是个小诡计，也许相对于设置个动画然后每次显示一帧而言，用移动手势来直接设置门的transform会更简单。

在这个例子中的确是这样，但是对于比如说关键这这样更加复杂的情况，或者有多个图层的动画组，相对于实时计算每个图层的属性而言，这就显得方便的多了。