Layer Time - iOS Core Animation - page 202

}
@end
Relative Time
As far as Core Animation is concerned, time is relative. Each animation has its own
representation of time, which can be independently sped up, delayed, or offset.
The beginTime property specifies the time delay before the animation begins. This delay is
measured from the point at which the animation is added to a visible layer. This defaults to
zero (that is, the animation will begin immediately).
The speed property is a time multiplier. It defaults to 1.0, but decreasing it will slow down
time for the layer/animation and increasing it will speed up time. With a speed of 2.0, an
animation with a nominal duration of 1 second will actually complete in 0.5 seconds.
The timeOffset property is similar to beginTime in that it time-shifts the animation. But
while increasing the beginTime increases the delay before an animation begins, increasing
the timeOffset fast-forwards to a specific point in the animation. For example, for an
animation that lasts 1 second, setting a timeOffset of 0.5 seconds would mean that the
animation starts halfway through.
Unlike beginTime , timeOffset is unaffected by speed . So, if you were to increase the
speed to 2.0 as well as setting the timeOffset to 0.5, you would have effectively skipped
to the end of the animation because a 1-second animation sped up by a factor of two lasts
for only 0.5 seconds. However, even if you skip to the end of the animation using the
timeOffset , it will still play for the same total duration; the animation simply loops around
and plays again up until the point where it originally started.
You can try this out with the simple test app in Listing 9.3. Just set the speed and
timeOffset sliders to the desired value, and then press Play to see the effect they have (see
Figure 9.3).
Listing 9.3 Testing the timeOffset and speed Properties
@interface ViewController ()
@property ( nonatomic , weak ) IBOutlet UIView *containerView;
@property ( nonatomic , weak ) IBOutlet UILabel *speedLabel;
@property ( nonatomic , weak ) IBOutlet UILabel *timeOffsetLabel;
@property ( nonatomic , weak ) IBOutlet UISlider *speedSlider;
@property ( nonatomic , weak ) IBOutlet UISlider *timeOffsetSlider;
@property ( nonatomic , strong ) UIBezierPath *bezierPath;
@property ( nonatomic , strong ) CALayer *shipLayer;