Graphics Reference
In-Depth Information
Keyframe Animations
CABasicAnimation
is interesting in that it shows us the underlying mechanism behind most
of the implicit animations on iOS, but adding a
CABasicAnimation
to a layer explicitly is a
lot of work for little benefit when there are simpler ways to achieve the same effect (either
using implicit animations for hosted layers, or
UIView
animation for views and backing
layers).
CAKeyframeAnimation
, however, is considerably more powerful and has no equivalent
interface exposed in UIKit.
CAKeyframeAnimation
is, like
CABasicAnimation
, a subclass
of
CAPropertyAnimation
. It still operates on a single property, but unlike
CABasicAnimation
it is not limited to just a single start and end value, and instead can be
given an arbitrary sequence of values to animate between.
The term
keyframe
originates from traditional animation, where a lead animator would
draw only the frames where something significant happens (the
key
frames), and then the
less highly skilled artists would draw the frames in between (which could be easily inferred
from the keyframes). The same principle applies with
CAKeyframeAnimation
: You provide
the significant frames, and Core Animation fills in the gaps using a process called
interpolation.
We can demonstrate this using our colored layer from the earlier example. We'll set up an
array of colors and play them back with a single command using a keyframe animation (see
Listing 8.5).
Listing 8.5
Applying a Sequence of Colors Using
CAKeyframeAnimation
- (
IBAction
)changeColor
{
//create a keyframe animation
CAKeyframeAnimation
*animation = [
CAKeyframeAnimation
animation
];
animation.
keyPath
=
@"backgroundColor"
;
animation.
duration
=
2.0
;
animation.
values
=
@[
(
__bridge
id
)[
UIColor
blueColor
].
CGColor
,
(
__bridge
id
)[
UIColor
redColor
].
CGColor
,
(
__bridge
id
)[
UIColor
greenColor
].
CGColor
,
(
__bridge
id
)[
UIColor
blueColor
].
CGColor
]
;
//apply animation to layer
[
self
.
colorLayer
addAnimation
:animation
forKey
:
nil
];
}
