The app animates stars on the screen by changing various View properties that control position, size, rotation, and translucency. This app shows how to create Property Animations, using ObjectAnimator, which are the basic building blocks of most Android animations. Property animations are used to animate (or change over time) the value of a property on an object, usually a UI object like an Android view.

Property animation involves changing of property values of the views over time like position, rotation, scale or transparency.

ObjectAnimator provides a simple set-and-forget mechanism for animating properties.

val animator = ObjectAnimator.ofFloat(star, View.ROTATION, -360f, 0f)

ObjectAnimator acts on the target “star” (ImageView). It runs an animation on the ROTATION property of star with start and end values of -360 to 0 degrees. The default duration of animation is 300 milliseconds.

The discontinuous motion which is caused when the animation restarts in the middle of current animation, is called Jank. When the next animation is requested (by user click) and the current animation isn't finished yet, the property resets to original value causing an abrupt motion. To resolve this we can start the new animation from current value or prevent user from requesting next animation until current one isn't finished.

To handle animation states we use AnimatorListenerAdapter that has callbacks for an animation starting, ending, pausing, resuming and repeating. Instead of implementing the raw AnimatorListener the adapter class is used because it provides default implementations of all the listener methods and we can override only those which are required. We prevent the Jank effect using AnimatorListenerAdapter.

It holds information about a single property, with values that the property animates between. However, it doesn't specify the target to it is applied. We use them to achieve parallel animations. An ObjectAnimator can hold multiple PropertyViewHolder objects and they all animate together, in parallel. The target to PropertyViewHolder is specified by ObjectAnimator.

val scaleX = PropertyValuesHolder.ofFloat(View.SCALE_X, 4f)
val scaleY = PropertyValuesHolder.ofFloat(View.SCALE_Y, 4f)
val animator = ObjectAnimator.ofPropertyValuesHolder(star, scaleX, scaleY)

ObjectAnimator not only animates view properties like alpha and rotation, but it can animate any object that has properties that object animator can call during animation. For example, the background color of a view is not exposed as android.util.property object, so we pass a string to object animator, which then uses setter and getter to set and get the value of that field. ObjectAnimator takes the string and looks for properties with the same name.

var animator = ObjectAnimator.ofArgb(view,"backgroundColor", Color.BLACK, Color.RED).start()

1. We can use animator.repeatCount to simply repeat the animation after initial run. We can specify if we want to reverse the animation by specify repeat mode.

   animator.repeatMode = ObjectAnimator.REVERSE
   

2. To animate between colors instead of using ObjectAnimator.ofInt() factory, we should use ObjectAnimator.ofArgb() to properly interpret colors (Requires API level 21 and above).

3. To run multiple animations together we can put them into AnimatorSet. It can be used to run animations in parallel or sequentially.