Android View Animation: A Blueprint for Motion

Why it exists

To provide a declarative and reusable way to animate UI elements in the traditional Android View system. Instead of hardcoding animation logic in Kotlin or Java, developers can define animations in XML, separating the "what" from the "how" and "when". This makes animations easier to manage and reuse.

The mental model

View-based animation is like a blueprint for motion. You create a plan in an XML file specifying what property should change (e.g., alpha), its start and end values (valueFrom, valueTo), and over how long (duration). In your code, you simply load this blueprint and apply it to a specific View, telling it to execute the plan.

How it works

Android provides two main systems: Property Animation and the older View Animation. Property Animation is more flexible. You define it in an XML file within the res/animator/ directory. The root element is typically <set>, which contains one or more <objectAnimator> elements. An <objectAnimator> targets a specific property of an object by its string name (e.g., "alpha", "translationX"). You can group multiple animators inside a <set> to run them together (together) or one after another (sequentially). The critical step is in your code: you call AnimatorInflater.loadAnimator() to get an Animator object from the XML, then call setTarget() on that object to link it to your target View, and finally call start().

When to use it

Use this for animations on UI components in apps built with the traditional Android View system (not Jetpack Compose). It's excellent for creating reusable, standard animations like fades, slides, or color changes that can be applied to multiple views across your app, often triggered by user interaction.

When not to use it

This system is not used in Jetpack Compose, which has its own modern animation APIs. For complex, dynamic animations where targets and values change frequently based on runtime logic, defining animations programmatically in code might be simpler. Also, prefer Property Animation over the older, less flexible View Animation (tween/frame), which only animates a View's drawing, not its actual properties.

One canonical example

To make a Button fade out, you create res/animator/fade_out.xml with an <objectAnimator> that has android:propertyName="alpha", android:duration="500", and android:valueTo="0.0". In your Kotlin code, you would inflate this animator, call animator.setTarget(myButton), and then call animator.start() to run the animation on that specific button.