To create an explicit animation (where you control the animation,
rather than letting the framework control it), perhaps
you can use one of the built-in explicit animations classes.
For more information, watch
Making your first directional animations with
built-in explicit animations.
(Also published as a companion article.)

If you need to build an explicit animation from scratch, watch
Creating custom explicit animations with
AnimatedBuilder and AnimatedWidget.
(Also published as a companion article.)

For a deeper understanding of just how animations work in Flutter, watch
Animation deep dive.
(Also published as a companion article.)

Codelabs, tutorials, and articles

The following resources are a good place to start learning
the Flutter animation framework. Each of these documents
shows how to write animation code.

Implicit animations codelab
Covers how to use implicit animations
using step-by-step instructions and interactive examples.