Complete staggered animation

Like all interactive widgets, the complete animation consists
of a widget pair: a stateless and a stateful widget.

The stateless widget specifies the Tweens,
defines the Animation objects, and provides a build() function
responsible for building the animating portion of the widget tree.

The stateful widget creates the controller, plays the animation,
and builds the non-animating portion of the widget tree.
The animation begins when a tap is detected anywhere in the screen.

Full code for basic_staggered_animation’s main.dart

Stateless widget: StaggerAnimation

AnimatedBuilder—a general purpose widget for building
animations. The

class StaggerAnimation extends StatelessWidget {
  StaggerAnimation({ Key key, this.controller }) :