build method
override
Declares this node's children and behavior.
Runs once on mount, and again on every rebuild.
super.build() is required, as skipping it drops whatever the superclass
declared.
Implementation
@override
void build() {
super.build();
for (var i = 0; i < effects.length; i += 1) {
effects[i].onFinish(() => _track(i));
}
_advance();
}