Life cycle in Flutter (from https://stackoverflow.com/a/53379776/3177846)

lifeCycle.dart

import 'package:flutter/material.dart';

class LifeCycle extends StatefulWidget {
  // 1 - createState(): When the Framework is instructed to build a StatefulWidget, it immediately calls createState()
  @override
  _LifeCycleState createState() => _LifeCycleState();
}

class _LifeCycleState extends State<LifeCycle> {
  // 2 - mounted is true: When createState creates your state class, a buildContext is assigned to that state.
  // BuildContext is, overly simplified, the place in the widget tree in which this widget is placed.
  // Here's a longer explanation:
  // All widgets have a bool this.mounted property.
  // It is turned true when the buildContext is assigned.
  // It is an error to call setState when a widget is unmounted.

  @override
  // 3 - initState(): This is the first method called when the widget is created (after the class constructor, of course.) initState is called once and only once. It must called super.initState().
  void initState() {
    super.initState();
  }

  @override
  // 4 - didChangeDependencies(): This method is called immediately after initState on the first time the widget is built.
  void didChangeDependencies() {
    super.didChangeDependencies();
  }

  @override
  // 5 - build(): This method is called often. It is required, and it must return a Widget.
  Widget build(BuildContext context) {
    return Text('Life cycle');
  }

  @override
  // 6 - didUpdateWidget(Widget oldWidget): If the parent widget changes and has to rebuild this widget
  // (because it needs to give it different data), but it's being rebuilt with the same runtimeType,
  // then this method is called.
  // This is because Flutter is re-using the state, which is long lived.
  // In this case, you may want to initialize some data again, as you would in initState.
  void didUpdateWidget(Widget oldWidget) {
    super.didUpdateWidget(Widget oldWidget);
  }

  @override
  // 7 - setState(): This method is called often from the framework itself and from the developer.
  // Its used to notify the framework that data has changed
  void setState() {
    super.setState();
  }

  @override
  // 8 - deactivate(): Deactivate is called when State is removed from the tree, but it might be reinserted
  // before the current frame change is finished.
  // This method exists basically because State objects can be moved from one point in a tree to another.
  void deactivate() {
    super.deactivate();
  }

  @override
  // 9 - dispose(): Dispose is called when the State object is removed, which is permanent.
  // This method is where you should unsubscribe and cancel all animations, streams, etc.
  void dispose() {
    super.dispose();
  }

  // 10 - mounted is false: The state object can never remount, and an error is thrown is setState is called.

}