What is the difference between initState and a class constructor in Flutter?

In some cases you will need to start an animation or change the state when you create your Widget, then is not possible to do that in your constructor because your Widget is not inserted in the tree yet.

Example of AnimationController

    AnimationController _animationController ;

    ...

      @override
        void initState() {
        ... instance the animationController

          _animationController.forward();
          super.initState();
        }

Another example, when you receive some params from another Widget, let say your StatefulWidget has a param named title and you want to create a local variable in your State class to handle the state, you will have to do something like this:

  class ExampleWidget extends StatefulWidget {

   final String title;

   ExampleWidget({this.title});

  ....


  YourStateClass extends State<ExampleWidget> {

  var localVariable;

  @override
        void initState() {
          localVariable = widget.title;
          super.initState();
        }

And now you could use your localVariable inside your widget tree to update the state.

The difference is (in the context of creating a State object) which has the initState() method:

• constructor simply create a new State instance

• initState() is called after the object is created and at this point you have access to the BuildContext or the StatefulWidget to which the State is attached to, respectively using the context and the widget properties. At this point the State is already mounted.

Reference State: https://api.flutter.dev/flutter/widgets/State-class.html

Reference mounted State: https://api.flutter.dev/flutter/widgets/State/mounted.html