Flutter design patterns - singleton

main.dart

import 'package:flutter/material.dart';

abstract class ExampleStateBase {
  @protected
  String initialText;
  @protected
  String stateText;
  String get currentText => stateText;

  void setStateText(String text) {
    stateText = text;
  }

  void reset() {
    stateText = initialText;
  }
}

//Singleton’s implementation by definition
class ExampleStateByDefinition extends ExampleStateBase {
  static ExampleStateByDefinition _instance;

  ExampleStateByDefinition._internal() {
    initialText = "A new 'ExampleStateByDefinition' instance has been created.";
    stateText = initialText;
    print(stateText);
  }

  static ExampleStateByDefinition getState() {
    if (_instance == null) {
      _instance = ExampleStateByDefinition._internal();
    }

    return _instance;
  }
}

//Class ExampleState implements a Singleton design pattern “the Dart way”
class ExampleState extends ExampleStateBase {
  static final ExampleState _instance = ExampleState._internal();

  factory ExampleState() {
    return _instance;
  }

  ExampleState._internal() {
    initialText = "A new 'ExampleState' instance has been created.";
    stateText = initialText;
    print(stateText);
  }
}

//Just a simple implementation of the state class without bothering it with Singleton or any other ”fancy-schmancy” design patterns.
class ExampleStateWithoutSingleton extends ExampleStateBase {
  ExampleStateWithoutSingleton() {
    initialText =
        "A new 'ExampleStateWithoutSingleton' instance has been created.";
    stateText = initialText;
    print(stateText);
  }
}


void main() {
  runApp(SingletonExample());
}

class SingletonExample extends StatefulWidget {
  @override
  _SingletonExampleState createState() => _SingletonExampleState();
}

class _SingletonExampleState extends State<SingletonExample> {
  final List<ExampleStateBase> stateList = [
    ExampleState(),
    ExampleStateByDefinition.getState(),
    ExampleStateWithoutSingleton()
  ];

  void _setTextValues([String text = 'Singleton']) {
    for (var state in stateList) {
      state.setStateText(text);
    }
    setState(() {});
  }

  void _reset() {
    for (var state in stateList) {
      state.reset();
    }
    setState(() {});
  }

  @override
  Widget build(BuildContext context) {
    return ScrollConfiguration(
      behavior: ScrollBehavior(),
      child: SingleChildScrollView(
        padding: const EdgeInsets.symmetric(horizontal: paddingL),
        child: Column(
          children: <Widget>[
            for (var state in stateList)
              Padding(
                padding: const EdgeInsets.only(bottom: paddingL),
                child: SingletonExampleCard(
                  text: state.currentText,
                ),
              ),
            const SizedBox(height: spaceL),
            PlatformButton(
              child: Text("Change states\' text to 'Singleton'"),
              materialColor: Colors.black,
              materialTextColor: Colors.white,
              onPressed: _setTextValues,
            ),
            PlatformButton(
              child: Text("Reset"),
              materialColor: Colors.black,
              materialTextColor: Colors.white,
              onPressed: _reset,
            ),
            const SizedBox(height: spaceXL),
            Text(
              'Note: change states\' text and navigate the application (e.g. go to the tab "description" or main menu, then go back to this example) to see how the Singleton state behaves!',
              textAlign: TextAlign.justify,
            ),
          ],
        ),
      ),
    );
  }
}