`ValueNotifierStatefulWidget` for Flutter

This Flutter package provides an extremely simple state management system for Flutter, particularly for use with widgets whose state may change in reaction to an event handler like onTap.

ValueNotifierStatefulWidget is more or less a merger of the classes ValueNotifier and ValueListenableBuilder into one class, which manages the lifecycle of the ValueNotifier for you.

ValueNotifierStatefulWidget allows you to avoid having to create a new StatefulWidget class every time you need to have the display of the widget respond to a simple state change. This allows you to set up stateful behavior even inside a StatelessWidget, which can dramatically simplify your widget code.

Only one state value can be managed per widget, but that covers a wide array of usecases.

Usage

Wrapping state-dependent widgets

Many Flutter widgets (checkboxes, radio buttons, switches) have to be wrapped in a StatefulWidget to track their selected state. ValueNotifierStatefulWidget dramatically simplifies this.

Before:

class SwitchWidget extends StatefulWidget {
  const SwitchWidget({super.key});

  @override
  State<SwitchWidget> createState() => _SwitchWidgetState();
}

class _SwitchWidgetState extends State<SwitchWidget> {
  bool switchedOn = false;

  @override
  Widget build(BuildContext context) {
    return Switch(
      value: switchedOn,
      onChanged: (bool value) => setState(() => switchedOn = value),
    );
  }
}

After:

ValueNotifierStatefulWidget(
  initialValue: false,
  builder: (context, switchedOn) => Switch(
    value: switchedOn.value,
    onChanged: (bool value) => switchedOn.value = value,
  ),
)

Making state changes after async gaps

ValueNotifierStatefulWidget<bool>(
  initialValue: false,
  builder: (BuildContext context, ValueNotifier<bool> isLoading) =>
    ElevatedButton.icon(
      style: ElevatedButton.styleFrom(
        backgroundColor: Colors.blue,
        foregroundColor: Colors.white,
        disabledBackgroundColor: Colors.blue.shade200,
        disabledForegroundColor: Colors.white,
      ),
      // Button icon: when loading, show a spinner, otherwise show a
      // checkbox icon
      icon: isLoading.value
          ? const SizedBox(
              width: 15,
              height: 15,
              child: CircularProgressIndicator(
                color: Colors.white,
                strokeWidth: 2.5,
              ),
            )
          : const Icon(
              Icons.check,
              color: Colors.white,
            ),
      // Button text
      label: const Text(
        'Submit',
        style: TextStyle(fontSize: 15),
      ),
      // Disable the button when loading
      onPressed: isLoading.value
          ? null
          : () async {
              try {
                // Set loading state to true
                isLoading.value = true;
                // Call asynchronous function
                await doSomethingAsynchronously();
              } finally {
                // Set loading state to false (done inside a finally
                // block in case the async function throws an error)
                isLoading.value = false;
              }
            },
    ),
);

Passing in your own `ValueNotifier`

The ValueNotifierStatefulWidget constructor can take a valueNotifier parameter, where you pass in your own ValueNotifier<T>, which allows the state to be managed externally. This effectively makes ValueNotifierStatefulWidget work similarly to ValueListenableBuilder.

`safeSetState`

This library also exposes safeSetState as an extension on State, which allows you to call setState anytime, including after awaiting an async function in a handler callback, without running into problems with the widget no longer being mounted. safeSetState is called when the ValueNotifier's value changes, which is important for making state updates in asynchronous functions, as shown above.

extension SafeUpdateState on State {
  void safeSetState(void Function() updaterFunction) {
    void callSetState() {
      // Can only call setState if mounted
      if (mounted) {
        // ignore: invalid_use_of_protected_member
        setState(updaterFunction);
      }
    }
    if (SchedulerBinding.instance.schedulerPhase == SchedulerPhase.persistentCallbacks) {
      // Currently building, can't call setState -- add post-frame callback
      SchedulerBinding.instance.addPostFrameCallback((_) => callSetState());
    } else {
      callSetState();
    }
  }
}

lukehutch/flutter_value_notifier_stateful_widget

ValueNotifierStatefulWidget for Flutter