computed_value_notifier.dart

computed_value_notifier.dart

import 'package:flutter/foundation.dart';

typedef ValueComputation<T, O> = T Function(O value);

class ComputedValueNotifier<T> extends ValueNotifier<T> {
  final Listenable _parent;
  final ValueGetter<T> _compute;

  factory ComputedValueNotifier.fromListenable(
    Listenable notifier,
    ValueGetter<T> callback,
  ) {
    return ComputedValueNotifier._(notifier, callback, callback());
  }

  static ComputedValueNotifier<T> fromValueNotifier<T, O>(
    ValueNotifier<O> parent,
    ValueComputation<T, O> compute,
  ) {
    return _ComputedValueNotifierFromValueNotifiers(parent, compute);
  }

  ComputedValueNotifier._(
    this._parent,
    this._compute,
    T initial,
  ) : super(initial) {
    _parent.addListener(_internalListener);
  }

  void _internalListener() {
    value = _compute();
  }

  @override
  void dispose() {
    _parent.removeListener(_internalListener);
    super.dispose();
  }
}

class _ComputedValueNotifierFromValueNotifiers<T, O> extends ValueNotifier<T>
    implements ComputedValueNotifier<T> {
  @override
  final ValueNotifier<O> _parent;
  @override
  final ValueGetter<T> _compute;

  factory _ComputedValueNotifierFromValueNotifiers(
    ValueNotifier<O> parent,
    ValueComputation<T, O> compute,
  ) {
    return _ComputedValueNotifierFromValueNotifiers._(
      parent,
      () => compute(parent.value),
      compute(parent.value),
    );
  }

  _ComputedValueNotifierFromValueNotifiers._(
    this._parent,
    this._compute,
    T initial,
  ) : super(initial) {
    _parent.addListener(_internalListener);
  }

  @override
  void _internalListener() {
    value = _compute();
  }

  @override
  void dispose() {
    _parent.removeListener(_internalListener);
    super.dispose();
  }
}