Wrap any TextField with TextEditingControllerBuilder to make it declarative

text_editing_controller_builder.dart

import 'package:flutter/widgets.dart';

class TextEditingControllerBuilder extends StatefulWidget {
  /// Exposes a [TextEditingController] to the child, which allows
  /// us to convert any [TextField] into a declarative version.
  ///
  /// Typically used for wiring up many state fields to form inputs
  /// and making sure everything stays in sync.
  ///
  /// If [text] is updated, the consuming [TextField] will also be updated.
  /// If the ancestor is rebuilt, the composing state will not be lost like it typically is.
  ///
  /// ```dart
  /// TextEditingControllerBuilder(
  ///   text: appState.name,
  ///   builder: (context, controller) {
  ///     return TextField(
  ///       controller: controller,
  ///       onChanged: (value) => appState.updateName(value),
  ///     );
  ///   },
  /// );
  /// ```
  const TextEditingControllerBuilder({
    Key key,
    @required this.text,
    @required this.builder,
  }) : super(key: key);

  /// The text to declaratively update in the text controller
  final String text;

  /// The builder which exposes the [TextEditingController] to the child
  final Widget Function(BuildContext, TextEditingController) builder;

  @override
  _TextEditingControllerBuilderState createState() =>
      _TextEditingControllerBuilderState();
}

class _TextEditingControllerBuilderState
    extends State<TextEditingControllerBuilder> {
  TextEditingController controller;

  @override
  void initState() {
    controller = TextEditingController(text: widget.text);
    super.initState();
  }

  @override
  void dispose() {
    controller?.dispose();
    super.dispose();
  }

  @override
  void didUpdateWidget(covariant TextEditingControllerBuilder oldWidget) {
    if (oldWidget.text != widget.text) {
      controller.value = controller.value.copyWith(text: widget.text);
    }

    super.didUpdateWidget(oldWidget);
  }

  @override
  Widget build(BuildContext context) {
    return widget.builder(context, controller);
  }
}