Flutter 组件集录 | 后悔药 UndoHistory

在Flutter中，UndoHistory是一个用于处理撤销操作的类。它通常不是直接使用的，而是作为Undo包中其他控件的一部分，如UndoStack。如果你需要实现一个简单的撤销功能，你可以使用ValueNotifier和ValueListenableBuilder来创建一个简单的撤销控件。

以下是一个简单的例子，展示了如何使用ValueNotifier和ValueListenableBuilder来创建一个简单的撤销记录系统：

import 'package:flutter/material.dart';
 
void main() {
  runApp(MyApp());
}
 
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomePage(),
    );
  }
}
 
class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}
 
class _HomePageState extends State<HomePage> {
  final _undoHistory = UndoHistory<String>();
 
  void _addItem(String item) {
    _undoHistory.add(item);
  }
 
  void _undo() {
    _undoHistory.undo();
  }
 
  void _redo() {
    _undoHistory.redo();
  }
 
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            ValueListenableBuilder<List<String>>(
              valueListenable: _undoHistory.undoStack,
              builder: (context, stack, child) {
                return Text('Undo Stack: ${stack.join(", ")}');
              },
            ),
            ValueListenableBuilder<List<String>>(
              valueListenable: _undoHistory.redoStack,
              builder: (context, stack, child) {
                return Text('Redo Stack: ${stack.join(", ")}');
              },
            ),
            Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                FlatButton(
                  child: Text('Add Item'),
                  onPressed: () => _addItem('Item'),
                ),
                FlatButton(
                  child: Text('Undo'),
                  onPressed: _undoHistory.canUndo ? _undo : null,
                ),
                FlatButton(
                  child: Text('Redo'),
                  onPressed: _undoHistory.canRedo ? _redo : null,
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}
 
class UndoHistory<T> {
  final _undoStack = <T>[];
  final _redoStack = <T>[];
 
  final ValueNotifier<List<T>> _undoStackNotifier = ValueNotifier([]);
  final ValueNotifier<List<T>> _redoStackNotifier = ValueNotifier([]);
 
  ValueListenable<List<T>> get undoStack =>