Flutter:如果存在则获取提供程序，否则返回null而不是异常

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

void main() {
  runApp(
    ChangeNotifierProvider(
      create: (context) => MyNotifier(),
      child: MyApp(),
    ),
  );
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('Provider Example')),
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // 使用?.操作符来避免异常
    final myNotifier = Provider.of<MyNotifier>(context, listen: false)?.value;

    return Text('Value: $myNotifier');
  }
}

class MyNotifier extends ChangeNotifier {
  int _value = 42;

  int get value => _value;

  void updateValue(int newValue) {
    _value = newValue;
    notifyListeners();
  }
}