当MyChangeNotifier更改时如何计算函数，而无需在构建方法中进行不必要的调用 Flutter

您可以使用StatefulWidget和ListenableBuilder的组合.确保您的小部件为StatefulWidget，因为您需要管理状态和生命周期方法.然后，使用ListenableBuilder来收听MyChangeNotifier中的变化.只有当通知器通知其监听器时，此小部件才会重建，这正是您所需要的.

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

class MyWidget extends StatefulWidget {
 @override
 _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
 Future<String>? _futureMd5;

 @override
 void initState() {
    super.initState();
    // Initialize _futureMd5 with a default value or null if not needed
    _futureMd5 = _computemd5(Provider.of<MyChangeNotifier>(context, listen: false));
 }

 @override
 Widget build(BuildContext context) {
    return ListenableBuilder(
      listenable: Provider.of<MyChangeNotifier>(context),
      builder: (context, child) {
        // Trigger _computemd5 only when MyChangeNotifier is updated
        _futureMd5 = _computemd5(Provider.of<MyChangeNotifier>(context, listen: false));
        return FutureBuilder<String>(
          future: _futureMd5,
          builder: (context, snapshot) {
            if (snapshot.connectionState == ConnectionState.waiting) {
              return CircularProgressIndicator(); // Show loading indicator while waiting
            } else if (snapshot.hasError) {
              return Text('Error: ${snapshot.error}');
            } else {
              return Text(snapshot.data!); // Display the result
            }
          },
        );
      },
    );
 }

 Future<String> _computemd5(MyChangeNotifier myChangeNotifier) async {
    // Your computation logic here
    // For demonstration, let's just return a dummy string after a delay
    await Future.delayed(Duration(seconds: 2));
    return 'MD5 Result';
 }
}