signals-and-flutter-hooks

Signals 和 Flutter Hooks

在使用 Flutter 处理数据时，最先接触到的内容之一就是 setState。

setState

import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(debugShowCheckedModeBanner: false, home: Counter()));
}

class Counter extends StatefulWidget {
  const Counter({super.key});

  @override
  State<Counter> createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  int count = 0;

  void increment() {
    if (mounted) {
      setState(() {
        count++;
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Counter')),
      body: Center(child: Text('Count: $count')),
      floatingActionButton: FloatingActionButton(
        onPressed: increment,
        child: const Icon(Icons.add),
      ),
    );
  }
}

每次调用 setState 时，它只会将 widget 标记为需要重建，但需要你（作为开发者）留意并明确这些更新发生的时机。如果修改数据时忘记调用 setState，widget 树就可能变得过时。

ValueNotifier

我们可以使用 ValueNotifier 来改进这一点，而不是直接存储值。这样，我们就可以在容器中读写值，并使用 ValueListenableBuilder 等辅助 widget，在值发生变化时更新 widget 树的部分区域。

import 'package:flutter/material.dart';

void main() {
  runApp(const MaterialApp(debugShowCheckedModeBanner: false, home: Counter()));
}

class Counter extends StatefulWidget {
  const Counter({super.key});

  @override
  State<Counter> createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  final count = ValueNotifier(0);

  void increment() {
    count.value++;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Counter')),
      body: Center(child: ValueListenableBuilder(
        valueListenable: count,
        builder: (context, value, child) {
          return Text('Count: $value');
        }
      )),
      floatingActionButton: FloatingActionButton(
        onPressed: increment,
        child: const Icon(Icons.add),
      ),
    );
  }
}

FlutterSignal

使用 signals 包，我们可以将 ValueNotifier 升级为基于 signal 的实现，它采用基于推送 / 拉取架构的响应式图。

import 'package:flutter/material.dart';
import 'package:signals/signals_flutter.dart';

void main() {
  runApp(const MaterialApp(debugShowCheckedModeBanner: false, home: Counter()));
}

class Counter extends StatefulWidget {
  const Counter({super.key});

  @override
  State<Counter> createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  final count = signal(0);

  void increment() {
    count.value++;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Counter')),
      body: Center(
        child: ValueListenableBuilder(
          valueListenable: count,
          builder: (context, value, child) {
            return Text('Count: $value');
          },
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: increment,
        child: const Icon(Icons.add),
      ),
    );
  }
}

6.0.0 之后创建的 Signals 也实现了 ValueNotifier，因此无需更改其他代码即可轻松迁移。

我们可以使用 Watch widget 或 .watch(context) 扩展来替代 ValueListenableBuilder。

import 'package:flutter/material.dart';
import 'package:signals/signals_flutter.dart';

void main() {
  runApp(const MaterialApp(debugShowCheckedModeBanner: false, home: Counter()));
}

class Counter extends StatefulWidget {
  const Counter({super.key});

  @override
  State<Counter> createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  final count = signal(0);

  void increment() {
    count.value++;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Counter')),
      body: Center(
        child: Text('Count: ${count.watch(context)}'),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: increment,
        child: const Icon(Icons.add),
      ),
    );
  }
}

flutter_hooks

使用 Flutter Hooks，我们可以改用 HookWidget，减少 StatefulWidget 的样板代码。借助 useState，我们可以直接在 build 方法中定义状态，并轻松地在 widget 之间共享。

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

void main() {
  runApp(const MaterialApp(debugShowCheckedModeBanner: false, home: Counter()));
}

class Counter extends HookWidget {
  const Counter({super.key});

  @override
  Widget build(BuildContext context) {
    final count = useState(0);
    return Scaffold(
      appBar: AppBar(title: const Text('Counter')),
      body: Center(child: Text('Count: ${count.value}')),
      floatingActionButton: FloatingActionButton(
        onPressed: () => count.value++,
        child: const Icon(Icons.add),
      ),
    );
  }
}

useState 会返回一个 ValueNotifier，值发生变化时会自动重建 widget。

signals_hooks

借助新的包 signals_hooks，我们现在可以在 HookWidget 中定义 Signals，并获得响应式图的优势，同时还能在 widget 之间共享生命周期。

import 'package:flutter/material.dart';
import 'package:flutter_hooks/flutter_hooks.dart';
import 'package:signals_hooks/signals_hooks.dart';

void main() {
  runApp(const MaterialApp(debugShowCheckedModeBanner: false, home: Counter()));
}

class Counter extends HookWidget {
  const Counter({super.key});

  @override
  Widget build(BuildContext context) {
    final count = useSignal(0);
    final countStr = useComputed(() => count.value.toString());
    useSignalEffect(() {
      print('count: $count');
    });
    return Scaffold(
      appBar: AppBar(title: const Text('Counter')),
      body: Center(child: Text('Count: $countStr')),
      floatingActionButton: FloatingActionButton(
        onPressed: () => count.value++,
        child: const Icon(Icons.add),
      ),
    );
  }
}