Flutter Provider - A Simple Yet Powerful State Management Tool

Flutter Provider
看似簡單卻⼜不簡單的
狀態管理⼯具
Kaohsiung
Johnny Sung

Full stack developer
Johnny Sung (宋岡諺)
https://fb.com/j796160836
https://blog.jks.co
ff
ee/
https://www.slideshare.net/j796160836
https://github.com/j796160836

https://pub.dev/packages/provider

講在 Provider 之前
— InheritedWidget
Kaohsiung

https://www.youtube.com/watch?v=Zbm3hjPjQMk

萬物皆 Widget
在對應的狀態產⽣對應的畫⾯

https://abhishekdoshi26.medium.com/deep-dive-into-
fl
utter-trees-542f7395df5c

Widget 兄弟其實還少⼀個
• StatefulWidget
• StatelessWidget
• InheritedWidget 就是我啦！

InheritedWidget
想解決
Widget tree 跨層傳遞資料不便的問題

https://docs.
fl
utter.dev/data-and-backend/state-mgmt/simple

class FrogColor extends InheritedWidget {
const FrogColor({
super.key,
required this.color,
required super.child,
});
final Color color;
static FrogColor? maybeOf(BuildContext context) {
return context.dependOnInheritedWidgetOfExactType<FrogColor>();
}
static FrogColor of(BuildContext context) {
final FrogColor? result = maybeOf(context);
assert(result != null, 'No FrogColor found in context');
return result!;
}
@override
bool updateShouldNotify(FrogColor oldWidget) => color != oldWidget.color;
}
https://api.
fl
utter.dev/
fl
utter/widgets/InheritedWidget-class.html
背後宣告

// continuing from previous example...
class MyPage extends StatelessWidget {
const MyPage({super.key});
@override
Widget build(BuildContext context) {
return Scaffold(
body: FrogColor(
color: Colors.green,
child: Builder(
builder: (BuildContext innerContext) {
return Text(
'Hello Frog',
style: TextStyle(color: FrogColor.of(innerContext).color),
);
},
),
),
);
}
}
https://api.
fl
utter.dev/
fl
呼叫使⽤

Provider
就是 InheritedWidget 的延伸（與包裝）
其實

不信？讓我們看看⼀⼩段 Provider 的原始碼

class FrogColor extends InheritedWidget {
const FrogColor({
super.key,
required this.color,
required super.child,
});
final Color color;
static FrogColor? maybeOf(BuildContext context) {
return context.dependOnInheritedWidgetOfExactType<FrogColor>();
}
static FrogColor of(BuildContext context) {
final FrogColor? result = maybeOf(context);
assert(result != null, 'No FrogColor found in context');
return result!;
}
@override
bool updateShouldNotify(FrogColor oldWidget) => color != oldWidget.color;
}
https://api.
fl
utter.dev/
fl
（剛剛看到的段落）

class Provider<T> extends InheritedProvider<T> {
Provider({
Key? key,
required Create<T> create,
Dispose<T>? dispose,
bool? lazy,
TransitionBuilder? builder,
Widget? child,
}) : super(
key: key,
lazy: lazy,
builder: builder,
create: create,
dispose: dispose,
debugCheckInvalidValueType: kReleaseMode
? null
: (T value) =>
Provider.debugCheckInvalidValueType?.call<T>(value),
child: child,
);
static T of<T>(BuildContext context, {bool listen = true}) {
assert(
context.owner!.debugBuilding ||
listen == false ||
debugIsInInheritedProviderUpdate,
'''
Tried to listen to a value exposed with provider, from outside of the widget tree. … 略
''',
);
final inheritedElement = _inheritedElementOf<T>(context);
if (listen) {
context.dependOnInheritedWidgetOfExactType<_InheritedProviderScope<T?>>();
}
final value = inheritedElement?.value;
if (_isSoundMode) {
if (value is! T) {
throw ProviderNullException(T, context.widget.runtimeType);
}
return value;
}
return value as T;
}
}
Provider 的原始碼

Provider 家族
• Provider：提供遠端存取的服務
• ChangeNoti
fi
erProvider：它會監聽 model 的變化。當資料發⽣變更，它會重繪
Consumer 的⼦ widget。
• FutureProvider：是 FutureBuilder 的⼀個封裝，提供⼀個 Future，FutureProvider 會在 Future 完成的時候
通知 Consumer 重繪它的 Widget ⼦樹。
• StreamProvider：是 StreamBuilder 的⼀個封裝，提供⼀個 Stream，然後 Consumer 會在 Stream 收到事
件時更新它的 Widget⼦樹。

import 'package:flutter/foundation.dart';
class Counter with ChangeNotifier {
int _count = 0;
int get count => _count;
void increment() {
_count++;
notifyListeners();
}
}

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'counter.dart';
class MyHomePage extends StatefulWidget {
const MyHomePage({super.key});
@override
State<MyHomePage> createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
@override
return Scaffold(
body: ChangeNotifierProvider<Counter>(
create: (context) => Counter(),
child: Builder(builder: (context) {
final counter = context.watch<Counter>().count;
return Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text('$counter'),
ElevatedButton(
onPressed: () {
final counter = context.read<Counter?>();
counter?.increment();
print('count=${counter?.count ?? 0}');
},
child: const Text('Push me')),
],
),
);
}),
),
);
}
}

Consumer<Counter>(builder: (context, myModel, child) {
return Text('${myModel.count}');
}),
或
三種寫法都可以
註：Consumer 的部份
Text('${context.watch<Counter>().count}')
Text('${Provider.of<Counter>(context).count}')
或

/// Exposes the [read] method.
extension ReadContext on BuildContext {
T read<T>() {
return Provider.of<T>(this, listen: false);
}
}
Provider 原始碼中
context.read() 對應到 Provider.of()

import 'package:flutter/foundation.dart';
class TabProvider with ChangeNotifier {
int _currentIndex = 0;
int get currentIndex => _currentIndex;
set currentIndex(int index) {
_currentIndex = index;
notifyListeners();
}
}

mixin & with
Mixins are a way of reusing a class’s code in multiple class hierarchies.
簡單來說就是：
我提供⼀些⽅法給你使⽤，但我不⽤成為你的⽗類別。
mixin ShowName {
void showName() {
print('mixin ShowName');
}
}
class Quadrilateral with ShowName {
void showAllSideLength() {
showName();
}
}
https://ithelp.ithome.com.tw/articles/10268368?sc=iThelpR

import 'package:flutter_provider_tab_demo/tab_provider.dart';
import 'package:flutter_provider_tab_demo/tab_screen.dart';
void main() {
runApp(const MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({super.key});
// This widget is the root of your application.
@override
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MultiProvider(
providers: [
ChangeNotifierProvider(create: (context) => TabProvider()),
],
child: TabScreen(),
),
);
}
}

import 'package:flutter_provider_tab_demo/tab_provider.dart';
class TabItem {
final String name;
final int index;
TabItem(this.name, this.index);
}
class TabScreen extends StatelessWidget {
final List<TabItem> tabItems = [
TabItem('Tab 1', 0),
];
TabScreen({super.key});
@override
// build
}
}
return Scaffold(
appBar: AppBar(
title: const Text('Tab Example'),
),
body: Column(
children: [
ListView.builder(
shrinkWrap: true,
itemCount: tabItems.length,
itemBuilder: (context, index) {
TabItem tabItem = tabItems[index];
return ListTile(
title: Text(tabItem.name),
onTap: () {
TabProvider p = context.read<TabProvider>();
p.currentIndex = tabItem.index;
},
);
},
),
Expanded(
child: IndexedStack(
index: context.watch<TabProvider>().currentIndex,
children: const [
Center(child: Text('Tab 1')),
],
),
),
],
),
);
選單按鈕
分⾴內⾴

Provider 使⽤情境
• 會員帳號權限管理
• 帳號登入
• Token 過期、被強迫登出
• Widget tree 平⾏傳遞資料（例如：分⾴切換）

全域的狀態機變數
Provider 我的理解：

全域變數
雖可恥但有⽤

https://stackover
fl
ow.com/questions/73226745/
fl
utter-many-bloc-providers-cluttering-widget-tree

MaterialApp(
theme: ThemeData(
),
home: BlocProvider<SigninCubit>(
create: (context) => SigninCubit(),
child: BlocProvider<UpdateCubit>(
create: (context) => UpdateCubit(),
child: BlocProvider<ProfileBloc>(
create: (context) => ProfileBloc(),
child: BlocProvider<VoteBloc>(
create: (context) => VoteBloc(),
child: BlocProvider<MessageBloc>(
create: (context) => MessageBloc(),
child: BlocProvider<LeaderboardBloc>(
create: (context) => LeaderboardBloc(),
child: BlocProvider<CommentBloc>(
create: (context) => CommentBloc(),
child: BlocProvider<EditbioCubit>(
create: (context) => EditbioCubit(),
child: BlocProvider<CommentCubit>(
create: (context) => CommentCubit(),
child: BlocProvider<SearchBloc>(
create: (context) => SearchBloc(),
child: BlocProvider<SwipeBloc>(
create: (context) => SwipeBloc(),
child: Scaffold(
body: Builder(builder: (context) {
return const Center(
child: Column(
children: <Widget>[
Text('Sample'),
],
),
);
}),
),
),
),
),
),
),
),
),
),
),
),
),
);
不免俗的，Flutter 沒寫好的話，很容易寫出⼀堆波動拳
https://captown.capcom.com/zh-TW/museums/street
fi
ghter/5/images/125#group=0&photo=0

部分的波動拳可以⽤ MultiXXXProvider 解決部分的問題
但這範例還有權責不明確問題
MaterialApp(
theme: ThemeData(
),
home: MultiBlocProvider(
providers: [
BlocProvider<SigninCubit>(create: (context) => SigninCubit()),
BlocProvider<UpdateCubit>(create: (context) => UpdateCubit()),
BlocProvider<ProfileBloc>(create: (context) => ProfileBloc()),
BlocProvider<VoteBloc>(create: (context) => VoteBloc()),
BlocProvider<MessageBloc>(create: (context) => MessageBloc()),
BlocProvider<LeaderboardBloc>(create: (context) => LeaderboardBloc()),
BlocProvider<CommentBloc>(create: (context) => CommentBloc()),
BlocProvider<EditbioCubit>(create: (context) => EditbioCubit()),
BlocProvider<CommentCubit>(create: (context) => CommentCubit()),
BlocProvider<SearchBloc>(create: (context) => SearchBloc()),
BlocProvider<SwipeBloc>(create: (context) => SwipeBloc()),
],
child: Scaffold(
body: Builder(builder: (context) {
return const Center(
child: Column(
children: <Widget>[
Text('Sample'),
],
),
);
}),
),
),
);

要慎⽤呀！
https://pic1.zhimg.com/80/v2-5af8d68f267c1ed3ece59d5bd919a870_1440w.webp

解決波動拳⼩建議
• View 分類好封裝進 StatelessWidget
• 邏輯與 View 分類分開存放
• ⽤ MultiProvider 或 MultiBlocProvider 等類似套件來拉平

Q: Provider 與 Bloc 有什麼差異？
我的回答：要解決的問題不同，使⽤不同的解決⽅式

當 Bloc 遇上 provider ?
http://teddy-chen-tw.blogspot.com/2012/03/blog-post_20.html
爭什麼，摻在⼀起做成撒尿⽜丸啊

https://pub.dev/packages/
fl
utter_bloc

https://pub.dev/packages/
fl
utter_bloc
😜

BlocProvider = Bloc + Provider

BlocProvider 家族
• BlocProvider
• BlocBuilder
• BlocListener
• BlocConsumer = BlocBuilder +
BlocListener

BlocProvider<LoginBloc>(
create: (BuildContext context) => LoginBloc()),
child: …,
),
在 Widget Tree 最頂層處使⽤ BlocProvider 並宣告⾃訂的 Bloc
（以下是⼀些 pseudo code）
BlocProvider 使⽤⽅式 (1/4)

BlocBuilder<LoginBloc, LoginState>(builder: (context, state) {
if (state is LoginStateLoggedIn) {
return …;
}
if (state is LoginStateInitial) {
return …;
}
return Container();
}
在所需 Widget 的 build() 處使⽤ BlocBuilder
這裡沒指定 bloc 參數時，預設會 Provider 往上找

在需要聆聽事件的地⽅使⽤ BlocListener
這裡沒指定 bloc 參數時，預設會 Provider 往上找
BlocListener<LoginBloc, LoginState>(
listener: (context, state) {
if (state is LoginStateError) {
// Do Something
}
if (state is LoginStateLoggedIn) {
// Do Something
}
},
child: … );

在需要變更狀態的地⽅使⽤ context.read() 並操作 Bloc
ElevatedButton(
child: const Text('Login'),
onPressed: () {
context.read<LoginBloc>().add(LoginEventLogin());
},
)

其他狀態管理比較
Kaohsiung

https://k.sina.cn/article_7407912364_1b98bc5ac00101jqaw.html?from=cul

https://pub.dev/packages/scoped_model

https://pub.dev/packages/riverpod

Flutter Provider - A Simple Yet Powerful State Management Tool

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