Flutter で複数の InheritedWidget の値を参照しているときに、参照先ウィジェットがどのタイミングで再ビルドされるのかを確認したかった。その記録。
２つの InheritedWidget の値が同時に更新される場合 #
結果：build() は１回だけ実行される。
import 'package:flutter/material.dart';
void main() {
runApp(const MyApp());
}
// -------------------
// Inherited Counter A
// -------------------
class InheritedCounterA extends InheritedWidget {
const InheritedCounterA({
super.key,
required this.data,
required super.child,
});
final CounterStateA data;
@override
bool updateShouldNotify(InheritedCounterA oldWidget) {
return true;
}
}
class CounterA extends StatefulWidget {
const CounterA({
super.key,
required this.child,
});
final Widget child;
@override
State<CounterA> createState() => CounterStateA();
static CounterStateA of(BuildContext context) {
final result =
context.dependOnInheritedWidgetOfExactType<InheritedCounterA>();
assert(result != null);
return result!.data;
}
}
class CounterStateA extends State<CounterA> {
int count = 0;
void increment() {
setState(() {
count++;
});
}
@override
Widget build(BuildContext context) {
return InheritedCounterA(
data: this,
child: widget.child,
);
}
}
// -------------------
// Inherited Counter B
// -------------------
class InheritedCounterB extends InheritedWidget {
const InheritedCounterB({
super.key,
required this.data,
required super.child,
});
final CounterStateB data;
@override
bool updateShouldNotify(InheritedCounterB oldWidget) {
return true;
}
}
class CounterB extends StatefulWidget {
const CounterB({
super.key,
required this.child,
});
final Widget child;
@override
State<CounterB> createState() => CounterStateB();
static CounterStateB of(BuildContext context) {
final result =
context.dependOnInheritedWidgetOfExactType<InheritedCounterB>();
assert(result != null);
return result!.data;
}
}
class CounterStateB extends State<CounterB> {
int count = 0;
void increment() {
setState(() {
count++;
});
}
@override
Widget build(BuildContext context) {
return InheritedCounterB(
data: this,
child: widget.child,
);
}
}
// ----------
// UI Widgets
// ----------
class MyApp extends StatelessWidget {
const MyApp({
super.key,
});
@override
Widget build(BuildContext context) {
return const MaterialApp(
home: CounterA(
child: CounterB(
child: MyWidget(),
),
),
);
}
}
class MyWidget extends StatelessWidget {
const MyWidget({
super.key,
});
@override
Widget build(BuildContext context) {
final counterA = CounterA.of(context);
final counterB = CounterB.of(context);
print("Re-build");
print("CounterA count: ${counterA.count}");
print("CounterB count: ${counterB.count}");
return Column(
children: [
Text("CounterA count: ${counterA.count}"),
Text("CounterB count: ${counterB.count}"),
IconButton(
icon: const Icon(Icons.add),
onPressed: () {
counterA.increment();
counterB.increment();
},
),
],
);
}
}
# 初回描画
Re-build
CounterA count: 0
CounterB count: 0
# カウントアップボタン押下１回目
Re-build
CounterA count: 1
CounterB count: 1
# カウントアップボタン押下２回目
Re-build
CounterA count: 2
CounterB count: 2
# カウントアップボタン押下３回目
Re-build
CounterA count: 3
CounterB count: 3
２つの InheritedWidget の値が時間差で更新される場合 #
結果：build() は２回実行される。InheritedWidget A に対して１回、InheritedWidget B に対して１回。
import 'package:flutter/material.dart';
void main() {
runApp(const MyApp());
}
// -------------------
// Inherited Counter A
// -------------------
class InheritedCounterA extends InheritedWidget {
const InheritedCounterA({
super.key,
required this.data,
required super.child,
});
final CounterStateA data;
@override
bool updateShouldNotify(InheritedCounterA oldWidget) {
return true;
}
}
class CounterA extends StatefulWidget {
const CounterA({
super.key,
required this.child,
});
final Widget child;
@override
State<CounterA> createState() => CounterStateA();
static CounterStateA of(BuildContext context) {
final result =
context.dependOnInheritedWidgetOfExactType<InheritedCounterA>();
assert(result != null);
return result!.data;
}
}
class CounterStateA extends State<CounterA> {
int count = 0;
void increment() {
Future.delayed(const Duration(seconds: 1), () {
setState(() {
count++;
});
});
}
@override
Widget build(BuildContext context) {
return InheritedCounterA(
data: this,
child: widget.child,
);
}
}
// -------------------
// Inherited Counter B
// -------------------
class InheritedCounterB extends InheritedWidget {
const InheritedCounterB({
super.key,
required this.data,
required super.child,
});
final CounterStateB data;
@override
bool updateShouldNotify(InheritedCounterB oldWidget) {
return true;
}
}
class CounterB extends StatefulWidget {
const CounterB({
super.key,
required this.child,
});
final Widget child;
@override
State<CounterB> createState() => CounterStateB();
static CounterStateB of(BuildContext context) {
final result =
context.dependOnInheritedWidgetOfExactType<InheritedCounterB>();
assert(result != null);
return result!.data;
}
}
class CounterStateB extends State<CounterB> {
int count = 0;
void increment() {
Future.delayed(const Duration(seconds: 2), () {
setState(() {
count++;
});
});
}
@override
Widget build(BuildContext context) {
return InheritedCounterB(
data: this,
child: widget.child,
);
}
}
// ----------
// UI Widgets
// ----------
class MyApp extends StatelessWidget {
const MyApp({
super.key,
});
@override
Widget build(BuildContext context) {
return const MaterialApp(
home: CounterA(
child: CounterB(
child: MyWidget(),
),
),
);
}
}
class MyWidget extends StatelessWidget {
const MyWidget({
super.key,
});
@override
Widget build(BuildContext context) {
final counterA = CounterA.of(context);
final counterB = CounterB.of(context);
print("Re-build");
print("CounterA count: ${counterA.count}");
print("CounterB count: ${counterB.count}");
return Column(
children: [
Text("CounterA count: ${counterA.count}"),
Text("CounterB count: ${counterB.count}"),
IconButton(
icon: const Icon(Icons.add),
onPressed: () {
counterA.increment();
counterB.increment();
},
),
],
);
}
}
# 初回描画
Re-build
CounterA count: 0
CounterB count: 0
# カウントアップボタン押下１回目
# （１秒後）
Re-build
CounterA count: 1
CounterB count: 0
# （２秒後）
Re-build
CounterA count: 1
CounterB count: 1
# カウントアップボタン押下２回目
# （１秒後）
Re-build
CounterA count: 2
CounterB count: 1
# （２秒後）
Re-build
CounterA count: 2
CounterB count: 2
# カウントアップボタン押下３回目
# （１秒後）
Re-build
CounterA count: 3
CounterB count: 2
# （２秒後）
Re-build
CounterA count: 3
CounterB count: 3
親ウィジェットでキャッシュしている場合 #
結果：親ウィジェットでキャッシュしていても、子ウィジェットが InheritedWidget を参照していれば子ウィジェットの build() は実行される。
import 'package:flutter/material.dart';
void main() {
runApp(const MyApp());
}
// -------------------
// Inherited Counter A
// -------------------
class InheritedCounterA extends InheritedWidget {
const InheritedCounterA({
super.key,
required this.data,
required super.child,
});
final CounterStateA data;
@override
bool updateShouldNotify(InheritedCounterA oldWidget) {
return true;
}
}
class CounterA extends StatefulWidget {
const CounterA({
super.key,
required this.child,
});
final Widget child;
@override
State<CounterA> createState() => CounterStateA();
static CounterStateA of(BuildContext context) {
final result =
context.dependOnInheritedWidgetOfExactType<InheritedCounterA>();
assert(result != null);
return result!.data;
}
}
class CounterStateA extends State<CounterA> {
int count = 0;
void increment() {
Future.delayed(const Duration(seconds: 1), () {
setState(() {
count++;
});
});
}
@override
Widget build(BuildContext context) {
return InheritedCounterA(
data: this,
child: widget.child,
);
}
}
// -------------------
// Inherited Counter B
// -------------------
class InheritedCounterB extends InheritedWidget {
const InheritedCounterB({
super.key,
required this.data,
required super.child,
});
final CounterStateB data;
@override
bool updateShouldNotify(InheritedCounterB oldWidget) {
return true;
}
}
class CounterB extends StatefulWidget {
const CounterB({
super.key,
required this.child,
});
final Widget child;
@override
State<CounterB> createState() => CounterStateB();
static CounterStateB of(BuildContext context) {
final result =
context.dependOnInheritedWidgetOfExactType<InheritedCounterB>();
assert(result != null);
return result!.data;
}
}
class CounterStateB extends State<CounterB> {
int count = 0;
void increment() {
Future.delayed(const Duration(seconds: 2), () {
setState(() {
count++;
});
});
}
@override
Widget build(BuildContext context) {
return InheritedCounterB(
data: this,
child: widget.child,
);
}
}
// ----------
// UI Widgets
// ----------
class MyApp extends StatelessWidget {
const MyApp({
super.key,
});
@override
Widget build(BuildContext context) {
return const MaterialApp(
home: CounterA(
child: CounterB(
child: MyWidget(),
),
),
);
}
}
class MyWidget extends StatefulWidget {
const MyWidget({
super.key,
});
@override
State<MyWidget> createState() => _MyWidgetState();
}
class _MyWidgetState extends State<MyWidget> {
Widget? _cachedWidget;
(int countA, int countB)? _previousCount;
Widget _build(BuildContext context) {
final counterA = CounterA.of(context);
final counterB = CounterB.of(context);
return Column(
children: [
Text("CounterA count: ${counterA.count}"),
Text("CounterB count: ${counterB.count}"),
IconButton(
icon: const Icon(Icons.add),
onPressed: () {
counterA.increment();
counterB.increment();
},
),
],
);
}
@override
Widget build(BuildContext context) {
final counterA = CounterA.of(context);
final counterB = CounterB.of(context);
print("Re-build");
print("CounterA count: ${counterA.count}");
print("CounterB count: ${counterB.count}");
return _cachedWidget = _cachedWidget == null ||
_previousCount != (counterA.count, counterB.count)
? _build(context)
: _cachedWidget!;
}
}
# 初回描画
Re-build
CounterA count: 0
CounterB count: 0
# カウントアップボタン押下１回目
# （１秒後）
Re-build
CounterA count: 1
CounterB count: 0
# （２秒後）
Re-build
CounterA count: 1
CounterB count: 1
# カウントアップボタン押下２回目
# （１秒後）
Re-build
CounterA count: 2
CounterB count: 1
# （２秒後）
Re-build
CounterA count: 2
CounterB count: 2
# カウントアップボタン押下３回目
# （１秒後）
Re-build
CounterA count: 3
CounterB count: 2
# （２秒後）
Re-build
CounterA count: 3
CounterB count: 3