Flutter - New StatefullWidget, why related createState() method not called? - flutter

Question

in the 'TestPageState', I put a button to call 'setState' method. new TestChildWidget() will create a new TestChildWidget,default construtor TestChildWidget() is called, but why 'createState()' method in TestChildWidget not called?
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
// This widget is the root of your application.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: new TestPage()
);
}
}
class TestPage extends StatefulWidget{
TestPageState createState(){
print('TestPageState createState');
return new TestPageState();
}
}
class TestPageState extends State<TestPage>{
#override
Widget build(BuildContext context) {
print('TestPageState Build');
// TODO: implement build
return Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
new TestChildWidget(),
RaisedButton(
child: Text("刷新"),
onPressed: () => setState(() => print('setState')), //build method will called, new TestChildWidget() will be called
)
],
);
}
}
class TestChildWidget extends StatefulWidget{
TestChildWidget(){
//new TestChildWidget(), this default construtor will be called, but why createState() not called?
}
#override
State<StatefulWidget> createState() {
// TODO: implement createState
print('TestChildWidget createState');
return new TestChildWidgetState();
}
}
class TestChildWidgetState extends State<TestChildWidget>{
#override
Widget build(BuildContext context) {
print('TestChildWidgetState build');
// TODO: implement build
return Text('1111');
}
}
in the 'TestPageState', I put a button to call 'setState' method. new TestChildWidget() will create a new TestChildWidget,default construtor TestChildWidget() is called, but why 'createState()' method in TestChildWidget not called?

Answer 1

It creates a state or calls the state build when the state is created.
class TestChildWidgetState extends State<TestChildWidget>{
TestChildWidgetState(){
print("Created only once");
}
}
The framework calls createState whenever it inflates a StatefulWidget, which means that multiple State objects might be associated with the same StatefulWidget if that widget has been inserted into the tree in multiple places. Similarly, if a StatefulWidget is removed from the tree and later inserted in to the tree again, the framework will call createState again to create a fresh State object, simplifying the lifecycle of State objects.
See StatefulWidget

Answer 2

print(...) log maybe not print to console but Flutter have called createState()
You can try use:
developer.debugger(when: true);
in createState()

Answer 3

when setState() called, it will call
abstract class Widget extends DiagnosticableTree {
const Widget({ this.key });
final Key key;
···
static bool canUpdate(Widget oldWidget, Widget newWidget) {
return oldWidget.runtimeType == newWidget.runtimeType
&& oldWidget.key == newWidget.key;
}
}
if canUpdate() return true, it means there is no need to create a new element.

Answer 4

The Stateful widget has a default parameter 'Key'. If you supply it with a different value each time - that would enforce new state to be created i.e createState() called.
Instead of new TestChildWidget(), try:
new TestChildWidget(
key: Key('different key for diff states'), // or key: UniqueKey()
),
I actually use the same key value for states that are same, in order to have the caching in those cases.