在'TestPageState'中,我放了一个按钮来调用'setState'方法。新的TestChildWidget()将创建一个新的TestChildWidget,默认构造函数TestChildWidget()被调用,但是为什么不调用TestChildWidget中的'createState()'方法呢?
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');
}
}
在'TestPageState'中,我放了一个按钮来调用'setState'方法。新的TestChildWidget()将创建一个新的TestChildWidget,默认构造函数TestChildWidget()被调用,但是为什么不调用TestChildWidget中的'createState()'方法呢?
创建状态或在创建状态时调用状态build
。
class TestChildWidgetState extends State<TestChildWidget>{
TestChildWidgetState(){
print("Created only once");
}
}
该框架在对StatefulWidget进行充气时会调用createState,这意味着如果该StatefulWidget已在多个位置插入到树中,则可能会将多个State对象与同一StatefulWidget关联。同样,如果将StatefulWidget从树中删除并随后再次插入到树中,则框架将再次调用createState来创建新的State对象,从而简化State对象的生命周期。参见StatefulWidget
print(...)日志可能无法打印到控制台,但Flutter已调用createState()
您可以尝试使用:
developer.debugger(when: true);
在createState()
中调用setState()时,它将调用
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;
}
}
如果canUpdate()返回true,则意味着无需创建新元素。