flutter, How to judge whether another widget is in the tree - flutter

Question

Actually, I have a parent widget, and It has some of the child widgets in its Column.
like this
Container(
width: double.infinity,
color: Colors.white,
padding: EdgeInsets.fromLTRB(20.0, 50.0, 20.0, 0.0),
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
Title(),
SizedBox(height: 80.0,),
confirmLoginType(),
SizedBox(height: 10.0),
LoginButton(),
PolicyTips(
key: IndexGlobalKey.policyTipsKey,
updateState: receiveMessageUpdateState
),
Bottom()
],
),
),
in the PolicyTips, I pass a key to it, and I want to get the key in the LoginButton , but It has always been null when I get currentState.
The code is below:LoginButton
class LoginButton extends StatefulWidget {
LoginButton({Key key}) : super(key: key);
#override
_LoginButtonState createState() => _LoginButtonState();
}
class _LoginButtonState extends State<LoginButton> {
#override
Widget build(BuildContext context) {
policyTipsKey = IndexGlobalKey.policyTipsKey.currentState;
return Container(
child: Text()
)
}
}
what can I do? help me please, thanks.
This is IndexGlobalKey code.
class IndexGlobalKey {
static final GlobalKey<_PolicyTipsState> policyTipsKey = GlobalKey<_PolicyTipsState>();
static GlobalKey<_FormState> phoneLoginKey = GlobalKey<_FormState>();
static GlobalKey<_FormForIdCardLoginState> idCardLoginKey = GlobalKey<_FormForIdCardLoginState>();
}

Answer 1

Build method of _LoginButtonState runs before PolicyTips renders and before IndexGlobalKey.policyTipsKey is actually set. The reason is LoginButton goes before PolicyTips in column. Thats why you get null when you call IndexGlobalKey.policyTipsKey.currentState from build of _LoginButtonState.
To solve this you need to call IndexGlobalKey.policyTipsKey.state right where you use it. For example, when you need to get policy tips state on button tap just use it inside onPressed callback:
class _LoginButtonState extends State<LoginButton> {
#override
Widget build(BuildContext context) {
// An example of your button
return TextButton(
onPressed: () {
final policyTipsState = IndexGlobalKey.policyTipsKey.currentState;
// Here you can use policyTipsState
},
child: Text('button'),
);
}
}