How to access values from another dart file?

Question

I am new to flutter.Here I stored a value to a variable doc_id ,I want to use this value in another file called comments.dart. So I did something like below but it gives null value in comment.dart.

await FirebaseFirestore.instance
    .collection('blogs')
    .add({
    'title': titleController.text,                         
}).then((value) {
  doc_id = value.id;

  comment(postid: docid);

  successAlert(context);
}).catchError((error) => 
 errorAlert(context));

Comment.dart

class comment extends StatefulWidget {
  final String? postid;
  const comment({Key? key, this.postid}) : super(key: key);
  _commentState createState() => _commentState();
}

class _commentState extends State<comment> {
  @override
  Widget build(BuildContext context) {
    return        
        Text(widget.postid);
  } 
}

Answer 1

Just create a global variable and assign from there

String postid = "";

class comment extends StatefulWidget {
  final String? postid;
  const comment({Key? key, this.postid}) : super(key: key);
  _commentState createState() => _commentState();
}

class _commentState extends State<comment> {
  @override
  Widget build(BuildContext context) {
    return        
        Text(postid);
  } 
}

void setPostID(String s) {  // get value
   postid = s;
}

Finally assign the value

await FirebaseFirestore.instance
    .collection('blogs')
    .add({
    'title': titleController.text,                         
}).then((value) {
  doc_id = value.id;
  
  setPostID(value.id);   // set value

  comment(postid: docid);

  successAlert(context);
}).catchError((error) => 
 errorAlert(context));

Answer 2

You can use: https://pub.dev/packages/shared_preferences

await FirebaseFirestore.instance
    .collection('blogs')
    .add({
    'title': titleController.text,                         
}).then((value) {
  doc_id = value.id;
  
  await prefs.setString('doc_id', postid);   // set value

  comment(postid: docid);

  successAlert(context);
}).catchError((error) => 
 errorAlert(context));

Finally use it in your class

class comment extends StatefulWidget {
  final String? postid;
  const comment({Key? key, this.postid}) : super(key: key);
  _commentState createState() => _commentState();
}

class _commentState extends State<comment> {

  @override
  void initState() {    
    super.initState();
    widget.postid = prefs.getString('doc_id');  // get the value
    setState(() {});
  }
  @override
  Widget build(BuildContext context) {
    return        
        Text(postid);
  } 
}