only static members can be accessed in initializers - dart

Question

I have an int value passed from the first screen to the second screen and I can have my value without any problem...My problem is that I want to add my recieved value to astring to complete and start working with my api which reqires to add the imported value of the first screen...Iam trapped between the recieved int which I can not change it to a static, and the api string which reqires the added value to be a static
the second Screen:
class CatsNews extends StatefulWidget {
#override
_CatsNewsState createState() => new _CatsNewsState();
}
class _CatsNewsState extends State<CatsNews> {
int _id ;
String root = "http://api.0mr.net";
String url = "/api/GetCategoryById/$_id";
#override
List data;
Future<String> getDrawerItem() async {
var response = await http.get(
Uri.encodeFull(url), headers: {"Accept": "application/json"});
setState(() {
var respondbody = json.decode(response.body);
data = respondbody["StoriesCategory"];
});
}
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('Cat Screen'),
),
body: Center()
I get the value through the shared preferences and it works fine if it was inside Widget build(BuildContext context)
#override
void initState() {
getIntPrefrence().then(updatId);
super.initState();
this.getDrawerItem();
}
void updatId(int id) {
setState(() {
this._id = id;
});
}
}
UPDATE:
I have added the Srting url to the initstate() and the code is as foloows:
class _CatsNewsState extends State<CatsNews> {
#override
List data;
int _id ;
var response;
String root = "http://-api.0mr.net";
String url ;
Future<String> getDrawerItem() async {
response = await http
.get(Uri.encodeFull(url), headers: {"Accept": "application/json"});
setState(() {
var respondbody = json.decode(response.body);
data = respondbody["StoriesCategory"];
});
}
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('$_id'),
),
body: Center()
}
Future<int> getIntPrefrence() async {
SharedPreferences pref = await SharedPreferences.getInstance();
int id = pref.getInt("id");
return id;
}
#override
void initState() {
super.initState();
getIntPrefrence().then(updatId);
updateurl();
this.getDrawerItem();
//initResponse();
}
updateurl() {
setState(() {
this.url= "$root/api/CategoryId/$_id";
});
}
void updatId(int id) {
setState(() {
this._id = id;
});
}
}
the previous issue was solved by adiing the String to the initstate(), but the updated values does not add to the String and deals witht the imported int id as null or zero,however the int id works fine and shows the imported value in any widget inside the Widget build(BuildContext context)

Answer 1

The offending code is probably
var response = await http.get(...
You can't have arbitrary code in a field initializer which is the part after = in above code.
Dart has a strict order in object creation steps.
Field initializers are executed before the constructor initialization list and before the constructor initialization list of super classes.
Only after all the constructor initialization lists of all super classes are executed, the constructor bodies are executed and only from then on is it allowed to access this, the instance of your class being created.
Your code accesses this (implicitly) before that and at this point object initialization isn't done and therefore access to this prohibited to prevent undefined bahavior.
Static members are safe to access, because they don't depend on object initialization. They are ready to use without an instance entirely.
What the error message is telling you is that your initializer is trying to do things that are not possible at this point and you need to move the code somewhere else.
The field initialization code can be moved to the constructor initialization list. This is usually done if the field is supposed to be final,
or to the constructor body, or to a method - as shown below.
var response;
#override
void initState() {
super.initState();
_initResponse();
}
void _initResponse() async {
response = await http.get(
Uri.encodeFull(url), headers: {"Accept": "application/json"});
setState(() {
var respondbody = json.decode(response.body);
data = respondbody["StoriesCategory"];
});
}
Because initState does not allow async we move the code to another method (_initResponse) that we call from initState.

Answer 2

the solution of my problem was adding both the imported value and the String which I want to edit according to it inside the method which will start my api request as follows:
void initState() {
super.initState();
this.getDrawerItem();
}
Future<String> getDrawerItem() async {
int _id ;
String url;
SharedPreferences pref = await
SharedPreferences.getInstance();
_id = (pref.getInt("id")?? 0);
url = "http://gomhuriaonline-api.0mr."
"net/api/GetCategoryById/$_id";
print(url);
response = await http
.get(Uri.encodeFull(url), headers:
{"Accept": "application/json"});
setState(() {
var respondbody = json.decode(response.body);
data = respondbody["StoriesCategory"];
});
}
}