In my use case i have to change the layout of the app with json data. I have a json file which i want to get and use the key values without using Future method in the next method rather i want to place the mapped json and place it in an empty curly brackets:

This is the json file i grab:

# test_json.json

{
"fontSize" : "10",
"fontFamily" : "A",
"fontWeigth" : "bold",
"fontColor" : "blue"
}

This is the file which grabs the json file and maps it:

# get_json.dart


class GetJson{
  Future<Map<String, dynamic>> getJson() async {
    String jsonData =
        await rootBundle.loadString('assets/json/test_json.json');
    Map<String, dynamic> data = jsonDecode(jsonData);
    return data;
  }


}

Then i grab this mapped json and i want to place it inside a variable called mappedData and place it inisde empty curly brackets. Then i want to get the number with getNumber() and inisde this method i convert the type of fontSize from string to double with another custom method called TypeConvertor.getDouble():

class Utility {
  var mappedData= {};

 setJson() async {
    mappedData = await GetJson().getJson();
  }

getNumber(String key) {
    var data = mappedData;
    return TypeConvertor.getDouble(data[key]);
  }
}

In my use case i need to this like this i have no other choice. I want to explicitly grab the json like that and i don't want getNumber() to be a Future because then i cannot place Utility().getNumber("fontSize") inside a stateful widget because then i have to use setState and i want to avoid that because i will have alot of keys beside fontSize and so then i have to use setState for every key values. I just want to use Utility().getNumber("fontSize") inside property fontSize and The rest also like this. In my usecase i have to do it like that:

class TestView extends StatefulWidget {
  const TestView({Key? key}) : super(key: key);

  @override
  State<TestView> createState() => _TestViewState();
}

class _TestViewState extends State<TestView> {
  @override
  Widget build(BuildContext context) {
    return Text(
      "test",
      style: TextStyle(fontSize: Utility().getNumber("fontSize")),
    );
  }
}

But in my app mappedData gives null. The full error is : Unhandled Exception: type 'Null' is not a subtype of type 'String' and the null value is inside mappedData. I want to grab the json data and place it inside an empty map and use the mapped json from there on. How do i resolve the null execption issue?

EDIT

Change variables to correct versions

CodePudding user response：

Probably it's because you don't call setJson before call getNumber. The following code is work.

final utility = Utility();
await utility.setJson();
print(utility.getNumber("fontSize"));

If you want to avoid similar mistakes, you have some options as solutions.

• Include mappedData to Utility's constructor.
• Change getNumber to static, and add argument mappedData.
• Use JsonSerializable(It's a little difficult but the best solution.)

CodePudding user response：

I found the solution which is partly contributed by @bakatsuyuki. So i did use await utility.setJson(); but i also initilized it with initState() so field utility has no null value. I also used FutureBuilder() to check if snapshot.hasData and then i display the Text() widget with the data from utilitiy else show empty Container. This way i can resolve the null Exception.

This is the view that worked for me:

class AppView extends StatefulWidget {
  const AppView({
    Key? key,
  }) : super(key: key);

  @override
  _AppViewState createState() => _AppViewState();
}

class _AppViewState extends State<AppView> {
  final utility = Utility();

  Future setUtility() async {
    await utility.setJson();
  }

  @override
  void initState() {
    setUtility();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: FutureBuilder(
            future: AppContent().getAppContent(),
            builder: (context, snapshot) {
              if (snapshot.hasData) {
                return Text("test",
                    style: TextStyle(
                      fontSize: utility.getNumber("fontSize"),
                    ));
              } else {
                return Container();
              }
            }),
      ),
    );
  }
}