Global Variables in Dart

问题:

I try to create a Dart single page application.

I have created a first custom element (custom-application) which contains the whole application. It has a container in it which is used to render views. And a side nav which will contain user informations and be updated when the user is log in.

I want to share informations between views. How can I define a global variable in custom-application and be able to share it with the other views ?

For example, when you start the app, you are not authenticated. When you call /login (login-view) you'll have a login form. I want, when you log in the application, the custom-application element stores the user informations loaded by the nested view login-view and update the side nav.

Is it possible to do it ?

回答1:

Just create a library file and create fields for globals you need there. Import this library everywhere you need access to these fields.

app.dart

import 'globals.dart' as globals;

main() {
  globals.isLoggedIn = true;
}

component1.dart

import 'globals.dart' as globals;

class MyComponent {
  view() {
    if(globals.isLoggedIn) {
      doSomething();
    else {
      doSomethingElse();
    }
  }
}

globals.dart

library my_prj.globals;

bool isLoggedIn = false;

You can also
- create a singleton in the globals library (see How do you build a Singleton in Dart? for more details).
- use observable to get notified about changes (see Implement an Observer pattern in Dart, How can i trigger a kind of onChange event in a class for more details)

回答2:

I had the same problem with global variables. Therefore I also needed different configuration for each app version (dev / prod ) and i don't want to write the configuration in the main_dev.dart or in the main_prod.dart file.

I wrote a simple flutter package that deals with having seperated configuration files and load them at app startup. The configuration is then available at each line of code in your app.

https://github.com/Ephenodrom/Flutter-Global-Config

How to use it :

Create a json file under assets/cfg/$file.json

Add assets/cfg to your pubspec.yaml

Loading different configuration files at app start :

import 'package:flutter/material.dart';
import 'package:global_configuration/global_configuration.dart';

void main() async{
  await GlobalConfiguration().loadFromAsset("app_settings");
  await GlobalConfiguration().loadFromAsset("env_dev_settings");
  runApp(MyApp());
}
class MyApp extends StatelessWidget {
  ...
}

Using the configuration in your app :

import 'package:flutter/material.dart';
import 'package:global_configuration/global_configuration.dart';

class CustomWidget extends StatelessWidget {

    CustomWiget(){
        // Access the config in the constructor
        print(GlobalConfiguration().getString("key1"); // prints value1
    }

    @override
     Widget build(BuildContext context) {
        // Access the config in the build method
        return new Text(GlobalConfiguration().getString("key2"));
     }
}