How to add Singleton in dart? - flutter

Question

i'm pretty new to flutter and i want to add singleton in my flutter app. I used shared preferences to save my private and public key, but also i want to take this info from it when i launch app
try {
userPubKey = getPublicKey() as String;
userPrivateKey = getPrivateKey() as String;
} catch (e) {
}
if (userPrivateKey == "null" || userPubKey == "null") {
var crypter = RsaCrypt();
var pubKey = crypter.randPubKey;
var privKey = crypter.randPrivKey;
String pubKeyString = crypter.encodeKeyToString(pubKey);
String privKeyString = crypter.encodeKeyToString(privKey);
setPublicKey(pubKeyString);
setPrivateKey(privKeyString);
userPubKey = pubKeyString;
userPrivateKey = privKeyString;
}
Here is my singleton screen. I need to add my pubKey, privateKey, UId and userName data in Singleton. I copied random singleton code with factory construct.
class Singleton {
Singleton.privateConstructor();
static final Singleton instance = Singleton.privateConstructor();
factory Singleton() {
return instance;
}
String pubKey;
String privateKey;
String userName;
String userID;
setPubKey(String key){
this.pubKey = key;
}
String getPubKey(){
return this.pubKey;
}
}

Answer 1

You don't need factory constructor as you can directly use instance variable being static. Here's how you do it.
class Singleton {
Singleton._();
static final Singleton instance = Singleton._();
String pubKey;
String privateKey;
String userName;
String userID;
void setPubKey(String key) => pubKey = key;
String getPubKey() => pubKey;
}
void main() {
var instance = Singleton.instance;
}