realmInstance.js

#### db.js ####
import Realm from 'realm'
import Log from './schema/Log';
import Setting from './schema/Setting';
import UserProfile from './schema/UserProfile';
import { asyncGetEncyptionKey } from '../src/utils';

class db {

    static realmInstance = null

    static schema = [
        Log, 
        Setting,
        UserProfile
    ]

    static async constructor(key=null) {
        this.key = key
    }

    static async getDb() {
        try {
            console.log(this.realmInstance);
            
            if (this.key === null) {
                this.key = await asyncGetEncyptionKey() # to get EncryptionKey from asyncronouse process (ex. from key store)
            }
    
            if (this.realmInstance !== null) {
                return this.realmInstance
            } else {
                this.realmInstance = new Realm({
                    schema: this.schema,
                    encryptionKey: this.key
                })
                return this.realmInstance
            }        

        } catch(error) {
            console.error("getDb", error);
        }
    }
}

export default db

### Screen.js ###
import React from 'react'
import db from './db'
class Screen extends React.Component {

    async componentDidMount() {
        const realm = await db.getDb()
        let users = db.getDb().objects("User")
    }
}