playground.js

const onReady = Symbol('onReady');

const PRESERVED_METHODS = ["property", "constructor", "__defineGetter__", "__defineSetter__", "hasOwnProperty", "__lookupGetter__", "__lookupSetter__", "isPrototypeOf", "propertyIsEnumerable", "toString", "valueOf", "__proto__", "toLocaleString"]

function getAllMethodNames (obj: any, depth = Infinity) {
    const methods = new Set<String>()
    while (depth-- && obj) {
        for (const key of Reflect.ownKeys(obj)) {
            if (typeof key === 'string' && PRESERVED_METHODS.indexOf(key) < 0 && typeof obj[key] === 'function')
            methods.add(key)
        }
        obj = Reflect.getPrototypeOf(obj)
    }
    return [...methods]
}

function classDecorator<T extends { new(...args: any[]): {} }>(constructor: T) {
    return class extends constructor {
        private _ready: boolean = true;
        private _onloads: any[] = [];

        [onReady]<T extends (...args: any[]) => any>(callback: T) {
            console.log('ready?')
            return this._ready ? callback : (e: Parameters<T>) => new Promise((resolve, reject) => {this._onloads.push((e: Parameters<T>) => callback(e).then(resolve).catch(reject))})
        }

        constructor(...args: any[]) {
            super(...args)
            getAllMethodNames(this).forEach((key) => {
                console.log('key: ', key)
                if (typeof this[key as keyof this] === 'function') {
                    const method = this[key as keyof this]
                    this[key as keyof this] = this[onReady](method)
                }
            })
        }
    }
}

@classDecorator
class Greeter {
    property = "property";
    hello: string;
    constructor(m: string) {
        this.hello = m;
    }
    bello() {
        console.log('yho')
    }
}

console.log(new Greeter("world").bello());

reim-service.js

const service = createService({
  async pre () {
    await prepareAPIKey()
    return api
  }
})({
  
})

service2.js

@service()
class GoogleSheet {
  pre: () => {
    new Promise(resolve => {
      this._gapi.load('client:auth2', resolve)
    }).then(() => {
      // blablabla
    })
  }
  
  read() {
  }
}