Typgin NativeScript's untyped Observable

Observable.ts

import * as Untyped from 'tns-core-modules/data/observable'
import deprecated from 'deprecated-decorator'  // npm install --save-dev deprecated-decorator

/**
 * ValueTypes<{foo: number, bar: string}> = number | string
 */
type ValueTypes<T> = T extends { [k: string]: infer I }
  ? I
  : never

/**
 * Field<'foo', {foo: number, bar: string}> = 'foo'
 */
type Field<K extends string, T> = K extends keyof T
  ? K
  : never

/**
 * Don't dirty your hands.
 * You must use this instead of [[Untyped.Observable]].
 *
 * Unfortunately, still this is a little type unsafe.
 * Please see the comment of [https://github.com/aiya000/workspace/blob/7cc4c5e53a74036e9c2339cc39d6ee0f47f8a315/TypeScript/ConditionalTypes/type-untyped-method.ts].
 */
export default class Observable<T extends object> extends Untyped.Observable {
  constructor() {
    super()
  }

  /**
   * A typed safety set()
   */
  public assign<K extends string>(name: Field<K, T>, value: ValueTypes<T>): void {
    super.set(name, value)
  }

  @deprecated('assign')
  public set(name: string, value: any): void {
    super.set(name, value)
  }
}