type FormCallback = (event: React.FormEvent<any>) => void;
interface CallbackMap {
  [stateKey: string]: FormCallback;
}

class CallbackFarm {
  constructor(readonly component: React.Component) {}
  updateFromTarget(stateKey: string): FormCallback {
    if (stateKey not in this.registeredCallbacks) {
      this.registeredCallbacks[stateKey] = (event: React.FormEvent<any>) => {
        this.component.setState({[stateKey]: event.currentTarget.value});
      };
    }
    return this.registeredCallbacks[stateKey];
  }
  private registeredCallbacks: {[stateKey: string]: (event: React.FormEvent<any>) => void} = {};
}