handleAction (name, handler) {
    if (!name || !handler || !isFunction(handler)) {
      throw new Error('Illegal arguments.')
    }
    handler = handler.bind(this)
    this._actionHandlers[name] = handler
  }
Define an action handler. Call this during construction/initialization of a component.

    @param {String} action name
    @param {Functon} a function of this component.