toggleLoading(toggle = null) {
      if (toggle == null) {
        return (this.mutableLoading = !this.mutableLoading)
      }
      return (this.mutableLoading = toggle)
    }
Toggle this.loading. Optionally pass a boolean
value. If no value is provided, this.loading
will be set to the opposite of it's current value.
@param toggle Boolean
@returns {*}