code: func NewBundleSource(bundle *Bundle, callbacks ...BundleActor) *BundleSource {
  	bs := &BundleSource{
  		callbacks: callbacks,
  	}
  	bs.Update(bundle)
  	return bs
  }
docs: // NewBundleSource creates a new BundleSource with an initial Bundle value
  // The callbacks will be invoked whenever the Update method is called for the
  // BundleSource.  Note, these callbacks are called immediately before this function
  // returns.