class_docstr: A Link defines some connection between a source and target object
  in their visualization. It is quite similar to a Stream as it
  allows defining callbacks in response to some change or event on
  the source object, however, unlike a Stream, it does not transfer
  data and make it available to user defined subscribers. Instead
  a Link directly causes some action to occur on the target, for JS
  based backends this usually means that a corresponding JS callback
  will effect some change on the target in response to a change on
  the source.
  
  A Link must define a source object which is what triggers events,
  but must not define a target. It is also possible to define bi-
  directional links between the source and target object.
code: def target(self):
      return self._target() if self._target else None
  
imports: import weakref
  from collections import defaultdict
  import param
  from ..core.util import dimension_sanitizer
label_desc: Use image node_id 4 for calling the Link obj's underlying member method code with example usage: obj.target() and returns: unknown