Expose bare minimum to add property effects dynamically.

[kaste]

As implemented #3460.

Following API:

  Polymer({
    is: 'x-custom-effect'
  });

  var el = document.createElement('x-custom-effect');

  var fx = el.addPropertyEffect('foo', function(path, value, old) {
    // path will be 'foo' or e.g. 'foo.splices' etc.
    // old value will only be avail if path is simple e.g. 'foo'
  });
  // do something
  // then later ... unregister
  el.removePropertyEffect(fx);

[dfreedm]

Can you describe what the use cases are for custom property effects with some examples?

[kaste]

Well I linked to numerous issues in the PR: #3420, #1796, #2131, #2638, #1778
Note: The PR just exposes the already implemented functionEffect. The only missing part is that Polymer does not define the equivalent path effect.

In a nutshell, this is just the straightforward code for what Polymer calls an annotationEffect:

// <x-foo prop="[[ bar ]]"></x-foo>
var el = document.createElement('x-foo');
this.addPropertyEffect('bar', function(path, value, oldValue) {
   el.prop = value;
});

And since a function effect is pretty low-level, you could rebuild all the effect kinds with it.

The problem with this PR is, adding elements gets damn easy, but removing them from the document requires that you also unregister all the custom effects, so you still have to keep track of that state.

The problem Polymer has, is that it somewhat fails in highly polymorphic situations: If you can't tell which elements/tags you will need to insert upfront, or if you just don't know which properties for example to bind to etc. Polymer also often forces you to use dom-repeats's or dom-if's although they don't perform that well and are highly opinionated in what they do and how they work and don't work.

[kaste]

I think this is a good use-case: #3547.

[TimvdLippe]

This has been shipped in Polymer 2: https://www.polymer-project.org/2.0/docs/api/elements/Polymer.Element#method-addPropertyEffect Thanks for the suggestion!