Maintain $parent safely within the core

[nikku]

Is your feature request related to a problem? Please describe

The child to parent pointer $parent is important for model traversal. We do have basic test coverage for it, however we do not ensure that it is being maintained as part of the core. Instead, libraries using it (i.e. moddle-xml, bpmn-js) manually maintain the property in various places. This is error prone and not properly done everywhere.

At best, we do automatically maintain the property:

// $parent set during modeling
const foo = moddle.create('bpmn:ExtensionElements');

// during creation
const blub = { };
foo.values.push(blub); // blub = { $parent: foo }
foo.bar = { }; // bar = { $parent: foo }

// during removal
foo.values.pop(); // blub = { }
blub.$parent = null;
foo.bar = null; // bar = { }

Describe the solution you'd like

• $parent is maintained automatically by moddle so that I can rely on it

Describe alternatives you've considered

Keep $parent maintenance out of scope for the library.

Additional context

Brought up in the context of bpmn-io/bpmnlint#69 (comment).

[pinussilvestrus]

We had a related user request recently: bpmn-io/bpmn-moddle#92

It included an interesting discussion around this (where to maintain what, what's the expectation) => the user expected that this is maintained in the core. 👍

[philippfromme]

user expected that this is maintained in the core

Good validation! 👍🏻