What constitutes a run loop side effect?

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Great question! I had an answer originally drafted, but in gathering the steps I realized that my mental model was slightly off.

Time for a code safari :zebra:!

// assume obj here is _something_ :raised_hand: :wave:  
Ember.set(obj, 'foo', 'bar');

When Ember.set runs, the following path is taken:

  1. Ember.set checks if foo were a computed property on obj (assume for now that it isn’t) – here
  2. Ember.set checks if obj.setUnknownProperty exists (assume that it isn’t) – here
  3. Ember.set checks if foo is a dependent key in an observer / computed or is otherwise “watched” (e.g. due to template usage) (assume that it isn’t) – here
  4. Ember.set does obj.foo = 'bar' – here
  5. If the value is different that the previous value, Ember.set calls notifyPropertyChange(obj, 'foo') – here
  6. notifyPropertyChange checks if the property is being watched, and notifies any observers or computed properties that foo has changed – here
  7. notifyPropertyChange then “marks the object as dirty” – here
  8. markObjectAsDirty grabs the underlying “reference” / “tag” for that object and property then marks it as dirty – here
  9. If there was a tag, markObjectAsDirty calls ensureRunLoop to ensure a runloop is scheduled – here
  10. ensureRunLoop checks if anything has been rendered, and if it has schedules a run loop

Footnotes:

  • markObjectAsDirty is an internal utility function primarily used with Glimmer 2’s reference system

tldr; The basic decision tree for when an Ember.set would schedule work to happen async is:

  • Has anything been rendered yet? If not, then Ember.set currently will not schedule any async.
  • Is the new value being set different than the old value? If not, then nothing is notified or scheduled.
  • Has the object (if an ObjectProxy) or property (for normal objects) been rendered in a template? If not, nothing is scheduled.

If the answers to all of those questions are “yes”, then async will be scheduled…

Phew, sorry about that (it was fun though, huh?)

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