Change `stage.loop()` so that movies are advanced in tree order

[davidaurelio]

Advance Timeline Tree Depth-First

Current State

stage.loop() is the central method that “moves the playhead” of all objects having a timeline and currently added to the stage.

It loops over all these objects using the movie registry. Objects are added to / removed from it by addChild() / removeChild().

The movie registry contains objects in insertion order. This allows movies inserted during the advancement / looping process to be advanced as well.

Change to Tree Traversal

The request is to change the logic to depth-first hierarchical tree traversal. This makes the actual order easier to predict for developers building things that depend on the tree structure.

• the Timeline.emitFrame() method should
  1. emit the "tick" event.
  2. emit the currentFrame and "advance" events, if playing
  3. call the emitFrame() method of every child timeline
• the Stage.loop() method should
  1. invoke emitFrame() on itself, thus also advancing all submovies
  2. invoke incrementFrame() method of all timelines. This method could work as emitFrame(), also invoking the method of child timelines. An optimization idea is described below.

     Consequences

• stage.registry.movies would go away.
• If optimization 2.) wouldn’t be used, it would get easier to play parts of the hierarchy seperately.
• Timelines added to an already advanced Timeline/DisplayList during the advancement phase would only be advanced on next "tick". This most probably isn’t a common usecase

  Optimization possibilities

  1. Each DisplayList could maintain an own array of child Timelines to make looping more efficient. It could simply be managed at the same code spots that currently add/remove objects with timeline to/from the movie registry.
  2. emitFrame() could take an array parameter which is filled with all timelines encountered. The second iteration (incrementing frame numbers) could be more efficient by simply looping over the filled array.
  3. Add a playFrame() method to Timeline that simply calls this.emitFrame() and this.incrementFrame() in order. Would make the whole interface nice and usable without producing unexpected results.

[padolsey]

These all sound like good changes.

I think optimisation numbers (1) and (2) are absolute necessities since we're ditching registry.movies. (3) sounds good too -- makes things more intuitive.

I guess what'll take the most time is ensuring we have good test coverage for this stuff.