Generic hierarchy propagation

[mockersf]

Objective

• There are two different hierarchy propagation for now: Transform/GlobalTransform and Visibility/ComputedVisibility
• They slightly differ 😱

Solution

• Make a generic system that do both
• That can also be used by users if they want to add their own hierarchy propagation without handling all the details

[DJMcNab]

Can you explain the differences between this approach and #4216 ?

[nicopap]

Very excited about this. Though I think we still need more things (that can be added in the future):

• Independent hierarchies (I want to be able to declare two distinct hierarchies that spans the same entities)
• Ability to tolerate missing ComputedGlobal::Local with a default and propagating downward to other entities with a ComputedGlobal. (people wanted to reduce the amount of "required" components I think)

[mockersf]

Can you explain the differences between this approach and #4216 ?

Well the biggest difference is that I hadn't seen it before...
Other than that, that PR is very similar.

• Heritable trait based property inheritance systems #4216 is adding a extension on app to automatically add the systems, this one let users do it themselves
• Heritable trait based property inheritance systems #4216 doesn't handle visibility (which wasn't propagated at the time) and is missing a few updates that were done since

[superdump]

I like this. Have you investigated/thought through whether it will work for RenderLayers too? I suppose Local would be RenderLayers, ToPropagate would be RenderLayers, the from_local would just be a clone, combine_with_local would be an intersection(?), and value_to_propagate would be *self? Seems pretty intuitive. And nice that it's reusable for any hierarchy someone might want to add.

[superdump]

Indeed it looks pretty similar to #4216 , but it not requiring the App extension thing seems like an improvement.

[bas-ie]

Backlog cleanup: given that ComputedVisibility was already split back in 0.12 I'm guessing this one is not quite so relevant anymore. I'm sure @mockersf will correct me if I'm wrong 🙂