Push root node ref down the tree

[Adlai-Holler]

Discuss.

Theory is, in the future all tree accesses will require the root lock. We could use a reader/writer mutex if we wanted. This diff uses the instance lock for convenience.

Right now, the down-propagation is murky because tree modifications don't require the root lock, so the ordering there is kind of unclear.

What's nice about this, is you could call a locked_ method directly on the root node, safely.

If we do composite operations on the tree, such as ASM updating, should we hold the root the whole time or separate it, for example? Interesting stuff.

[Adlai-Holler]

cc @wiseoldduck @wsdwsd0829

[Adlai-Holler]

The _rootNode ivar is right now only guarded by self->__instanceLock__ but in reality (once we add it) it is ALSO guarded by the root node's lock, since tree modifications are not allowed without the root locked.

[appleguy]

Could you say more about this? How will it be enforced, and what would third-party code have to look like? I'm hoping there is a way to ensure that methods like addYogaChild / addSubnode: can be called without the lock and the framework internally manages acquiring the root lock.

...since tree modifications are not allowed without the root locked.

[wiseoldduck]

So ... the idea here would be to replace yogaRoot and return to the idea of just locking _rootNode to synchronize tree access, probably doing away with lockToRootIfNeeded altogether?

[Adlai-Holler]

@appleguy The public API wouldn't change.

@wiseoldduck Exactly

[wsdwsd0829]

YogaTree is different from normal node tree. Meaning when we create a yoga tree, the rootnode will always be self. So even during yoga layout in calculateLayouFromYogaRoot the _rootNode may not be updated timely.

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