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Update asenvironment.md

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Hannah Troisi
2016-07-24 15:41:09 -07:00
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@@ -6,10 +6,12 @@ prevPage: debug-tool-ASRangeController.html
nextPage: asrunloopqueue.html
---
`ASEnvironment` is an optimized state propagation system that allows the framework to distrubute a variety of important "evironmental" information up and down the node hierarchy.
Compared to UIKit, this system is very efficient and one of the reasons why nodes are much lighter weight than UIViews. This is achieved by using simple structures to store data rather than creating objects. For example, `UITraitCollection` is an object, but `ASEnvironmentTraitCollection` is just a struct.
This means that whenever a node needs to query something about its environment, for example its interface state or checking whether it is part of a rasterized hierarchy (whether any parent has .shouldRasterizeDescendents enabled), instead of climbing the entire tree or checking all of their children below them, they can go to one spot and read the value that was propogated to them.
A key operating principle of ASEnvironment is to update values when new subnodes are added or removed.
`ASEnvironment` allows objects that conform to the `<ASEnvironment>` protocol to be able to propagate specific states defined in an ASEnvironmentState up and/or down the ASEnvironment tree. To define how merges of States should happen, specific merge functions can be provided.
One of AsyncDisplayKit's built in propogation states is the `ASEnvironmentTraitCollection`. This allows nodes to propagate information about the UIDevice down the ASEnvironment tree, to their subnodes. This performance optimization means that we only have to check our UIDevice settings once.
### Addding your own ASEnvironmentState
Coming Soon...