Document the different graphs & hierarchies in play along with their lifecycles

[vmanot]

SwiftUI is a complex beast. Going through the codebase as well as other articles by people who've tried to understand how it concretely works, I recognize the following distinct graphs and hierarchies conceptually:

• The attribute graph (OGGraph and OGSubgraph, subgraphs can own subgraphs and attributes)
• ViewGraph

Some questions that I don't have a good answer to despite going over the codebase a dozen times:

• How do ViewInputs and ViewOutputs relate to AttributeGraph? I see sifting through a lot of indirection that they're creating Attributes in many places, I'm assuming the initialization/mutation of these attributes are tracked by some AttributeGraph-owned closure that they are being executed within.
• What is the lifecycle of a ViewGraph?
• Does ViewGraph maintain its hierarchy solely through PreferenceBridge?
• DisplayList?
• What other hierarchies are known to you?

[Kyle-Ye]

Part of the info has already outdated. I'm planning to realign the part to iOS 18 recently.

Keep tuned.

[Kyle-Ye]

DisplayList

SwiftUI will construct DisplayList in UIHostingView/ViewGraph and many other places. And will eventually kick off a render process via RendererHost (Diff & delta time & addSubview/insertLayer).

See more detail on #136.