Allowing custom reconciler host-context in React 18

[drcmda]

in a custom renderer we would like to do something like this:

<inject foo="bar">
  <group>
    <group>
      <obj />
    </group>
  </group>
</inject>

✅ obj is supposed to know it is within inject, this is possible

❌ inject is supposed to make its props (foo="bar") the host context for all of its nested contents, obj should be able to access it. this is not possible

we create a host context like this:

getRootHostContext(rootContainer) { return {} }
getChildHostContext(parentHostContext, type, rootContainer) {
 if (type === "inject") {}
 else return parentHostContext

when getChildHostContext gets called for inject, we only have access to string type, not props. i thought i can return an empty object {} and fill it later in:

finalizeInitialChildren(instance, type, props, rootContainer, hostContext) {
  if (type === "inject) hostContext.props = props

but when finalizeInitialChildren is called for inject it refers to root context, it can't refer to its own host context that was created by getChildHostContext.

i find this puzzling, we don't seem to have any relation between a host context and the element that created it.

solution

if we can access the current context it would be very useful to create a richer host context, that will also update.

finalizeInitialChildren(instance, type, props, rootContainer, hostContext, currentContext)
prepareUpdate(instance, type, oldProps, newProps, rootContainer, hostContext, currentContext)

alternatively, allowing us to receive props would at least allow for a static host context that doesn't update but at least carrier some deeper information.

getChildHostContext(parentHostContext, type, rootContainer, props)