Function switchContext

packages/react-server/src/ReactFizzNewContext.js:152–178

(newSnapshot: ContextSnapshot)

// updating all the context's current values. That way reads, always just read the current value.
// At the cost of updating contexts even if they're never read by this subtree.
export function switchContext(newSnapshot: ContextSnapshot): void {
  // The basic algorithm we need to do is to pop back any contexts that are no longer on the stack.
  // We also need to update any new contexts that are now on the stack with the deepest value.
  // The easiest way to update new contexts is to just reapply them in reverse order from the
  // perspective of the backpointers. To avoid allocating a lot when switching, we use the stack
  // for that. Therefore this algorithm is recursive.
  // 1) First we pop which ever snapshot tree was deepest. Popping old contexts as we go.
  // 2) Then we find the nearest common ancestor from there. Popping old contexts as we go.
  // 3) Then we reapply new contexts on the way back up the stack.
  const prev = currentActiveSnapshot;
  const next = newSnapshot;
  if (prev !== next) {
    if (prev === null) {
      // $FlowFixMe[incompatible-call]: This has to be non-null since it's not equal to prev.
      pushAllNext(next);
    } else if (next === null) {
      popAllPrevious(prev);
    } else if (prev.depth === next.depth) {
      popToNearestCommonAncestor(prev, next);
    } else if (prev.depth > next.depth) {
      popPreviousToCommonLevel(prev, next);
    } else {
      popNextToCommonLevel(prev, next);
    }
    currentActiveSnapshot = next;
  }
}

export function pushProvider<T>(
  context: ReactContext<T>,