Feature 19: Admin PWA (#187-#210) (#211)

Co-authored-by: dev_agent <17+dev_agent@code.cubecraftcreations.com>
Co-committed-by: dev_agent <17+dev_agent@code.cubecraftcreations.com>

admin/src/api/useTransitions.tsx

@@ -0,0 +1,61 @@
+/**
+ * One live transition stream per signed-in shell, fanned out to the screens.
+ *
+ * <TransitionsProvider> opens the stream on mount and aborts it on unmount;
+ * useTransitions(handler) registers a screen's handler for as long as the
+ * screen is mounted. Without a provider the hook does nothing, so a screen
+ * rendered on its own (e.g. in a test) never opens a stream.
+ */
+import { createContext, useContext, useEffect, useRef, useState, type ReactNode } from "react";
+import { openTransitionStream, type TransitionEvent } from "./events";
+
+type Listener = (event: TransitionEvent) => void;
+
+interface TransitionHub {
+  subscribe(listener: Listener): () => void;
+}
+
+const TransitionsContext = createContext<TransitionHub | null>(null);
+
+export function TransitionsProvider({ children }: { children: ReactNode }) {
+  const [hub] = useState(() => {
+    const listeners = new Set<Listener>();
+    return {
+      listeners,
+      subscribe(listener: Listener) {
+        listeners.add(listener);
+        return () => {
+          listeners.delete(listener);
+        };
+      },
+    };
+  });
+
+  useEffect(
+    () =>
+      openTransitionStream({
+        onTransition: (event) => {
+          for (const listener of [...hub.listeners]) {
+            try {
+              listener(event);
+            } catch {
+              // One screen's handler must not tear down the shared stream.
+            }
+          }
+        },
+      }),
+    [hub],
+  );
+
+  return <TransitionsContext.Provider value={hub}>{children}</TransitionsContext.Provider>;
+}
+
+/** Call `handler` for every transition while the calling component is mounted. */
+export function useTransitions(handler: Listener): void {
+  const hub = useContext(TransitionsContext);
+  const handlerRef = useRef(handler);
+  useEffect(() => {
+    handlerRef.current = handler;
+  });
+  useEffect(() => hub?.subscribe((event) => handlerRef.current(event)), [hub]);
+}