The watch handler is called whenever route files change. It receives route entries and generates files accordingly:

async watchHandler(entries, event) {
  // entries: Array<RouteResolverEntry>
  // event: WatcherEvent | undefined

  for (const entry of entries) {
    if (entry.kind === "api") {
      // Process API routes
      const route = entry.route; // ApiRoute
    } else {
      // Process page routes
      const route = entry.route; // PageRoute
    }
  }
}

On initial call (when the dev server starts), event is undefined and entries contains all routes. This is when you should generate all files from scratch.

On subsequent calls, event contains information about what changed:

type WatcherEvent = {
  kind: "create" | "update" | "delete";
  file: string; // Absolute path to changed file
};

You can use this to perform incremental updates, regenerating only affected files rather than everything.