FallbackMiddleware class

Middleware that falls back to an alternative handler when a response matches a condition.

This is particularly useful for serving Single Page Applications (SPAs) where missing files should fall back to index.html to enable client-side routing.

Basic SPA Usage

final webDir = Directory('web');
final indexFile = File('web/index.html');

pod.webServer.addMiddleware(
  FallbackMiddleware(
    fallback: StaticRoute.file(indexFile),
    on: (response) => response.statusCode == 404,
  ),
  '/**',
);
pod.webServer.addRoute(StaticRoute.directory(webDir), '/**');

Custom Condition Example

pod.webServer.addMiddleware(
  FallbackMiddleware(
    fallback: errorPageRoute,
    on: (response) => response.statusCode >= 500,
  ),
  '/api/**',
);

How It Works

1. The middleware wraps the inner handler (typically a StaticRoute)
2. Calls the inner handler and receives its response
3. If the response matches the condition (e.g., status == 404), calls the fallback handler
4. Otherwise, returns the original response

The fallback route automatically has access to the session because all routes in the WebServer are wrapped with session middleware.