darto-write-middleware

Write middleware in Darto

A middleware receives the same Context as handlers, plus a Next callback. Call await next() to continue the pipeline; return without calling next() to short-circuit (reject the request).

typedef Middleware = FutureOr<void> Function(Context c, Next next);
typedef Next       = Future<void>   Function();

Writing one (factory pattern)

Define a function that returns a Middleware closure. This lets the middleware take configuration:

Middleware timer() => (Context c, Next next) async {
  final sw = Stopwatch()..start();
  await next();                       // run downstream
  print('${c.req.method} ${c.req.path}  ${sw.elapsedMilliseconds}ms');
};

Short-circuit (reject before the handler)

Set a response and return without calling next():

Middleware requireAdmin() => (Context c, Next next) async {
  if (c.user?['role'] != 'admin') {
    c.forbidden({'error': 'Admins only'});
    return;                            // pipeline stops here
  }
  await next();
};

Sharing data with handlers

Use per-request state, set before next() and read downstream:

Middleware loadUser() => (Context c, Next next) async {
  c.set('userId', '42');               // or: c.user = {...}
  await next();
};
// in a handler: final id = c.get<String>('userId');

Registering middleware

Pick the narrowest scope that fits:

// Global — runs on every request. Call use() once per middleware.
app.use(logger());
app.use(timer());

// Path-scoped — runs on matching paths. Call mount() once per middleware.
app.mount('/api/*', cors());
app.mount('/api/*', jwtMiddleware);

// Route-level — only this route (the required middleware list):
app.get('/admin', [requireAdmin()], handler);
app.post('/upload', [bodyLimit(maxSize: 5 * 1024 * 1024)], handler);

Order matters: middleware runs in registration order, outermost first.

Built-in middleware

Darto ships many; each lives in its own sub-library, imported individually: