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Creating Middleware

A middleware is a piece of code that is run before or after a request is handled. It might modify the Request or Response. Create a middleware by conforming to the Middleware protocol. It has a single function intercept which takes a Request and next closure. It returns an EventLoopFuture<Response>.

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Accessing the Request

If you’d like to do something with the Request before it is handled, you can do so before calling next. Be sure to call and return next when you’re finished! 
/// Logs all requests that come through this middleware.
struct LogRequestMiddleware: Middleware {
    func intercept(_ request: Request, next: @escaping Next) -> EventLoopFuture<Response> {
        Log.info("Got a request to \(request.path).")
        return next(request)
    }
}
You may also do something with the request asynchronously, just be sure to continue the chain with next(req) when you are finished. 
/// Runs a database query before passing a request to a handler.
struct QueryingMiddleware: Middleware {
    func intercept(_ request: Request, next: @escaping Next) -> EventLoopFuture<Response> {
        return User.all()
            .flatMap { users in
                // Do something with `users` then continue the chain
                next(request)
            }
    }
}

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Setting Data on a Request

Sometimes you may want a Middleware to add some data to a Request. For example, you may want to authenticate an incoming request with a Middleware and then add a User to it for handlers down the chain to access. You can set generic data on a Request using Request.set and then access it in subsequent Middleware or handlers via Request.get. For example, you might be doing some experiments with a homegrown ExperimentConfig type. You’d like to assign random configurations of that type on a per-request basis. You might do so with a Middleware: 
struct ExperimentMiddleware: Middleware {
    func intercept(_ request: Request, next: @escaping Next) -> EventLoopFuture<Response> {
        let config: ExperimentConfig = ... // load a random experiment config
        return next(request.set(config))
    }
}
You would then intercept requests with that Middleware and utilize the set ExperimentConfig in your handlers. 
app
    .use(ExperimentalMiddleware())
    .get("/experimental_endpoint") { request in
        // .get() will throw an error if a value with that type hasn't been `set()` on the `Request`.
        let config: ExperimentConfig = try request.get()
        if config.shouldUseLoudCopy {
            return "HELLO WORLD!!!!!"
        } else {
            return "hey, world."
        }
    }

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Accessing the Response

If you’d like to do something with the Response of the handled request, you can plug into the future returned by next. 
/// Logs all responses that come through this middleware.
struct LogResponseMiddleware: Middleware {
    func intercept(_ request: Request, next: @escaping Next) -> EventLoopFuture<Response> {
        return next(request)
            // Use `flatMap` if you want to do something asynchronously.
            .map { response in
                Log.info("Got a response \(response.status) from \(request.path).")
                return response
            }
    }
}

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Adding Middleware to Your Application

There are a few ways to have a Middleware intercept requests.

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Global Intercepting

If you’d like a middleware to intercept all requests on an Application, you can add it via Application.useAll. 
struct ExampleApp: Application {
    func boot() {
        self
            .useAll(LoggingMiddleware())
            // LoggingMiddleware will intercept all of these, as well as any unhandled requests.
            .get("/foo") { request in "Howdy foo!" }
            .post("/bar") { request in "Howdy bar!" }
            .put("/baz") { request in "Howdy baz!" }
    }
}

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Specific Intercepting

A Middleware can be setup to only intercept requests to specific handlers via the .use(_ middleware: Middleware) function on an Application. The Middleware will intercept all requests to the subsequently defined handlers. 
app
    .post("/password_reset", handler: ...)
    // Because this middleware is provided after the /password_reset endpoint,
    // it will only affect subsequent routes. In this case, only requests to
    // `/user` and `/todos` would be intercepted by the LoggingMiddleware.
    .use(LoggingMiddleware())
    .get("/user", handler: ...)
    .get("/todos", handler: ...)
There is also a .group function that takes a Middleware. The Middleware will only intercept requests handled by handlers defined in the closure. 
app
    .post("/user", handle: ...)
    .group(middleware: CustomAuthMiddleware()) {
        // Each of these endpoints will be protected by the
        // `CustomAuthMiddleWare`...
        $0.get("/todo", handler: ...)
            .put("/todo", handler: ...)
            .delete("/todo", handler: ...)
    }
    // ...but this one will not.
    .post("/reset", handler: ...)