Middleware

Middleware can be used to edit requests before they are forwared to the router, edit the responses returned by the route handlers or even shortcut the router and return their own responses. Middleware is added to the application as follows.

let app = HBApplication()
app.middleware.add(MyMiddlware())

Groups

Middleware can also be applied to a specific set of routes using groups. Below is a example of applying an authentication middleware BasicAuthenticatorMiddleware to routes that need protected.

let app = HBApplication()
app.router.put("/user", createUser)
app.router.group()
    .add(middleware: BasicAuthenticatorMiddleware())
    .post("/user", loginUser)

The first route that calls createUser does not have the BasicAuthenticatorMiddleware applied to it. But the route calling loginUser which is inside the group does have the middleware applied.

Writing Middleware

All middleware has to conform to the protocol HBMiddleware. This requires one function apply(to:next) to be implemented. At some point in this function unless you want to shortcut the router and return your own response you are required to call next.respond(to: request) and return the result, or a result processed by your middleware. The following is a simple logging middleware that outputs every URI being sent to the server

public struct LogRequestsMiddleware: HBMiddleware {
    public func apply(to request: HBRequest, next: HBResponder) -> EventLoopFuture<HBResponse> {
        // log request URI
        request.logger.log(level: .debug, String(describing:request.uri.path))
        // pass request onto next middleware or the router
        return next.respond(to: request)
    }
}

If you want to process the response after it has been returned by the route handler you will need to use run a function on the EventLoopFuture returned by next.respond. Swift NIO provide documentation EventLoopFuture here.

public struct ResponseProcessingMiddleware: HBMiddleware {
    public func apply(to request: HBRequest, next: HBResponder) -> EventLoopFuture<HBResponse> {
        return next.respond(to: request).map { response in
            // process responses from handler and middleware further down the chain
            return processResponse(response)
        }
        .flatMapError { error in
            // if an error is thrown by handler or middleware further down the 
            // chain process that
            return processError(error)
        }
    }
}

Already available

Hummingbird comes with a number of middleware already implemented.

• HBCORSMiddleware: Sets CORS headers
• HBLogRequestsMiddleware: Outputs request details to the log
• HBMetricsMiddleware: Outputs request details to a metrics server

HummingbirdFoundation also provides some middleware

• HBFileMiddleware: Serves static files
• HBDateResponseMiddleware: Sets the date header in the response in an optimal manner.