Calico offers a router as an optional module. A router helps you build a single-page application composed of sub-pages with distinct URLs and support for session history (e.g., back/forward). It is implemented via the History API.

libraryDependencies += "com.armanbilge" %%% "calico-router" % "0.2.0-M1"

calico-router is built only with Cats Effect, FS2, http4s, and scala-js-dom, and has no dependency on calico core, such that it is framework-agnostic. Integration with Calico is seamless.

Special thanks to @jberggg whose fs2-spa-router inspired this project.

Concepts and demo

The Router API offers methods for interacting with the browser location and session history and for dispatching Routes into components that can be placed in your application. It is possible to dispatch multiple Routes within a single application and even nest them arbitrarily.

The method can be used to create a single "endpoint" by:

  1. matching on an http4s Uri
  2. extracting some state A from the Uri
  3. building a component with a Signal of A

This design enables an important optimization: when navigating to a new location, if the URI matches the currently rendered endpoint, then the existing component is reused and the Signal propagates any changes to dynamic content. This is demonstrated in the demo below where a pair of counters track every time an endpoint is re-rendered from scratch.

Endpoints are combined via the |+| operator, because Routes form a monoid.

In the following demo all routing is done by matching on the query portion of the URI since this is compatible with GitHub Pages hosting. A production application can match on the path portion for a friendlier UX.

import{*, given}
import calico.router.*
import calico.syntax.*
import calico.unsafe.given
import cats.effect.*
import cats.effect.syntax.all.*
import cats.syntax.all.*
import fs2.*
import fs2.concurrent.*
import fs2.dom.*
import org.http4s.*
import org.http4s.syntax.all.*

val app = Resource.eval(Router(Location[IO], History[IO, Unit])).flatMap { router =>
  (SignallingRef[IO].of(0), SignallingRef[IO].of(0)).tupled.toResource.flatMap {
    (helloCounter, countCounter) =>

      def helloUri(who: String) =
        uri"" +? ("page" -> "hello") +? ("who" -> who)

      def countUri(n: Int) =
        uri"" +? ("page" -> "count") +? ("n" -> n)

      val helloRoute =[IO] {
        case uri if uri.query.params.get("page").contains("hello") =>
          uri.query.params.getOrElse("who", "world")
      } { who => Resource.eval(helloCounter.update(_ + 1)) *> div("Hello, ", who) }

      val countRoute =[IO] {
        case uri if uri.query.params.get("page").contains("count") =>
      } { n =>
        Resource.eval(countCounter.update(_ + 1)) *>
            "Sheep: ",
            " ",
              onClick --> {
                _.foreach(_ => => countUri(i + 1)).flatMap(router.navigate))

      val content = (helloRoute |+| countRoute).toResource.flatMap(router.dispatch)

        p("Created hello page ",, " times."),
        p("Created count page ",, " times."),
        p("Watch the URL change in your browser address bar!"),
          List("Shaun", "Shirley", "Timmy", "Nuts").map { who =>
                href := "#",
                onClick --> (_.foreach(_ => router.navigate(helloUri(who)))),
                s"Hello, $who"
              href := "#",
              onClick --> (_.foreach(_ => router.navigate(countUri(0)))),
              "Let's count!"