Router scrolls to top (by default) on route change

see #57

Author: japgolly

extra/ROUTER.md

@@ -111,7 +111,7 @@ val RootComponent = ReactComponentB[MyPage.Router]("Root")
 Note that the `Router` type is prefixed as `ReactComponentB[MyPage.Router]` and not `ReactComponentB[Router]`.
 Routers are not interchangable between routing rule sets.
 
-#### Renmdering your Router
+#### Rendering your Router
 
 Before a `Router` can be created it needs to the base URL, which is the prefix portion of the URL that is the same for all your pages routes.
 
@@ -176,7 +176,7 @@ override protected def interceptRender(i: InterceptionR): ReactElement =
       i.element)
 ```
 
-### URL rewriting
+#### URL rewriting
 
 `RoutingRules` comes with a built-in (although inactive) URL rewriting rule called `removeTrailingSlashes`.
 It can be installed via `register()` and is a good example of how to create dynamic matching rules.
@@ -188,3 +188,12 @@ def removeTrailingSlashes: DynamicRoute = {
   parser { case regex(p) => p }.redirection(p => (Path(p), Redirect.Replace))
 }
 ```
+
+#### Callback: `onRouteChange`
+By default, when a new route is activated the window is scrolled to the top.
+This behaviour can be removed or customised by providing a callback in your routing rules called `onRouteChange`.
+
+```scala
+onRouteChange { loc =>
+  ...
+}

extra/src/main/scala/japgolly/scalajs/react/extra/router/Router.scala

@@ -41,7 +41,8 @@ object Router {
  * @tparam P Routing rules context. Prevents different routing rule sets being mixed up.
  */
 final class Router[P](val baseUrl: BaseUrl,
-                      val pathAction: Path => RouteAction[P],
+                      pathAction: Path => RouteAction[P],
+                      onSync: Location[P] => IO[Unit],
                       logger: Router.Logger) extends Broadcaster[Unit] {
 
   type Cmd[A]  = RouteCmd[P, A]
@@ -87,7 +88,7 @@ final class Router[P](val baseUrl: BaseUrl,
     )
 
   def syncToWindowUrlS: ReactST[IO, Loc, Unit] =
-    ReactS.setM(syncToWindowUrl)
+    ReactS.setM(syncToWindowUrl) addCallbackS onSync
 
   def wrongBase(wrongUrl: AbsUrl): Prog[Loc] = {
     val url = AbsUrl(baseUrl.value)

extra/src/main/scala/japgolly/scalajs/react/extra/router/RoutingRules.scala

@@ -1,5 +1,7 @@
 package japgolly.scalajs.react.extra.router
 
+import org.scalajs.dom
+import scalaz.effect.IO
 import scalaz.{-\/, \/-, \/}
 import japgolly.scalajs.react.{ReactElement, ReactComponentC, TopNode}
 
@@ -29,18 +31,30 @@ trait RoutingRules {
   private[this] var dynRoutes = Vector.empty[DRouteFn]
   private[this] val parseD: DRouteFn = p => dynRoutes.foldLeft(None: Option[RouteAction[P]])(_ orElse _(p))
 
+  private[this] var onRouteChangeAction: Loc => IO[Unit] =
+    Function const IO(dom.window.scrollTo(0, 0))
+
   // ===================================================================================================================
   // Interception
 
   protected case class InterceptionR(loc: Loc, router: Router, element: ReactElement)
 
+  /** Customise all renderable routes. */
   protected def interceptRender(i: InterceptionR): ReactElement = i.element
 
   private def mkloc(path: Path, render: Renderer): Loc = {
     lazy val l: Loc = Location(path, r => interceptRender(InterceptionR(l, r, render(r))))
     l
   }
 
+  /** Perform an action when a new route is activated. */
+  protected def onRouteChange(a: Loc => Unit): Unit =
+    onRouteChangeIO(l => IO(a(l)))
+
+  /** Perform an action when a new route is activated. */
+  protected def onRouteChangeIO(a: Loc => IO[Unit]): Unit =
+    onRouteChangeAction = a
+
   // ===================================================================================================================
   // Actions
 
@@ -140,7 +154,7 @@ trait RoutingRules {
   import Router.{Logger, nopLogger}
 
   final def routingEngine(base: BaseUrl, logger: Logger = nopLogger): Router =
-    new Router(base, totalParser, logger)
+    new Router(base, totalParser, onRouteChangeAction, logger)
 
   final def router(base: BaseUrl, logger: Logger = nopLogger): Router.Component[P] =
     Router.component(routingEngine(base, logger))