Replace polling with notifications by web sockets

This remove the polling delay and is overall a much nicer solution.
Add `websocket-jetty-server` dependency for web socket support for
the server.

build.gradle.kts

@@ -34,6 +34,7 @@ dependencies {
 
     implementation("org.eclipse.jetty:jetty-server:11.0.12")
     implementation("org.eclipse.jetty:jetty-servlet:11.0.12")
+    implementation("org.eclipse.jetty.websocket:websocket-jetty-server:11.0.12")
 
     runtimeOnly("org.slf4j:slf4j-simple:2.0.3")
     runtimeOnly("org.jetbrains.kotlin:kotlin-scripting-jsr223:1.7.20")

src/main/kotlin/nl/avisi/structurizr/site/generatr/ServeCommand.kt

@@ -4,6 +4,7 @@
 package nl.avisi.structurizr.site.generatr
 
 import com.sun.nio.file.SensitivityWatchEventModifier
+import jakarta.servlet.ServletContext
 import kotlinx.cli.*
 import nl.avisi.structurizr.site.generatr.site.copySiteWideAssets
 import nl.avisi.structurizr.site.generatr.site.generateDiagrams
@@ -13,8 +14,12 @@ import org.eclipse.jetty.server.Server
 import org.eclipse.jetty.servlet.DefaultServlet
 import org.eclipse.jetty.servlet.ServletContextHandler
 import org.eclipse.jetty.servlet.ServletHolder
+import org.eclipse.jetty.websocket.api.WebSocketAdapter
+import org.eclipse.jetty.websocket.server.JettyWebSocketServerContainer
+import org.eclipse.jetty.websocket.server.config.JettyWebSocketServletContainerInitializer
 import java.io.File
 import java.nio.file.*
+import java.time.Duration
 import kotlin.io.path.extension
 import kotlin.io.path.isDirectory
 import kotlin.io.path.isRegularFile
@@ -30,6 +35,8 @@ class ServeCommand : Subcommand("serve", "Start a development server") {
         ArgType.String, "site-dir", "s", "Directory for the generated site"
     ).default("build/serve")
 
+    private val eventSockets = mutableListOf<EventSocket>()
+
     override fun execute() {
         updateSite()
         val server = runServer()
@@ -68,6 +75,8 @@ class ServeCommand : Subcommand("serve", "Start a development server") {
             forServing = true
         )
 
+        sendSiteUpdatedNotifications()
+        clearInvalidEventSockets()
         println("Successfully generated diagrams and site")
     }
 
@@ -86,13 +95,23 @@ class ServeCommand : Subcommand("serve", "Start a development server") {
         ServletContextHandler().apply {
             contextPath = "/"
             addServlet(createStaticResourceServlet(), "/*")
+            addWebSocketServlet(this, "/_events")
         }
 
     private fun createStaticResourceServlet() =
         ServletHolder("default", DefaultServlet()).apply {
             setInitParameter("resourceBase", siteDir)
         }
 
+    private fun addWebSocketServlet(context: ServletContextHandler, pathSpec: String) =
+        JettyWebSocketServletContainerInitializer
+            .configure(context) { _: ServletContext?, container: JettyWebSocketServerContainer ->
+                container.idleTimeout = Duration.ZERO
+                container.addMapping(pathSpec) { _, _ ->
+                    EventSocket().also { eventSockets.add(it) }
+                }
+            }
+
     private fun startWatchService(): WatchService {
         val path = File(workspaceFile).absoluteFile.parentFile.toPath()
         val watchService = FileSystems.getDefault().newWatchService()
@@ -153,4 +172,25 @@ class ServeCommand : Subcommand("serve", "Start a development server") {
             SensitivityWatchEventModifier.HIGH
         )
     }
+
+    private fun sendSiteUpdatedNotifications() = eventSockets
+        .filter { it.isValid() }
+        .forEach { it.sendSiteUpdatedNotification() }
+
+    private fun clearInvalidEventSockets() = eventSockets
+        .toList() // Do a copy first since we might remove elements
+        .filterNot { it.isValid() }
+        .forEach { eventSockets.remove(it) }
+
+    private class EventSocket : WebSocketAdapter() {
+        fun sendSiteUpdatedNotification() {
+            if (session == null)
+                return
+
+            if (session.isOpen)
+                this.session.remote?.sendString("site-updated")
+        }
+
+        fun isValid() = isConnected
+    }
 }

src/main/resources/assets/js/auto-reload.js

@@ -25,4 +25,20 @@ function reloadIfNeeded() {
         });
 }
 
-setInterval(reloadIfNeeded, 200);
+function connectToWs() {
+    const ws = new WebSocket("ws://" + location.host + "/_events");
+    ws.onopen = () => console.log("Connected to socket.");
+    ws.onclose = (event) => {
+        console.log('Socket has been closed. Attempting to reconnect ...', event.reason);
+        setTimeout(() => connectToWs(), 1000);
+    };
+    ws.onmessage = (event) => {
+        if (event.data === "site-updated") {
+            console.log("Site update detected, detect page content change ...")
+            reloadIfNeeded();
+        }
+    }
+}
+
+reloadIfNeeded();
+connectToWs();