feat: add NumberSignal basic support

packages/java/endpoint/pom.xml

@@ -243,6 +243,11 @@
             <artifactId>hamcrest-all</artifactId>
             <scope>test</scope>
         </dependency>
+        <dependency>
+            <groupId>io.projectreactor</groupId>
+            <artifactId>reactor-test</artifactId>
+            <scope>test</scope>
+        </dependency>
 
         <dependency>
             <groupId>com.vaadin</groupId>

packages/java/endpoint/src/main/java/com/vaadin/hilla/EndpointController.java

@@ -85,6 +85,8 @@ public class EndpointController {
      */
     public static final String ENDPOINT_MAPPER_FACTORY_BEAN_QUALIFIER = "endpointMapperFactory";
 
+    private static final String SIGNALS_HANDLER_BEAN_NAME = "signalsHandler";
+
     private final ApplicationContext context;
 
     EndpointRegistry endpointRegistry;
@@ -140,6 +142,15 @@ public void registerEndpoints(URL openApiResource) {
                     .registerEndpoint(endpointBean));
         }
 
+        // make sure that signalsHandler endpoint is always registered:
+        if (endpointRegistry.get(SIGNALS_HANDLER_BEAN_NAME) == null) {
+            var signalHandlerBean = endpointBeans
+                    .get(SIGNALS_HANDLER_BEAN_NAME);
+            if (signalHandlerBean != null) {
+                endpointRegistry.registerEndpoint(signalHandlerBean);
+            }
+        }
+
         if (!endpointRegistry.isEmpty()) {
             HillaStats.reportHasEndpoint();
         }

packages/java/endpoint/src/main/java/com/vaadin/hilla/signals/NumberSignal.java

@@ -0,0 +1,154 @@
+package com.vaadin.hilla.signals;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.DoubleNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+
+import com.fasterxml.jackson.databind.node.TextNode;
+import com.vaadin.hilla.signals.core.StateEvent;
+import jakarta.annotation.Nullable;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Sinks;
+
+import java.util.HashSet;
+import java.util.Set;
+import java.util.UUID;
+import java.util.concurrent.locks.ReentrantLock;
+
+/**
+ * A signal that holds a number value.
+ */
+public class NumberSignal {
+
+    private static final Logger LOGGER = LoggerFactory
+            .getLogger(NumberSignal.class);
+
+    private final ReentrantLock lock = new ReentrantLock();
+
+    private final ObjectMapper mapper;
+
+    private final UUID id = UUID.randomUUID();
+
+    private Double value;
+
+    private final Set<Sinks.Many<ObjectNode>> subscribers = new HashSet<>();
+
+    /**
+     * Creates a new NumberSignal with the provided default value.
+     *
+     * @param defaultValue
+     *            the default value
+     */
+    public NumberSignal(@Nullable Double defaultValue) {
+        this.value = defaultValue;
+        this.mapper = new ObjectMapper();
+    }
+
+    /**
+     * Creates a new NumberSignal with the default value of 0.
+     */
+    public NumberSignal() {
+        this(0.0);
+    }
+
+    /**
+     * Subscribes to the signal.
+     *
+     * @return a Flux of JSON events
+     */
+    public Flux<ObjectNode> subscribe() {
+        Sinks.Many<ObjectNode> sink = Sinks.many().multicast()
+                .onBackpressureBuffer();
+
+        return sink.asFlux().doOnSubscribe(ignore -> {
+            LOGGER.debug("New Flux subscription...");
+            lock.lock();
+            try {
+                var currentValue = createSnapshot();
+                sink.tryEmitNext(currentValue);
+                subscribers.add(sink);
+            } finally {
+                lock.unlock();
+            }
+        }).doFinally(ignore -> {
+            lock.lock();
+            try {
+                LOGGER.debug("Unsubscribing from NumberSignal...");
+                subscribers.remove(sink);
+            } finally {
+                lock.unlock();
+            }
+        });
+    }
+
+    /**
+     * Submits an event to the signal and notifies subscribers about the change
+     * of the signal value.
+     *
+     * @param event
+     *            the event to submit
+     */
+    public void submit(ObjectNode event) {
+        lock.lock();
+        try {
+            processEvent(event);
+            // Notify subscribers
+            subscribers.removeIf(sink -> {
+                var updatedValue = createSnapshot();
+                boolean failure = sink.tryEmitNext(updatedValue).isFailure();
+                if (failure) {
+                    LOGGER.debug("Failed push");
+                }
+                return failure;
+            });
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    /**
+     * Returns the signal UUID.
+     *
+     * @return the id
+     */
+    public UUID getId() {
+        return this.id;
+    }
+
+    /**
+     * Returns the signal's current value.
+     *
+     * @return the value
+     */
+    @Nullable
+    public Double getValue() {
+        return this.value;
+    }
+
+    private ObjectNode createSnapshot() {
+        var snapshot = new StateEvent<>(this.id, StateEvent.EventType.SNAPSHOT,
+                this.value);
+        return snapshot.toJson();
+    }
+
+    private void processEvent(ObjectNode event) {
+        try {
+            var stateEvent = new StateEvent<Double>(event);
+            if (isSetEvent(stateEvent)) {
+                this.value = stateEvent.getValue();
+            } else {
+                throw new UnsupportedOperationException(
+                        "Unsupported event: " + event);
+            }
+        } catch (StateEvent.InvalidEventTypeException e) {
+            throw new UnsupportedOperationException(
+                    "Unsupported JSON: " + event, e);
+        }
+    }
+
+    private boolean isSetEvent(StateEvent<?> event) {
+        return StateEvent.EventType.SET.equals(event.getEventType());
+    }
+}

packages/java/endpoint/src/main/java/com/vaadin/hilla/signals/config/SignalsConfiguration.java

@@ -0,0 +1,54 @@
+package com.vaadin.hilla.signals.config;
+
+import com.vaadin.hilla.ConditionalOnFeatureFlag;
+import com.vaadin.hilla.EndpointInvoker;
+import com.vaadin.hilla.signals.handler.SignalsHandler;
+import com.vaadin.hilla.signals.core.SignalsRegistry;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+
+/**
+ * Spring beans configuration for signals.
+ */
+@Configuration
+public class SignalsConfiguration {
+
+    private SignalsRegistry signalsRegistry;
+    private SignalsHandler signalsHandler;
+    private final EndpointInvoker endpointInvoker;
+
+    public SignalsConfiguration(EndpointInvoker endpointInvoker) {
+        this.endpointInvoker = endpointInvoker;
+    }
+
+    /**
+     * Initializes the SignalsRegistry bean when the fullstackSignals feature
+     * flag is enabled.
+     *
+     * @return SignalsRegistry bean instance
+     */
+    @ConditionalOnFeatureFlag("fullstackSignals")
+    @Bean
+    public SignalsRegistry signalsRegistry() {
+        if (signalsRegistry == null) {
+            signalsRegistry = new SignalsRegistry();
+        }
+        return signalsRegistry;
+    }
+
+    /**
+     * Initializes the SignalsHandler endpoint when the fullstackSignals feature
+     * flag is enabled.
+     *
+     * @return SignalsHandler endpoint instance
+     */
+    @ConditionalOnFeatureFlag("fullstackSignals")
+    @Bean
+    public SignalsHandler signalsHandler() {
+        if (signalsHandler == null) {
+            signalsHandler = new SignalsHandler(signalsRegistry(),
+                    endpointInvoker);
+        }
+        return signalsHandler;
+    }
+}