Add http endpoint events with SSE

src/main/java/org/asamk/signal/http/ServerSentEventSender.java

@@ -0,0 +1,55 @@
+package org.asamk.signal.http;
+
+import java.io.BufferedWriter;
+import java.io.IOException;
+import java.io.OutputStream;
+import java.io.OutputStreamWriter;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
+
+/**
+ * This class send Server-sent events payload to an OutputStream.
+ * See <a href="https://html.spec.whatwg.org/multipage/server-sent-events.html">spec</a>
+ */
+public class ServerSentEventSender {
+
+    private final BufferedWriter writer;
+
+    public ServerSentEventSender(final OutputStream outputStream) {
+        this.writer = new BufferedWriter(new OutputStreamWriter(outputStream, StandardCharsets.UTF_8));
+    }
+
+    /**
+     * @param id    Event id
+     * @param event Event type
+     * @param data  Event data, each entry must not contain newline chars.
+     */
+    public synchronized void sendEvent(String id, String event, List<String> data) throws IOException {
+        if (id != null) {
+            writer.write("id:");
+            writer.write(id);
+            writer.write("\n");
+        }
+        if (event != null) {
+            writer.write("event:");
+            writer.write(event);
+            writer.write("\n");
+        }
+        if (data.size() == 0) {
+            writer.write("data\n");
+        } else {
+            for (final var d : data) {
+                writer.write("data:");
+                writer.write(d);
+                writer.write("\n");
+            }
+        }
+        writer.write("\n");
+        writer.flush();
+    }
+
+    public synchronized void sendKeepAlive() throws IOException {
+        writer.write(":\n");
+        writer.flush();
+    }
+}