add pinned announcement listener (#938)

* listens for pinned announcement in helper forum
* removes those announcements
* resolves #921

* improved javadoc and method name

* refactor class name

Author: ankitsmt211

application/src/main/java/org/togetherjava/tjbot/features/Features.java

@@ -118,6 +118,7 @@ public static Collection<Feature> createFeatures(JDA jda, Database database, Con
         features.add(new CodeMessageAutoDetection(config, codeMessageHandler));
         features.add(new CodeMessageManualDetection(codeMessageHandler));
         features.add(new SlashCommandEducator());
+        features.add(new PinnedNotificationRemover(config));
 
         // Event receivers
         features.add(new RejoinModerationRoleListener(actionsStore, config));

application/src/main/java/org/togetherjava/tjbot/features/help/PinnedNotificationRemover.java

@@ -0,0 +1,55 @@
+package org.togetherjava.tjbot.features.help;
+
+import net.dv8tion.jda.api.entities.Message;
+import net.dv8tion.jda.api.entities.MessageType;
+import net.dv8tion.jda.api.entities.channel.concrete.ThreadChannel;
+import net.dv8tion.jda.api.entities.channel.unions.MessageChannelUnion;
+import net.dv8tion.jda.api.events.message.MessageReceivedEvent;
+
+import org.togetherjava.tjbot.config.Config;
+import org.togetherjava.tjbot.features.MessageReceiverAdapter;
+
+import java.util.function.Predicate;
+import java.util.regex.Pattern;
+
+/**
+ * Removes notification-announcements about pinned messages from the helper forum.
+ */
+public final class PinnedNotificationRemover extends MessageReceiverAdapter {
+
+    private final Predicate<String> isHelpForumName;
+
+    /**
+     * Creates a new listener to receive all pinned announcement messages sent in help channels.
+     *
+     * @param config the config to use for this
+     */
+    public PinnedNotificationRemover(Config config) {
+        isHelpForumName =
+                Pattern.compile(config.getHelpSystem().getHelpForumPattern()).asMatchPredicate();
+    }
+
+    @Override
+    public void onMessageReceived(MessageReceivedEvent event) {
+        handlePinnedAnnouncement(event);
+    }
+
+    private boolean isHelpThread(MessageChannelUnion channel) {
+        if (!channel.getType().isThread()) {
+            return false;
+        }
+        ThreadChannel thread = channel.asThreadChannel();
+        String rootChannelName = thread.getParentChannel().getName();
+        return isHelpForumName.test(rootChannelName);
+    }
+
+    private boolean isPinnedAnnouncement(Message message) {
+        return message.getType() == MessageType.CHANNEL_PINNED_ADD;
+    }
+
+    private void handlePinnedAnnouncement(MessageReceivedEvent event) {
+        if (isPinnedAnnouncement(event.getMessage()) && isHelpThread(event.getChannel())) {
+            event.getMessage().delete().queue();
+        }
+    }
+}