Implement inline replies feature

src/main/java/net/dv8tion/jda/api/entities/Message.java

@@ -199,6 +199,9 @@ public interface Message extends ISnowflake, Formattable
             "(?:\\?\\S*)?(?:#\\S*)?",                                      // Useless query or URN appendix
             Pattern.CASE_INSENSITIVE);
 
+    @Nullable
+    Message getReferencedMessage();
+
     /**
      * An immutable list of all mentioned {@link net.dv8tion.jda.api.entities.User Users}.
      * <br>If no user was mentioned, this list is empty. Elements are sorted in order of appearance. This only

src/main/java/net/dv8tion/jda/api/entities/MessageType.java

@@ -88,6 +88,8 @@ public enum MessageType
      */
     CHANNEL_FOLLOW_ADD(12),
 
+    INLINE_REPLY(19),
+
     /**
      * Unknown MessageType.
      */

src/main/java/net/dv8tion/jda/api/requests/restaction/MessageAction.java

@@ -22,6 +22,7 @@
 import net.dv8tion.jda.api.entities.MessageEmbed;
 import net.dv8tion.jda.api.requests.RestAction;
 import net.dv8tion.jda.api.utils.AttachmentOption;
+import net.dv8tion.jda.api.utils.MiscUtil;
 import net.dv8tion.jda.internal.requests.restaction.MessageActionImpl;
 import net.dv8tion.jda.internal.utils.Checks;
 
@@ -181,6 +182,24 @@ static EnumSet<Message.MentionType> getDefaultMentions()
     @CheckReturnValue
     MessageAction apply(@Nullable final Message message);
 
+    @Nonnull
+    @CheckReturnValue
+    MessageAction referenceById(long messageId);
+
+    @Nonnull
+    @CheckReturnValue
+    default MessageAction referenceById(@Nonnull String messageId)
+    {
+        return referenceById(MiscUtil.parseSnowflake(messageId));
+    }
+
+    @Nonnull
+    @CheckReturnValue
+    default MessageAction reference(@Nonnull Message message)
+    {
+        return referenceById(message.getIdLong());
+    }
+
     /**
      * Enable/Disable Text-To-Speech for the resulting message.
      * <br>This is only relevant to MessageActions that are not {@code isEdit() == true}!

src/main/java/net/dv8tion/jda/internal/entities/AbstractMessage.java

@@ -106,6 +106,13 @@ protected void appendFormat(Formatter formatter, int width, int precision, boole
         }
     }
 
+    @Override
+    public Message getReferencedMessage()
+    {
+        unsupported();
+        return null;
+    }
+
     @Nonnull
     @Override
     public Bag<User> getMentionedUsersBag()

src/main/java/net/dv8tion/jda/internal/entities/EntityBuilder.java

@@ -1139,10 +1139,18 @@ public Message createMessage(DataObject jsonObject, MessageChannel chan, boolean
 
         MessageType type = MessageType.fromId(jsonObject.getInt("type"));
         ReceivedMessage message;
+        Message referencedMessage = null;
+        if (!jsonObject.isNull("referenced_message"))
+        {
+            referencedMessage = createMessage(jsonObject.getObject("referenced_message"), chan, false);
+            if (type == MessageType.DEFAULT)
+                type = MessageType.INLINE_REPLY;
+        }
         switch (type)
         {
+            case INLINE_REPLY:
             case DEFAULT:
-                message = new ReceivedMessage(id, chan, type, fromWebhook,
+                message = new ReceivedMessage(id, chan, type, referencedMessage, fromWebhook,
                     mentionsEveryone, mentionedUsers, mentionedRoles, tts, pinned,
                     content, nonce, user, member, activity, editTime, reactions, attachments, embeds, flags);
                 break;

src/main/java/net/dv8tion/jda/internal/entities/ReceivedMessage.java

@@ -55,6 +55,7 @@ public class ReceivedMessage extends AbstractMessage
     protected final long id;
     protected final MessageType type;
     protected final MessageChannel channel;
+    protected final Message referencedMessage;
     protected final boolean fromWebhook;
     protected final boolean mentionsEveryone;
     protected final boolean pinned;
@@ -81,14 +82,15 @@ public class ReceivedMessage extends AbstractMessage
     protected List<String> invites = null;
 
     public ReceivedMessage(
-        long id, MessageChannel channel, MessageType type,
+        long id, MessageChannel channel, MessageType type, Message referencedMessage,
         boolean fromWebhook, boolean mentionsEveryone, TLongSet mentionedUsers, TLongSet mentionedRoles, boolean tts, boolean pinned,
         String content, String nonce, User author, Member member, MessageActivity activity, OffsetDateTime editTime,
         List<MessageReaction> reactions, List<Attachment> attachments, List<MessageEmbed> embeds, int flags)
     {
         super(content, nonce, tts);
         this.id = id;
         this.channel = channel;
+        this.referencedMessage = referencedMessage;
         this.type = type;
         this.api = (channel != null) ? (JDAImpl) channel.getJDA() : null;
         this.fromWebhook = fromWebhook;
@@ -113,6 +115,12 @@ public JDA getJDA()
         return api;
     }
 
+    @Override
+    public Message getReferencedMessage()
+    {
+        return referencedMessage;
+    }
+
     @Override
     public boolean isPinned()
     {

src/main/java/net/dv8tion/jda/internal/entities/SystemMessage.java

@@ -34,7 +34,7 @@ public SystemMessage(
         String content, String nonce, User author, Member member, MessageActivity activity, OffsetDateTime editTime,
         List<MessageReaction> reactions, List<Attachment> attachments, List<MessageEmbed> embeds, int flags)
     {
-        super(id, channel, type, fromWebhook, mentionsEveryone, mentionedUsers, mentionedRoles,
+        super(id, channel, type, null, fromWebhook, mentionsEveryone, mentionedUsers, mentionedRoles,
             tts, pinned, content, nonce, author, member, activity, editTime, reactions, attachments, embeds, flags);
     }
 

src/main/java/net/dv8tion/jda/internal/requests/restaction/MessageActionImpl.java

@@ -62,6 +62,7 @@ public class MessageActionImpl extends RestActionImpl<Message> implements Messag
     protected EnumSet<Message.MentionType> allowedMentions;
     protected Set<String> mentionableUsers = new HashSet<>();
     protected Set<String> mentionableRoles = new HashSet<>();
+    protected long messageReference;
 
     public static void setDefaultMentions(@Nullable Collection<Message.MentionType> allowedMentions)
     {
@@ -185,6 +186,14 @@ public MessageActionImpl apply(final Message message)
         return content(content).tts(message.isTTS());
     }
 
+    @Nonnull
+    @Override
+    public MessageActionImpl referenceById(long messageId)
+    {
+        messageReference = messageId;
+        return this;
+    }
+
     @Nonnull
     @Override
     @CheckReturnValue
@@ -480,6 +489,8 @@ protected DataObject getJSON()
             if (nonce != null)
                 obj.put("nonce", nonce);
         }
+        if (messageReference != 0)
+            obj.put("message_reference", messageReference);
         obj.put("tts", tts);
         if (allowedMentions != null || !mentionableUsers.isEmpty() || !mentionableRoles.isEmpty())
         {