Fixes and Improvements

+ Fixed sign showing up after the GUI is closed.
+ Added lombok (makes my life easier.)
+ Made async packet process sync (so it can interact with the world directly without the need of implementing your own solution or running up the task id number.)

pom.xml

@@ -70,5 +70,11 @@
             <version>4.7.0</version>
             <scope>provided</scope>
         </dependency>
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+            <version>RELEASE</version>
+            <scope>compile</scope>
+        </dependency>
     </dependencies>
 </project>

src/main/java/dev/pns/signapi/Async2Sync.java

@@ -0,0 +1,29 @@
+package dev.pns.signapi;
+
+import org.bukkit.Bukkit;
+import org.bukkit.plugin.java.JavaPlugin;
+
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+
+public class Async2Sync {
+    private final List<SignTask> signTasks = new ArrayList<>();
+
+    public Async2Sync(JavaPlugin plugin) {
+        Bukkit.getScheduler().runTaskTimer(plugin, () -> {
+            if (signTasks.isEmpty()) return;
+            Iterator<SignTask> iterator = signTasks.iterator();
+            while (iterator.hasNext()) {
+                SignTask signTask = iterator.next();
+                signTask.getPlayer().sendBlockChange(signTask.getBlock().getLocation(), signTask.getBlock().getBlockData());
+                signTask.getOnClose().method(signTask.getPlayer(), signTask.getLines());
+                iterator.remove();
+            }
+        }, 0, 1);
+    }
+
+    public void add(SignTask signTask) {
+        signTasks.add(signTask);
+    }
+}

src/main/java/dev/pns/signapi/SignAPI.java

@@ -4,6 +4,7 @@
 import com.comphenix.protocol.ProtocolLibrary;
 import com.comphenix.protocol.events.PacketAdapter;
 import com.comphenix.protocol.events.PacketEvent;
+import org.bukkit.Bukkit;
 import org.bukkit.plugin.java.JavaPlugin;
 
 import java.util.HashMap;
@@ -12,19 +13,25 @@
 import java.util.UUID;
 
 public class SignAPI {
-    private final Map<UUID, SignGUI.onClose> openGUIs = new HashMap<>();
+    private final Map<UUID, SignTask> openGUIs = new HashMap<>();
+    private final Async2Sync async2Sync;
 
     public SignAPI(JavaPlugin plugin) {
+        async2Sync = new Async2Sync(plugin);
+
         ProtocolLibrary.getProtocolManager().addPacketListener(new PacketAdapter(plugin, PacketType.Play.Client.UPDATE_SIGN) {
             @Override
             public void onPacketReceiving(PacketEvent event) {
-                UUID uuid = event.getPlayer().getUniqueId();
+                Bukkit.getScheduler().scheduleSyncDelayedTask(plugin, () -> {
+                    UUID uuid = event.getPlayer().getUniqueId();
+                    if (!openGUIs.containsKey(uuid)) return;
+
+                    SignTask signTask = openGUIs.get(uuid);
+                    signTask.setLines(event.getPacket().getStringArrays().read(0));
 
-                if (!openGUIs.containsKey(uuid)) return;
-                SignGUI.onClose closeEvent = openGUIs.get(uuid);
-
-                openGUIs.remove(uuid);
-                closeEvent.method(event.getPlayer(), event.getPacket().getStringArrays().read(0));
+                    openGUIs.remove(uuid);
+                    async2Sync.add(signTask);
+                }, 0);
             }
         });
     }

src/main/java/dev/pns/signapi/SignGUI.java

@@ -17,10 +17,10 @@
 public class SignGUI {
     private final onClose onClose;
     private final List<String> text;
-    private final Map<UUID, onClose> openGUIs;
+    private final Map<UUID, SignTask> openGUIs;
     private final ProtocolManager protocolManager = ProtocolLibrary.getProtocolManager();
 
-    public SignGUI(Map<UUID, onClose> openGUIs, List<String> text, onClose closeFunction) {
+    public SignGUI(Map<UUID, SignTask> openGUIs, List<String> text, onClose closeFunction) {
         this.openGUIs = openGUIs;
         this.text = text;
         this.onClose = closeFunction;
@@ -59,7 +59,7 @@ public void open(Player player) {
         sendPacket(player, packet);
 
         // Store the method to be called when the sign is closed
-        openGUIs.put(player.getUniqueId(), onClose);
+        openGUIs.put(player.getUniqueId(), new SignTask(player, onClose, player.getWorld().getBlockAt(bp.getX(), bp.getY(), bp.getZ())));
     }
 
     public interface onClose {

src/main/java/dev/pns/signapi/SignTask.java

@@ -0,0 +1,17 @@
+package dev.pns.signapi;
+
+import lombok.Getter;
+import lombok.RequiredArgsConstructor;
+import lombok.Setter;
+import org.bukkit.block.Block;
+import org.bukkit.entity.Player;
+
+@Getter
+@RequiredArgsConstructor
+public class SignTask {
+    private final Player player;
+    private final SignGUI.onClose onClose;
+    private final Block block;
+    @Setter
+    private String[] lines;
+}