Add support for API load callbacks and initialization checks

src/main/java/org/mvplugins/multiverse/portals/MultiversePortalsApi.java

@@ -1,34 +1,85 @@
 package org.mvplugins.multiverse.portals;
 
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+import java.util.function.Consumer;
+
+import com.dumptruckman.minecraft.util.Logging;
 import org.bukkit.Bukkit;
 import org.bukkit.plugin.ServicePriority;
 import org.jetbrains.annotations.ApiStatus;
 import org.jetbrains.annotations.NotNull;
 import org.mvplugins.multiverse.core.inject.PluginServiceLocator;
+import org.mvplugins.multiverse.external.vavr.control.Try;
 import org.mvplugins.multiverse.portals.action.ActionHandlerProvider;
 import org.mvplugins.multiverse.portals.config.PortalsConfig;
 import org.mvplugins.multiverse.portals.utils.PortalFiller;
 import org.mvplugins.multiverse.portals.utils.PortalManager;
 
-import java.util.Objects;
-
 public final class MultiversePortalsApi {
 
     private static MultiversePortalsApi instance;
+    private static final List<Consumer<MultiversePortalsApi>> WHEN_LOADED_CALLBACKS = new ArrayList<>();
 
     static void init(@NotNull MultiversePortals multiversePortals) {
         if (instance != null) {
             throw new IllegalStateException("MultiversePortalsApi has already been initialized!");
         }
         instance = new MultiversePortalsApi(multiversePortals.getServiceLocator());
         Bukkit.getServicesManager().register(MultiversePortalsApi.class, instance, multiversePortals, ServicePriority.Normal);
+
+        List<Consumer<MultiversePortalsApi>> callbacks = List.copyOf(WHEN_LOADED_CALLBACKS);
+        WHEN_LOADED_CALLBACKS.clear();
+        MultiversePortalsApi loadedApi = instance;
+        callbacks.forEach(callback -> runLoadCallback(callback, loadedApi));
+    }
+
+    private static void runLoadCallback(
+            @NotNull Consumer<MultiversePortalsApi> callback,
+            @NotNull MultiversePortalsApi loadedApi) {
+        Try.run(() -> callback.accept(loadedApi))
+                .onFailure(exception -> Logging.warning(
+                        "A Multiverse-Portals API load callback failed: %s", exception.getMessage()));
     }
 
     static void shutdown() {
+        if (instance == null) {
+            return;
+        }
         Bukkit.getServicesManager().unregister(instance);
         instance = null;
     }
 
+    /**
+     * Executes a callback once the Multiverse-Portals API has been initialized.
+     * The callback is executed immediately if the API is already initialized.
+     *
+     * @param consumer The callback to execute
+     *
+     * @since 5.3
+     */
+    @ApiStatus.AvailableSince("5.3")
+    public static void whenLoaded(@NotNull Consumer<MultiversePortalsApi> consumer) {
+        if (instance != null) {
+            consumer.accept(instance);
+        } else {
+            WHEN_LOADED_CALLBACKS.add(consumer);
+        }
+    }
+
+    /**
+     * Checks whether the Multiverse-Portals API has been initialized.
+     *
+     * @return {@code true} if the API has been initialized, otherwise {@code false}
+     *
+     * @since 5.3
+     */
+    @ApiStatus.AvailableSince("5.3")
+    public static boolean isLoaded() {
+        return instance != null;
+    }
+
     /**
      * Gets the MultiversePortalsApi. This will throw an exception if the Multiverse-Portals has not been initialized.
      *