Added registry

eco-api/src/main/java/com/willfp/eco/core/registry/Registrable.java

@@ -0,0 +1,32 @@
+package com.willfp.eco.core.registry;
+
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * An object that can be registered.
+ *
+ * @see Registry
+ */
+public interface Registrable {
+    /**
+     * Get the ID of the element.
+     *
+     * @return The ID.
+     */
+    @NotNull
+    String getID();
+
+    /**
+     * Called when the element is registered.
+     */
+    default void onRegister() {
+        // Do nothing by default.
+    }
+
+    /**
+     * Called when the element is removed.
+     */
+    default void onRemove() {
+        // Do nothing by default.
+    }
+}

eco-api/src/main/java/com/willfp/eco/core/registry/Registry.java

@@ -0,0 +1,102 @@
+package com.willfp.eco.core.registry;
+
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+
+/**
+ * A registry for {@link Registrable}s.
+ *
+ * @param <T> The type of {@link Registrable}.
+ */
+public abstract class Registry<T extends Registrable> {
+    /**
+     * The registry.
+     */
+    private final Map<String, T> registry = new ConcurrentHashMap<>();
+
+    /**
+     * Instantiate a new registry.
+     */
+    protected Registry() {
+
+    }
+
+    /**
+     * Register a new element.
+     *
+     * @param element The element to register.
+     * @return The element.
+     */
+    @NotNull
+    public T register(@NotNull final T element) {
+        registry.put(element.getID(), element);
+
+        element.onRegister();
+
+        return element;
+    }
+
+    /**
+     * Remove an element.
+     *
+     * @param element The element.
+     * @return The element.
+     */
+    public T remove(@NotNull final T element) {
+        element.onRemove();
+
+        registry.remove(element.getID());
+
+        return element;
+    }
+
+    /**
+     * Remove an element by ID.
+     *
+     * @param id The ID.
+     * @return The element.
+     */
+    @Nullable
+    public T remove(@NotNull final String id) {
+        T element = registry.get(id);
+
+        if (element != null) {
+            element.onRemove();
+        }
+
+        return registry.remove(id);
+    }
+
+    /**
+     * Get an element by ID.
+     *
+     * @param id The ID.
+     * @return The element, or null if not found.
+     */
+    @Nullable
+    public T get(@NotNull final String id) {
+        return registry.get(id);
+    }
+
+    /**
+     * Clear the registry.
+     */
+    public void clear() {
+        for (T value : registry.values()) {
+            remove(value);
+        }
+    }
+
+    /**
+     * Get all elements.
+     *
+     * @return All elements.
+     */
+    public Set<T> values() {
+        return Set.copyOf(registry.values());
+    }
+}