Added a CapabilityService to api module

Plan/api/build.gradle

@@ -2,7 +2,7 @@ plugins {
     id "com.jfrog.bintray" version "1.8.1"
 }
 
-ext.apiVersion = '0.0.3'
+ext.apiVersion = '0.0.4'
 
 bintray {
     user = System.getenv('BINTRAY_USER')

Plan/api/src/main/java/com/djrapitops/plan/capability/Capability.java

@@ -0,0 +1,49 @@
+/*
+ *  This file is part of Player Analytics (Plan).
+ *
+ *  Plan is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Lesser General Public License v3 as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  Plan is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public License
+ *  along with Plan. If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.djrapitops.plan.capability;
+
+import java.util.Optional;
+
+/**
+ * List of different capabilities current version provides.
+ * <p>
+ * The enum is package private to restrict direct access. This is to avoid NoClassDefFoundError in case
+ * a wanted Capability is not provided in an earlier version.
+ * <p>
+ * Use {@link CapabilityService#hasCapability(String)} with name of a Capability to figure out if an API is available.
+ * Example usage: {@code CapabilityService.getInstance().hasCapability("DATA_EXTENSION_VALUES")}.
+ * <p>
+ * If a capability is not available, attempting to use the capability might lead to exceptions.
+ *
+ * @author Rsl1122
+ */
+enum Capability {
+
+    /**
+     * ExtensionService, DataExtension API base package, PluginInfo, Conditional, Tab, TabInfo, TabOrder and BooleanProvider, DoubleProvider, PercentageProvider, NumberProvider, StringProvider annotations.
+     */
+    DATA_EXTENSION_VALUES;
+
+    static Optional<Capability> getByName(String name) {
+        try {
+            return Optional.of(valueOf(name));
+        } catch (IllegalArgumentException e) {
+            return Optional.empty();
+        }
+    }
+
+}

Plan/api/src/main/java/com/djrapitops/plan/capability/CapabilityService.java

@@ -0,0 +1,76 @@
+/*
+ *  This file is part of Player Analytics (Plan).
+ *
+ *  Plan is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Lesser General Public License v3 as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  Plan is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public License
+ *  along with Plan. If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.djrapitops.plan.capability;
+
+import java.util.Optional;
+import java.util.function.Consumer;
+
+/**
+ * Service for figuring out provided API capabilities.
+ * <p>
+ * {@link CapabilityService#registerEnableListener(Consumer)} to be notified of Plan reloads
+ * {@link CapabilityService#hasCapability(String)} to check if a capability is available.
+ * <p>
+ * See {@link Capability} for list of capabilities provided by the current version.
+ *
+ * @author Rsl1122
+ */
+public interface CapabilityService {
+
+    /**
+     * Obtain instance of CapabilityService.
+     *
+     * @return CapabilityService implementation.
+     * @throws NoClassDefFoundError  If Plan is not installed and this class can not be found or if older Plan version is installed.
+     * @throws IllegalStateException If Plan is installed, but not enabled.
+     */
+    static CapabilityService getInstance() {
+        return Optional.ofNullable(CapabilityServiceHolder.service)
+                .orElseThrow(() -> new IllegalStateException("CapabilityService has not been initialised yet."));
+    }
+
+    /**
+     * Register a method to be called when Plan reloads.
+     *
+     * @param isEnabledListener The boolean given to the method tells if Plan has enabled successfully.
+     */
+    void registerEnableListener(Consumer<Boolean> isEnabledListener);
+
+    /**
+     * Check if the API on the current version provides a capability.
+     *
+     * @param capabilityName Name of a capability
+     * @return true if the capability is available.
+     * @see Capability for different capabilityNames.
+     */
+    default boolean hasCapability(String capabilityName) {
+        return Capability.getByName(capabilityName).isPresent();
+    }
+
+    class CapabilityServiceHolder {
+        static CapabilityService service;
+
+        private CapabilityServiceHolder() {
+            /* Static variable holder */
+        }
+
+        static void set(CapabilityService service) {
+            CapabilityServiceHolder.service = service;
+        }
+    }
+
+}

Plan/bukkit/src/main/java/com/djrapitops/plan/system/listeners/BukkitListenerSystem.java

@@ -19,6 +19,7 @@ package com.djrapitops.plan.system.listeners;
 import com.djrapitops.plan.Plan;
 import com.djrapitops.plan.PlanPlugin;
 import com.djrapitops.plan.api.events.PlanBukkitEnableEvent;
+import com.djrapitops.plan.capability.CapabilityServiceImplementation;
 import com.djrapitops.plan.system.listeners.bukkit.*;
 import com.djrapitops.plan.system.status.Status;
 import org.bukkit.Bukkit;
@@ -83,7 +84,9 @@ public class BukkitListenerSystem extends ListenerSystem {
 
     @Override
     public void callEnableEvent(PlanPlugin plugin) {
-        PlanBukkitEnableEvent event = new PlanBukkitEnableEvent(plugin.isSystemEnabled());
+        boolean isEnabled = plugin.isSystemEnabled();
+        PlanBukkitEnableEvent event = new PlanBukkitEnableEvent(isEnabled);
         Bukkit.getServer().getPluginManager().callEvent(event);
+        CapabilityServiceImplementation.notifyAboutEnable(isEnabled);
     }
 }

Plan/bungeecord/src/main/java/com/djrapitops/plan/system/listeners/BungeeListenerSystem.java

@@ -19,6 +19,7 @@ package com.djrapitops.plan.system.listeners;
 import com.djrapitops.plan.PlanBungee;
 import com.djrapitops.plan.PlanPlugin;
 import com.djrapitops.plan.api.events.PlanBungeeEnableEvent;
+import com.djrapitops.plan.capability.CapabilityServiceImplementation;
 import com.djrapitops.plan.system.listeners.bungee.PlayerOnlineListener;
 
 import javax.inject.Inject;
@@ -46,7 +47,9 @@ public class BungeeListenerSystem extends ListenerSystem {
 
     @Override
     public void callEnableEvent(PlanPlugin plugin) {
-        PlanBungeeEnableEvent event = new PlanBungeeEnableEvent(plugin.isSystemEnabled());
+        boolean isEnabled = plugin.isSystemEnabled();
+        PlanBungeeEnableEvent event = new PlanBungeeEnableEvent(isEnabled);
         ((PlanBungee) plugin).getProxy().getPluginManager().callEvent(event);
+        CapabilityServiceImplementation.notifyAboutEnable(isEnabled);
     }
 }

Plan/common/src/main/java/com/djrapitops/plan/capability/CapabilityServiceImplementation.java

@@ -0,0 +1,58 @@
+/*
+ *  This file is part of Player Analytics (Plan).
+ *
+ *  Plan is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU Lesser General Public License v3 as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  Plan is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU Lesser General Public License for more details.
+ *
+ *  You should have received a copy of the GNU Lesser General Public License
+ *  along with Plan. If not, see <https://www.gnu.org/licenses/>.
+ */
+package com.djrapitops.plan.capability;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Consumer;
+
+/**
+ * Singleton instance implementation for {@link CapabilityService}.
+ * <p>
+ * Only one instance exists per runtime in order to notify others when the plugin enables.
+ *
+ * @author Rsl1122
+ */
+public class CapabilityServiceImplementation implements CapabilityService {
+
+    private List<Consumer<Boolean>> enableListeners;
+
+    private CapabilityServiceImplementation() {
+        /* Inject required for dagger */
+        CapabilityServiceHolder.set(this);
+        enableListeners = new ArrayList<>();
+    }
+
+    private static CapabilityServiceImplementation get() {
+        CapabilityService instance = CapabilityService.getInstance();
+        if (instance == null) {
+            return new CapabilityServiceImplementation();
+        }
+        return (CapabilityServiceImplementation) instance;
+    }
+
+    public static void notifyAboutEnable(boolean isEnabled) {
+        for (Consumer<Boolean> enableListener : get().enableListeners) {
+            enableListener.accept(isEnabled);
+        }
+    }
+
+    @Override
+    public void registerEnableListener(Consumer<Boolean> enableListener) {
+        enableListeners.add(enableListener);
+    }
+}

Plan/sponge/src/main/java/com/djrapitops/plan/system/listeners/SpongeListenerSystem.java

@@ -20,6 +20,7 @@ import com.djrapitops.plan.PlanPlugin;
 import com.djrapitops.plan.PlanSponge;
 import com.djrapitops.plan.SpongeServerShutdownSave;
 import com.djrapitops.plan.api.events.PlanSpongeEnableEvent;
+import com.djrapitops.plan.capability.CapabilityServiceImplementation;
 import com.djrapitops.plan.system.listeners.sponge.*;
 import org.spongepowered.api.Sponge;
 import org.spongepowered.api.event.Event;
@@ -94,5 +95,6 @@ public class SpongeListenerSystem extends ListenerSystem {
         } catch (IllegalStateException ignore) {
             /* Ignore, Sponge is not initialized */
         }
+        CapabilityServiceImplementation.notifyAboutEnable(plugin.isSystemEnabled());
     }
 }

Plan/velocity/src/main/java/com/djrapitops/plan/system/listeners/VelocityListenerSystem.java

@@ -19,6 +19,7 @@ package com.djrapitops.plan.system.listeners;
 import com.djrapitops.plan.PlanPlugin;
 import com.djrapitops.plan.PlanVelocity;
 import com.djrapitops.plan.api.events.PlanVelocityEnableEvent;
+import com.djrapitops.plan.capability.CapabilityServiceImplementation;
 import com.djrapitops.plan.system.listeners.velocity.PlayerOnlineListener;
 
 import javax.inject.Inject;
@@ -52,7 +53,9 @@ public class VelocityListenerSystem extends ListenerSystem {
 
     @Override
     public void callEnableEvent(PlanPlugin plugin) {
-        PlanVelocityEnableEvent event = new PlanVelocityEnableEvent(plugin.isSystemEnabled());
+        boolean isEnabled = plugin.isSystemEnabled();
+        PlanVelocityEnableEvent event = new PlanVelocityEnableEvent(isEnabled);
         ((PlanVelocity) plugin).getProxy().getEventManager().fireAndForget(event);
+        CapabilityServiceImplementation.notifyAboutEnable(isEnabled);
     }
 }