package com.atlassian.plugin.tracker;
   
   import com.atlassian.plugin.ModuleDescriptor;
   import com.atlassian.plugin.PluginAccessor;
   import com.atlassian.plugin.event.PluginEventListener;
   import com.atlassian.plugin.event.PluginEventManager;
   import com.atlassian.plugin.event.events.PluginDisabledEvent;
   import com.atlassian.plugin.event.events.PluginModuleDisabledEvent;
   import com.atlassian.plugin.event.events.PluginModuleEnabledEvent;
  import com.google.common.base.Function;
  
  import java.util.concurrent.CopyOnWriteArraySet;
  
  import static com.google.common.collect.Iterables.filter;
  import static com.google.common.collect.Iterables.transform;
  import static com.google.common.collect.Iterables.unmodifiableIterable;
  import static java.util.Collections.singleton;
  
  /**
   * Tracks enabled plugin module descriptors, focusing on fast reads
   *
   * @since 2.6.0
   */
  public class DefaultPluginModuleTracker<M, T extends ModuleDescriptor<M>> implements PluginModuleTracker<M, T> {
      private final PluginEventManager pluginEventManager;
      private final Class<T> moduleDescriptorClass;
      private final Customizer<M, T> pluginModuleTrackerCustomizer;
      private final CopyOnWriteArraySet<T> moduleDescriptors = new CopyOnWriteArraySet<T>();
      private final ModuleTransformer<M, T> moduleTransformer = new ModuleTransformer<M, T>();
  
      //
      // ctors
      //
  
      public DefaultPluginModuleTracker(final PluginAccessor pluginAccessor, final PluginEventManager pluginEventManager, final Class<T> moduleDescriptorClass) {
          this(pluginAccessor, pluginEventManager, moduleDescriptorClass, new NoOpPluginModuleTrackerCustomizer<M, T>());
      }
  
      public DefaultPluginModuleTracker(final PluginAccessor pluginAccessor, final PluginEventManager pluginEventManager, final Class<T> moduleDescriptorClass, final Customizer<M, T> pluginModuleTrackerCustomizer) {
          this.pluginEventManager = pluginEventManager;
          this.moduleDescriptorClass = moduleDescriptorClass;
          this.pluginModuleTrackerCustomizer = pluginModuleTrackerCustomizer;
  
          // TODO: registering an event handler in constructor isn't reliable:
          //       if we're invoked in another thread before we're fully constructed, the event handler might throw exceptions.
          //       Won't break the plugin manager (event publisher catches Throwable and logs) but we may miss that event.
          // https://extranet.atlassian.com/display/CONF/2015/04/28/Publishing+unconstructed+objects+into+main+memory+considered+harmful
          pluginEventManager.register(this);
  
          addDescriptors(pluginAccessor.getEnabledModuleDescriptorsByClass(moduleDescriptorClass));
      }
  
      //
      // PluginModuleTracker impl
      //
  
      public Iterable<T> getModuleDescriptors() {
          return unmodifiableIterable(moduleDescriptors);
      }
  
      public Iterable<M> getModules() {
          return transform(getModuleDescriptors(), moduleTransformer);
      }
  
      public int size() {
          return moduleDescriptors.size();
      }
  
      public void close() {
          pluginEventManager.unregister(this);
      }
  
      //
      // plugin event listening
      //
  
      @PluginEventListener
      public void onPluginModuleEnabled(final PluginModuleEnabledEvent event) {
          addDescriptors(singleton((ModuleDescriptor<?>) event.getModule()));
      }
  
      @PluginEventListener
      public void onPluginModuleDisabled(final PluginModuleDisabledEvent event) {
          removeDescriptors(singleton((ModuleDescriptor<?>) event.getModule()));
      }
  
      @PluginEventListener
      public void onPluginDisabled(final PluginDisabledEvent event) {
          removeDescriptors(event.getPlugin().getModuleDescriptors());
      }
  
      //
      // module descriptor management
      //
  
      private void addDescriptors(final Iterable<? extends ModuleDescriptor<?>> descriptors) {
          for (final T descriptor : filtered(descriptors)) {
              final T customized = pluginModuleTrackerCustomizer.adding(descriptor);
              if (customized != null) {
                 moduleDescriptors.add(customized);
             }
         }
     }
 
     private void removeDescriptors(final Iterable<? extends ModuleDescriptor<?>> descriptors) {
         for (final T descriptor : filtered(descriptors)) {
             if (moduleDescriptors.remove(descriptor)) {
                 pluginModuleTrackerCustomizer.removed(descriptor);
             }
         }
     }
 
     /**
      * The descriptors that match the supplied class.
      */
     private Iterable<T> filtered(final Iterable<? extends ModuleDescriptor<?>> descriptors) {
         return filter(descriptors, moduleDescriptorClass);
     }
 
     //
     // inner classes
     //
 
     private static class NoOpPluginModuleTrackerCustomizer<M, T extends ModuleDescriptor<M>> implements PluginModuleTracker.Customizer<M, T> {
         public T adding(final T descriptor) {
             return descriptor;
         }
 
         public void removed(final T descriptor) {
         }
     }
 
     /**
      * Safely get the Module from a {@link ModuleDescriptor}.
      */
     private static class ModuleTransformer<M, T extends ModuleDescriptor<M>> implements Function<T, M> {
         public M apply(final T from) {
             return from.getModule();
         }
     }
 
     /**
      * Static factory method for constructing trackers generically where M is not known.
      *
      * @param <M>                   The module class, generically inferred.
      * @param <T>                   The module descriptor class.
      * @param pluginAccessor        For getting the enabled descriptors of a certain type.
      * @param pluginEventManager    For being told about changes to the enabled plugins.
      * @param moduleDescriptorClass The type of module descriptors we are interested in.
      * @return a PluginModuleTracker useful for fast and upd to date caching of the currently enabled module descriptors.
      * @since 2.7.0
      */
     public static <M, T extends ModuleDescriptor<M>> PluginModuleTracker<M, T> create(final PluginAccessor pluginAccessor, final PluginEventManager pluginEventManager, final Class<? extends ModuleDescriptor<?>> moduleDescriptorClass) {
         @SuppressWarnings("unchecked")
         final Class<T> klass = (Class<T>) moduleDescriptorClass;
         return new DefaultPluginModuleTracker<M, T>(pluginAccessor, pluginEventManager, klass);
     }
 }