class LazyPluginCollection

abstract class LazyPluginCollection implements \IteratorAggregate, \Countable {
    
    /**
     * Stores all instantiated plugins.
     *
     * @var array
     */
    protected $pluginInstances = [];
    
    /**
     * Stores the IDs of all potential plugin instances.
     *
     * @var array
     */
    protected $instanceIds = [];
    
    /**
     * Initializes and stores a plugin.
     *
     * @param string $instance_id
     *   The ID of the plugin instance to initialize.
     */
    protected abstract function initializePlugin($instance_id);
    
    /**
     * Gets the current configuration of all plugins in this collection.
     *
     * @return array
     *   An array of up-to-date plugin configuration.
     */
    public abstract function getConfiguration();
    
    /**
     * Sets the configuration for all plugins in this collection.
     *
     * @param array $configuration
     *   An array of up-to-date plugin configuration.
     *
     * @return $this
     */
    public abstract function setConfiguration($configuration);
    
    /**
     * Clears all instantiated plugins.
     */
    public function clear() {
        $this->pluginInstances = [];
    }
    
    /**
     * Determines if a plugin instance exists.
     *
     * @param string $instance_id
     *   The ID of the plugin instance to check.
     *
     * @return bool
     *   TRUE if the plugin instance exists, FALSE otherwise.
     */
    public function has($instance_id) {
        return isset($this->pluginInstances[$instance_id]) || isset($this->instanceIds[$instance_id]);
    }
    
    /**
     * Gets a plugin instance, initializing it if necessary.
     *
     * @param string $instance_id
     *   The ID of the plugin instance being retrieved.
     */
    public function &get($instance_id) {
        if (!isset($this->pluginInstances[$instance_id])) {
            $this->initializePlugin($instance_id);
        }
        return $this->pluginInstances[$instance_id];
    }
    
    /**
     * Stores an initialized plugin.
     *
     * @param string $instance_id
     *   The ID of the plugin instance being stored.
     * @param mixed $value
     *   An instantiated plugin.
     */
    public function set($instance_id, $value) {
        $this->pluginInstances[$instance_id] = $value;
        $this->addInstanceId($instance_id);
    }
    
    /**
     * Removes an initialized plugin.
     *
     * The plugin can still be used; it will be reinitialized.
     *
     * @param string $instance_id
     *   The ID of the plugin instance to remove.
     */
    public function remove($instance_id) {
        unset($this->pluginInstances[$instance_id]);
    }
    
    /**
     * Adds an instance ID to the available instance IDs.
     *
     * @param string $id
     *   The ID of the plugin instance to add.
     * @param array|null $configuration
     *   (optional) The configuration used by this instance. Defaults to NULL.
     */
    public function addInstanceId($id, $configuration = NULL) {
        if (!isset($this->instanceIds[$id])) {
            $this->instanceIds[$id] = $id;
        }
    }
    
    /**
     * Gets all instance IDs.
     *
     * @return array
     *   An array of all available instance IDs.
     */
    public function getInstanceIds() {
        return $this->instanceIds;
    }
    
    /**
     * Removes an instance ID.
     *
     * @param string $instance_id
     *   The ID of the plugin instance to remove.
     */
    public function removeInstanceId($instance_id) {
        unset($this->instanceIds[$instance_id]);
        $this->remove($instance_id);
    }
    public function getIterator() {
        $instances = [];
        foreach ($this->getInstanceIds() as $instance_id) {
            $instances[$instance_id] = $this->get($instance_id);
        }
        return new \ArrayIterator($instances);
    }
    
    /**
     * {@inheritdoc}
     */
    public function count() {
        return count($this->instanceIds);
    }

}