[2.0.3] Source usability : better interfaces for sources relying on child plugins

I am not sure about the naming...

Proposal inspired from [ObjectWithPluginCollectionInterface](https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Plugin%21...) but not identical because we have a single collection and not many collections:

interface SourceWithCollectionInterface {

  /**
   * Get collection of items.
   *
   * @return array
   *   Array of items definitions.
   */
  public function getCollection(): array;

  /**
   * Get the minimal source configuration for an item.
   *
   * @param string $item_id
   *   An item ID.
   *
   * @return array
   *   A source configuration.
   */
  public function getSourceConfiguration(string $item_id): array;

  /**
   * Get the currently select item from the collection.
   *
   * @return string|null
   *   The item ID.
   */
  public function getSelectedItem(): ?string;

Do we also need a getItemLabel(string $item_id): string ?

getMinimalConfiguration does not need any parameter, unless it is static, which could be a nice idea after all.

i don’t like collection, because it reminds of the plugin collections in drupal which are managed differently…

also, the term configuration for the source refers to the global configuration, whereas we wanted the settings, i would keep settings to avoid confusion

getMinimalConfiguration does not need any parameter, unless it is static, which could be a nice idea after all.

Oops, a copy paste mistake, I fix it in my comment

• getChildPluginDefinitions >> getItems
• getSettingsSourceWithChildPlugin >> getMinimalSetting
• getChildPluginId >>getSelectedItem

interface SourceWithItemsInterface {

  /**
   * Get items.
   *
   * @return array
   *   Associative array of items definitions with ID as the key.
   */
  public function getItems(): array;

  /**
   * Get the minimal settings for an item.
   *
   * @param string $item_id
   *   An item ID.
   *
   * @return array
   *   A source setting.
   */
  public function getMinimalSetting(string $item_id): array;

  /**
   * Get the currently select item.
   *
   * @return string|null
   *   The item ID.
   */
  public function getSelectedItem(): ?string;

Is it better?

Also:

• Do we also need a getItemLabel(string $item_id): string ?
• it may be good to move ::getMinimalSetting() to SourceInterface and to implemtn it in SourcePluginBase as a proxy to PluginSettingsInterface::defaultSettings(). What do you think?

i am not so fan of items neither :)