Returns the cached plugin definitions of the decorated discovery class.
mixed On success this will return an array of plugin definitions. On failure this should return NULL, indicating to other methods that this has not yet been defined. Success with no values should return as an empty array and would actually be returned by the getDefinitions() method.
protected function getCachedDefinitions() {
if (!isset($this->definitions) && isset($this->cacheKey) && ($cache = cache($this->cacheBin)
->get($this->cacheKey))) {
$this->definitions = $cache->data;
}
return $this->definitions;
}