public static function isAppRegistered(?string $app = null): bool
    {
        self::initialize();
        if ($app === null) {
            $app = '*';
        }
        return isset(self::$repositories[$app]);
    }
public static function isAppRegistered(?string $app = null): bool
    {
        self::initialize();
        if ($app === null) {
            $app = '*';
        }
        return isset(self::$repositories[$app]);
    }
public static function isAppRegistered ( ? string $ app = null ) : bool { self :: initialize ( ) ; if ( $ app === null ) { $ app = '*' ; } return isset ( self :: $ repositories [ $ app ] ) ; }
Is app registered.

@param string $app Default null which means default app '*'.
@return bool
Is app registered .