共享單個訂閱（Publish RefCount）

Created: November-22, 2018

此程式碼將訂閱 emails observable 兩次：

emails.Where(email => email.From == "John").Subscribe(email => Console.WriteLine("A"));
emails.Where(email => email.From == "Mary").Subscribe(email => Console.WriteLine("B"));

要共享 emails 的單一訂閱，我們可以使用 Publish 和 RefCount 代替：

var _emails = emails.Publish().RefCount();
_emails.Where(email => email.From == "John").Subscribe(email => Console.WriteLine("A"));
_emails.Where(email => email.From == "Mary").Subscribe(email => Console.WriteLine("B"));