共享单个订阅（Publish RefCount）

Created: November-22, 2018

此代码将订阅 emails observable 两次：

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 的单一订阅，我们可以使用 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"));