system.reactive 共享一个订阅（发布）
示例
给定一个IObservable<Offer>的offers从商家购买或以固定价格出售某些类型的项目，我们可以按照如下匹配对买家和卖家的：
var sellers = offers.Where(offer => offer.IsSell).Select(offer => offer.Merchant); var buyers = offers.Where(offer => offer.IsBuy).Select(offer => offer.Merchant); var trades = Observable.Zip(sellers, buyers, (seller, buyer) => new Trade(seller, buyer));
问题在于，每个订阅trades将订阅offers两次。我们可以sellers和buyers共享一个单一订阅offers使用Publish：
var trades = offers.Publish(_offers =>
{
var sellers = _offers.Where(offer => offer.IsSell).Select(offer => offer.User);
var buyers = _offers.Where(offer => offer.IsBuy).Select(offer => offer.User);
return Observable.Zip(sellers, buyers, (seller, buyer) => new Trade(seller, buyer));
});
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