之前对 "非正式协议"一词不甚理解，查阅一番后就自己的理解简单梳理一下，
下面是苹果官方文档Cocoa Core Competencies一文中对非正式协议的介绍：
An informal protocol is a category on NSObject, which implicitly makes almost all objects adopters of the protocol. (A category is a language feature that enables you to add methods to a class without subclassing it.) Implementation of the methods in an informal protocol is optional. Before invoking a method, the calling object checks to see whether the target object implements it. Until optional protocol methods were introduced in Objective-C 2.0, informal protocols were essential to the way Foundation and AppKit classes implemented delegation.
可见非正式协议就是我们常用的类别(Category),而正式协议就是一个以@protocol方式命名的方法列表，与非正式协议相比不同的是，它要求显示的遵守协议。
原文：http://www.cnblogs.com/moyazi/p/5099920.html