以微信公众平台接口网站为例探讨在ruby中去掉case..when..判断的一些方法
很多条件判断，给人的第一感觉是，嗯，这里要用case..when..了，我个人是很不喜欢用case..when..的，它往往伴随着长串的不易管理的代码，在判断条件增加时会显得杂乱无章，而且很难看。下面的例子来自我用rails搭建的微信公共账号的公众平台接口网站，主要从如何根据微信的内容返回相关的信息来讲述避免使用case..when..判断的一些方法，不涉及如何搭建微信公众帐号接口网站，如果你需要这方面的信息，请参看万神劫写的用Rails搭建微信公众平台 API。
1、用判断的条件动态创建相应的类来处理。
首先，接收到信息的controller，假设为WechatController，那么它的create方法就是处理接受信息并给出返回的啦，刚开始时你可能会这样写
def create
case params[:xml][:MsgType]
when "text"
@return_content = WechatTextHandler.new(params[:xml]).return
render "text", :formats => :xml
when "event"
@return_content = WechatEventHandler.new(params[:xml]).return
render "event", :formats => :xml
when "xxx"
.
.
.
end
end
这里WechatTextHandler和WechatEventHandler分别是你用来处理text信息和event信息的类，ok,这里已经不少when了，而且随着MsgType的增加，你还要不断的增加when判断代码。用ruby，你可以不这么写。我们注意到这些用来处理的类名和when的判断条件有所关联，譬如处理"text"的就是WechatTextHandler,而render的模板名称也叫"text"(事实上这些类名及模板是我故意命名成如此的)，那么我们可以根据params[:xml][:MsgType]的值来构建相应的类并处理就可以了。
def create
@return_content = Object.const_get("Wechat#{params[:xml][:MsgType].capitalize}Handler").new(params[:xml]).return
render params[:xml][:MsgType], :formats => :xml
end
Object.const_get("Xxx")会返回一个Xxx类(Object的子类，如果父类是其它如FatherClass，则用FathClass.const_get("Xxx"))，所以在这里我们直接用params[:xml][:MsgType]构建了对应的处理类，volia！case..when..不见了，而且以后有新的MsgType时，譬如"click", 你不需要再去动controller里的代码，只需要写好处理click信息的类WechatClickHandler就可以了，它会自动构建WechatClickHandler类然后用params[:xml]参数实例化并调用return方法返回处理后的返回信息。
这个方法需要你规划好自己的类名。
2、使用对应的条件->方法hash来调用方法。
我们以WechatTextHandler为例讲如何不用case..when..判断来响应用户发来的命令信息。
譬如说我们的微信公共账号有三个功能，分别是：
查联系人电话号码（格式为"电话 关键字"或"d 关键字"）,调用方法 query_phone
查花名 （格式为"花 关键字"或"h 关键字"），调用方法 query_flower
随机返回一个笑话 (格式为"笑话"或"j"), 调用方法 tell_a_joke
那么你的return_content方法很可能看起来是这样的：
# 假设你已经在initialize里把命令和关键字拆开并将命令赋值给了command变量
def return_content
if valid_command? # 假设你有个valid_command?方法来检查命令的合法性
case command
when "电话"
query_phone
when "d"
query_phone
when "花"
query_flower
when "h"
query_flower
when "笑话"
tell_a_joke
when "j"
tell_a_joke
end
end
end
天啊，才三个命令就已经写了这么多行了，而且以后要添加功能的话还得不停的加when判断，这...一点都不好看也不利于扩展对吧?
这种时候我们可以用一个hash来维护命令和要调用的方法之间的关系，并替换掉这些case..when..判断。
Commands = { "电话" => { :method => :query_phone},
"q" => { :method => :query_phone},
"花" => { :method => :query_flower},
"h" => { :method => :query_flower},
"笑话" => { :method => :tell_a_joke},
"j" => { :method => :tell_a_joke}
}
def return_content
if valid_command?
send Commands[command][:method]
end
end
volia!只剩下一条语句，case..when..没有了！send("xmethod")将会调用xmethod这个方法，在这里我们就是在hash里找到命令对应的方法并调用，就是这么简单，而且你的valid_command?方法只需要
def valid_command?
Comamnds.has_key? command
end
就可以实现了。以后添加功能也不需要你再去修改return_content方法的代码，只需要写好要调用的方法，并把命令和调用方法的关系加入hash中就可以了，逻辑对应关系也是一目了然，这不是很好吗？ 甚至这个hash你也可以修改一下结构放到其他地方（譬如数据库中）。
以上的两种方法只是提供了一些思路，我相信还会有更多更好的方法，希望大家一起探讨。
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