关于MVVM的理解 - 前端之家

一直想聊聊这个话题，也有朋友跟我留言，让我讲讲MVVM，只可惜一直没整明白，不敢轻易下笔。针对MVVM，网上有很多不错的文章，比如MVVM介绍、被误解的 MVC 和被神化的 MVVM以及Look at MVVM from a different perspective等等

文章前我想先提几个问题

MVVM到底是什么？它和MVC有什么区别？
MVVM中VM到底是个什么角色？它和Controller或者Manager有什么区别？
ViewController在MVVM中扮演怎样角色？Api数据请求放在哪里？数据流向如何？

MVVM简介

关于MVVM，相信大家或多或少都有了解。引用MVVM介绍文中一图

受MVC或MVP架构的影响，对MVVM最初印象以为这是一个以viewmodel为核心，处理View和Model的开发架构。于是乎在原有MVC的基础上，创建了一个所谓的viewmodel对象，然后把ViewController中的代码移到viewmodel中，在viewmodel里面处理View以及Model的所有逻辑。毕竟大家都在说MVVM可以为ViewController瘦身，这ViewController就剩创建viewmodel的代码，嗯，够瘦身，这就是MVVM！

慢慢的，发现有什么地方不对，哪里不对？第一想法就是ViewController的定位，View？不是，Controller？也不是！毕竟它就创建viewmodel，好像与View、Model也没啥关系。抛开ViewController不谈，突然发现这样的viewmodel、Model以及View不就是MVC，一个以viewmodel为中心的MVC！

错在哪里

核心问题就在于对viewmodel角色的定位不清！基于MVVM设计思路，viewmodel存在目的在于抽离ViewController中展示业务逻辑，而不是替代ViewController，其它视图操作业务等还是应该放在ViewController中实现

既然不负责视图操作逻辑，viewmodel中就不应该存在任何View对象，更不应该存在Push/Present等视图跳转逻辑。因此，viewmodel中绝不应该存在任何视图操作相关的代码

 
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         @interface viewmodel : NSObject 
       
         // viewmodel中切不可存在view对象，更不该出现push或者present代码 
       
         - (instancetype)initWithTableView:(UITableView *)tableView; 
       
         @end

viewmodel做啥

很简单，处理视图展示逻辑，viewmodel负责将数据业务层提供的数据转化为界面展示所需的VO。其与View一一对应，没有View就没有viewmodel

比如，数据业务层传递一个含有性别属性sex的DO对象，0表示男， 1表示女。viewmodel的职责就是将其转化为展示层可显示的VO对象

 
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         self.personVO.sex = personDO.sex == 0 ? @"男": @"女";

viewmodel和View一起组成DDD(Model-Driven Design)领域驱动架构体系中的Presentation展示层。在iOS中，数据流向可以表示为viewmodel->ViewController->View，ViewController负责连接VO及其对应的View对象

领域驱动设计

领域驱动设计(DDD)对于安卓童鞋可能非常熟悉，有兴趣的童鞋可以参考这篇文章，本文不做过多讲解，借用其描述介绍几个名词

VO（View Object）：视图对象，用于展示层，它的作用是把某个指定页面（或组件）的所有数据封装起来
DO（Domain Object）：领域对象，就是从现实世界中抽象出来的有形或无形的业务实体
PO（Persistent Object）：持久化对象，它跟持久层（通常是关系型数据库）的数据结构形成一一对应的映射关系，如果持久层是关系型数据库，那么，数据表中的每个字段（或若干个）就对应PO的一个（或若干个）属性
Domain：领域驱动层，是用户与数据库交互的核心中转站，控制用户数据收集，控制请求转向等

MVVM架构中，viewmodel连接视图View和数据业务Model层，而Domain和Data数据持久层共同组成整个Model层。完整结构如图所示

Model并不表示Model

MVVM架构中的M，并不表示Model对象，而是表示整个数据业务层，对应DDD架构中的Domain层以及数据Data层。业务开发中，一般考虑Api或者DB对象，极少考虑Domain层设计，也不会区分DO或者PO对象。笼统定义Model ，将其传递给展示层viewmodel，久而久之，Model对象承载的信息越来越多，更有甚者，在Model中处理业务逻辑，导致项目维护成本增加，代码中出现if..else的概率也会越来越大

 
   
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         @interface PersonModel : NSObject 
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         @property (nonatomic,assign) NSInteger sex; 
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         @property (nonatomic,readonly) NSString *sexDescription; 
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         @end 
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         @implementation PersonModel 
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         // model中不应该存在业务逻辑<a href="https://www.jb51.cc/tag/daima/" target="_blank" class="keywords">代码</a> 
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         - (NSString *)sexDescription { 
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         return self.sex == 0 ? @"男": @"女"; 
       </div> 
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         } 
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         @end 
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<p style="margin-top: 0px; margin-bottom: 20px; padding-top: 0px; padding-bottom: 0px; border: 0px; font-size: 14px; vertical-align: baseline; line-height: 1.71429; color: rgb(68,sans-serif;">当然，Domain层并不是必须的，实际开发中，需要根据具体复杂度和需求来决定。比如只是纯粹的请求展示界面，设计过多的层次结构反而会<a href="https://www.jb51.cc/tag/zengjia/" target="_blank" class="keywords">增加</a>项目的维护成本。同时，Domain层不应该存在任何状态变量！</p> 
<h3 style="margin: 24px 0px; padding: 0px; border: 0px; font-size: 16px; vertical-align: baseline; clear: both; line-height: 1.84615; color: rgb(68,sans-serif;">Data数据层</h3> 
<p style="margin-top: 0px; margin-bottom: 20px; padding-top: 0px; padding-bottom: 0px; border: 0px; font-size: 14px; vertical-align: baseline; line-height: 1.71429; color: rgb(68,sans-serif;"><a href="https://www.jb51.cc/tag/viewmode/" target="_blank" class="keywords">viewmode</a>l负责展示层逻辑，而Data层则对应数据层逻辑，一般以Manager或者Service身份存在，数据来源主要<a href="https://www.jb51.cc/tag/baokuo/" target="_blank" class="keywords">包括</a>Api、DB或者Cache等。Data数据层操作对象主要为PO持久化对象，对象一旦创建，原则上不可<a href="https://www.jb51.cc/tag/xiugai/" target="_blank" class="keywords">修改</a></p> 
<p style="margin-top: 0px; margin-bottom: 20px; padding-top: 0px; padding-bottom: 0px; border: 0px; font-size: 14px; vertical-align: baseline; line-height: 1.71429; color: rgb(68,sans-serif;">Data数据层具有独立可测试性，其不依赖视图层而存在。<span style="margin: 0px; padding: 0px; border: 0px; vertical-align: baseline; text-decoration: underline;">切记不可在Data层操作任何视图对象！</span></p> 
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         @implementation PersonDBAccess 
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         // Data层不应该存在任何视图相关<a href="https://www.jb51.cc/tag/daima/" target="_blank" class="keywords">代码</a> 
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         - (NSArray *)fetchPersonModels { 
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         [SVProgressHUD showWithStatus:@"加载中。。。"]; 
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         } 
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         @end 
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<p style="margin-top:0px; margin-bottom:20px; padding-top:0px; padding-bottom:0px; border:0px; font-size:14px; vertical-align:baseline; line-height:1.71429; color:rgb(68,sans-serif"> MVVM奇葩说</h3> 
<p style="margin-top:0px; margin-bottom:20px; padding-top:0px; padding-bottom:0px; border:0px; font-size:14px; vertical-align:baseline; line-height:1.71429; color:rgb(68,sans-serif"> <a href="https://www.jb51.cc/tag/wenzhang/" target="_blank" class="keywords">文章</a>到此，想必各位对MVVM架构已经有了大致了解。对比安卓童鞋对MVP架构的钟爱，iOS童鞋也许更加青睐MVVM，拌上<a target="_blank" href="javascript:void()" class="outlink" data="/link?url=https://github.com/ReactiveCocoa/ReactiveCocoa" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; outline:none; color:rgb(11,234); text-decoration:none" rel="nofollow">ReactiveCocoa</a>或者<a target="_blank" href="javascript:void()" class="outlink" data="/link?url=https://github.com/ReactiveX/RxSwift" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; outline:none; color:rgb(11,234); text-decoration:none" rel="nofollow">RxSwift</a>，这道菜可以做的更加绚烂多彩！当然，正如唐巧在<a target="_blank" href="javascript:void()" class="outlink" data="/link?url=http://blog.devtang.com/2015/11/02/mvc-and-mvvm/" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; outline:none; color:rgb(11,234); text-decoration:none" rel="nofollow">文中</a>所言：ReactiveCocoa 和 MVVM 不应该被神化，我们需要保持的是一个拥抱变化的心，以及理性分析的态度。在新技术的面前，不盲从，也不守旧，一切的决策都应该建立在认真分析的基础上，这样才能应对技术的变化！</p> 
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<p style="margin-top:0px; margin-bottom:20px; padding-top:0px; padding-bottom:0px; border:0px; font-size:14px; vertical-align:baseline; line-height:1.71429; color:rgb(68,sans-serif"> <span style="margin:0px; padding:0px; border:0px; vertical-align:baseline">写在文后：</span></p> 
<p style="margin-top:0px; margin-bottom:20px; padding-top:0px; padding-bottom:0px; border:0px; font-size:14px; vertical-align:baseline; line-height:1.71429; color:rgb(68,sans-serif"> <span style="margin:0px; padding:0px; border:0px; vertical-align:baseline">为期两天的<a target="_blank" href="javascript:void()" class="outlink" data="/link?url=http://www.swiftconchina.com/" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; outline:none; color:rgb(11,234); text-decoration:none" rel="nofollow">SwiftCon</a>已经落幕，虽然离我只有两站地铁，只可惜依然没能前往聆听各位大师的技术<a href="https://www.jb51.cc/tag/fenxiang/" target="_blank" class="keywords">分享</a>，特别还有我同事兼朋友<a target="_blank" href="javascript:void()" class="outlink" data="/link?url=http://guanshanliu.typed.com/" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; outline:none; color:rgb(11,234); text-decoration:none" rel="nofollow">刘冠杉</a>的个人首秀。虽然，现在网上也出现了各种不好的声音，但是我仍然相信，大多数<a href="https://www.jb51.cc/tag/fenxiang/" target="_blank" class="keywords">分享</a>者还是为此付出了不少心血，值得我们为之鼓掌！</span></p> 
<p style="margin-top:0px; margin-bottom:20px; padding-top:0px; padding-bottom:0px; border:0px; font-size:14px; vertical-align:baseline; line-height:1.71429; color:rgb(68,sans-serif"> <span style="margin:0px; padding:0px; border:0px; vertical-align:baseline">个人技术的成长不在一朝一夕，而是长年累月的付出积累沉淀的结果！Swift3.0不久即将发布，而我个人也会逐渐转移☞对Swift语言的学习。与此同时，随着公司<a target="_blank" href="javascript:void()" class="outlink" data="/link?url=http://reactnative.cn/" style="margin:0px; padding:0px; border:0px; vertical-align:baseline; outline:none; color:rgb(11,234); text-decoration:none" rel="nofollow">ReactNative</a>项目的上马，我也会加强对RN技术的专研和学习中</span></p><i class="glyphicon glyphicon-link"></i> 原文链接：https://www.f2er.com/swift/322565.html</div>
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