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atom.xml
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<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<title><![CDATA[NO END FOR LEARNING]]></title>
<link href="http://benweizhu.github.io/atom.xml" rel="self"/>
<link href="http://benweizhu.github.io/"/>
<updated>2018-05-09T00:21:36+08:00</updated>
<id>http://benweizhu.github.io/</id>
<author>
<name><![CDATA[ZHU Benwei]]></name>
</author>
<generator uri="http://octopress.org/">Octopress</generator>
<entry>
<title type="html"><![CDATA[React的思考（十二）- 组织state和reducer（4）]]></title>
<link href="http://benweizhu.github.io/blog/2018/05/08/deep-thinking-in-react-12/"/>
<updated>2018-05-08T23:45:26+08:00</updated>
<id>http://benweizhu.github.io/blog/2018/05/08/deep-thinking-in-react-12</id>
<content type="html"><![CDATA[<h2>不同的页面也可能有相同领域，不同状态的数据，也算是一个Domain吧？</h2>
<h2>没有BFF的情况下</h2>
<p>基于Redux的应用程序中最常见的state结构是一个简单的JavaScript对象，它最外层的每个key中拥有特定域的数据。</p>
<p>然而，BFF可以是来自不同Domain的数据库合并后返回到前端的。</p>
<h2>暴露的服务和接口</h2>
<p>思考后端的操作逻辑，微服务的场景，按照业务的Restful API，Redux的这种设计模式导致这种CRUD和Service的冲突</p>
<h2>Normalize State</h2>
<h2>BFF层的数据结构</h2>
<h2>一个action要操作多个reducer中的state</h2>
<h2>redux的state和页面的关系</h2>
<h2>开发过程中state的演进过程</h2>
]]></content>
</entry>
<entry>
<title type="html"><![CDATA[React的思考（十一）- 组织state和reducer（3）]]></title>
<link href="http://benweizhu.github.io/blog/2018/05/04/deep-thinking-in-react-11/"/>
<updated>2018-05-04T23:09:38+08:00</updated>
<id>http://benweizhu.github.io/blog/2018/05/04/deep-thinking-in-react-11</id>
<content type="html"><![CDATA[<p>以React为首的“数据驱动”的开发方式，让我们从过去的DOM操作，慢慢转化成对应用状态和数据的管理，这些状态和数据在应用的生命周期中被持久化，被应用管理和使用，怎么样有效的在SPA中管理这些数据变的特别重要。</p>
<h2>前后端不同的数据模型</h2>
<p>在后端开发中，我们设计数据库或者对象模型，通常会根据领域模型来建立不同的数据表和对象，以反映我们对客观现实的抽象，这种抽象在MVC的世界里通常由Model表示。</p>
<p>而在前端开发中，我们一般会从UI的角度出发，去设计前端展示所需要的视图对象模型，我们也称之为View Model。</p>
<p>而往往在大多数情况下，后端的数据模型Model和前端的数据模型View Model是不对等的。</p>
<h2>来自后方的数据</h2>
<p>当一个应用的生命周期启动，应用会调用后台的API获取后端数据，然后以某种方式转换成前端所需数据模型，最后展示在应用的页面上。</p>
<p>作为一个后端的开发人员，通常以什么样的方式暴露后端的资源给别的应用使用呢？</p>
<p>如果需要API的通用度比较高，一般最简单直接的方法是开放领域资源的CRUD操作（你可以是RESTful API也可以是别的方式）。</p>
<figure class='code'><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
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<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
</pre></td><td class='code'><pre><code class=''><span class='line'>https://api.example.com/v1/zoos
</span><span class='line'>https://api.example.com/v1/animals
</span><span class='line'>https://api.example.com/v1/employees
</span><span class='line'>
</span><span class='line'>GET /zoos：列出所有动物园
</span><span class='line'>POST /zoos：新建一个动物园
</span><span class='line'>GET /zoos/ID：获取某个指定动物园的信息
</span><span class='line'>PUT /zoos/ID：更新某个指定动物园的信息（提供该动物园的全部信息）</span></code></pre></td></tr></table></div></figure>
<p>我们目前就暂且以这种方式为例，目测这种方式暴露的API比较常见，后面我们再讨论其他的。</p>
<h2>combineReducer下的拆分</h2>
<p>让我们再回到Redux中，基于Redux的应用程序中，比较常见的state结构是一个简单的JavaScript对象，它最外层的每个key中拥有特定域的数据，这其实是Redux官方文档上的一句话，我在其他的一些博客上也看到了采用基于业务领域的方式组织state结构。</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
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</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="p">{</span>
</span><span class='line'> <span class="s2">"zoos"</span><span class="o">:</span> <span class="p">{</span>
</span><span class='line'> <span class="p">...</span>
</span><span class='line'> <span class="p">},</span>
</span><span class='line'> <span class="s2">"animals"</span><span class="o">:</span> <span class="p">{</span>
</span><span class='line'> <span class="p">...</span>
</span><span class='line'> <span class="p">},</span>
</span><span class='line'> <span class="s2">"employees"</span><span class="o">:</span> <span class="p">{</span>
</span><span class='line'> <span class="p">...</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p>在combineReducer的帮助下，构建上面这个结构，其实就是拆分成多个reducer，拆分之后的reducer独立负责管理该特定切片state的更新。好处是，它提供了一种非常直接的代码逻辑拆分管理的方式，职责独立，物理位置独立（文件独立）。</p>
<h2>响应同一个操作</h2>
<p>有了这样一层结构，我们该如何操作它呢？</p>
<p>现在我们假设有这样一个操作，它既需要更新zoos下面的数据，又需要更新animals下面的数据。</p>
<p>如果放在后端代码中，你会怎么做？</p>
<p>我的思路会是写一个函数A，在这个函数里面调用zoos和animals的service或者repository的方法完成更新操作，那么，真正使用的时候，只需要调用这个函数A即可，思路很明朗直接。</p>
<p>在combineReducer和redux结合情况下，我们就需要转换一下思路了，不同业务领域下的数据被放置到了不同的reducer，而你能做的只是发送action。</p>
<p>combineReducer神奇的地方就是，被发送的action会被所有的reducer接收到。（有点像发布订阅模式）</p>
<p>这样一个过去直觉上同步有序的操作过程，在redux中，被分发到多个拆分之后的reducer中，每个reducer都去响应这个action，在需要的情况下独立的更新他们自己的切片state，最后组合成新的state。</p>
<h2>计算衍生（derived）数据</h2>
<p>后端的业务数据被存储在了redux的store中，然而它是以后端model的形式保存在那。我们的前端页面需要的数据模型，一般和后端model不完全一样，也许是多个后端model组合在一起才得到可以使用的view model，这种情况很常见。</p>
<p>这个时候，我们就需要从后端数据模型中计算衍生数据，得到我们最终需要的View Model。</p>
<p>一般的做法是在mapStateToProps中进行，mapStateToProps中的state是根节点上的state，所以可以拿到所有的领域数据，此时我们就能根据它衍生出我们需要的视图模型。</p>
<p>而在这里一般都会推荐使用reselect库来做，好处是：</p>
<p>第一，能够借机将这个计算衍生数据的逻辑拆分到另一个模块中 <br/>
第二，它能帮你记住之前的计算过的数据，避免二次计算，同时避免无意义的重新渲染（关于什么时候重新渲染，在前面已经介绍过了）。</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
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</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kr">import</span> <span class="p">{</span> <span class="nx">createSelector</span> <span class="p">}</span> <span class="nx">from</span> <span class="s1">'reselect'</span>
</span><span class='line'>
</span><span class='line'><span class="kr">const</span> <span class="nx">getVisibilityFilter</span> <span class="o">=</span> <span class="p">(</span><span class="nx">state</span><span class="p">)</span> <span class="o">=></span> <span class="nx">state</span><span class="p">.</span><span class="nx">visibilityFilter</span>
</span><span class='line'><span class="kr">const</span> <span class="nx">getTodos</span> <span class="o">=</span> <span class="p">(</span><span class="nx">state</span><span class="p">)</span> <span class="o">=></span> <span class="nx">state</span><span class="p">.</span><span class="nx">todos</span>
</span><span class='line'>
</span><span class='line'><span class="kr">export</span> <span class="kr">const</span> <span class="nx">getVisibleTodos</span> <span class="o">=</span> <span class="nx">createSelector</span><span class="p">(</span>
</span><span class='line'> <span class="p">[</span> <span class="nx">getVisibilityFilter</span><span class="p">,</span> <span class="nx">getTodos</span> <span class="p">],</span>
</span><span class='line'> <span class="p">(</span><span class="nx">visibilityFilter</span><span class="p">,</span> <span class="nx">todos</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="k">switch</span> <span class="p">(</span><span class="nx">visibilityFilter</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="k">case</span> <span class="s1">'SHOW_ALL'</span><span class="o">:</span>
</span><span class='line'> <span class="k">return</span> <span class="nx">todos</span>
</span><span class='line'> <span class="k">case</span> <span class="s1">'SHOW_COMPLETED'</span><span class="o">:</span>
</span><span class='line'> <span class="k">return</span> <span class="nx">todos</span><span class="p">.</span><span class="nx">filter</span><span class="p">(</span><span class="nx">t</span> <span class="o">=></span> <span class="nx">t</span><span class="p">.</span><span class="nx">completed</span><span class="p">)</span>
</span><span class='line'> <span class="k">case</span> <span class="s1">'SHOW_ACTIVE'</span><span class="o">:</span>
</span><span class='line'> <span class="k">return</span> <span class="nx">todos</span><span class="p">.</span><span class="nx">filter</span><span class="p">(</span><span class="nx">t</span> <span class="o">=></span> <span class="o">!</span><span class="nx">t</span><span class="p">.</span><span class="nx">completed</span><span class="p">)</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">)</span>
</span></code></pre></td></tr></table></div></figure>
<h2>总结</h2>
<p>这么分析下来，每个部分的职责都很清晰，开发的模式也比较明确，然而理想和设计在业务不复杂的时候都很美好，现实往往比它们骨感很多，这一篇文章只是给了一个简单的例子，没有分析复杂的场景。</p>
<p>在下一篇，我们可以在此基础上分析开发中的复杂场景，一起思考下怎么样管理是合适的。同时，我们也来思考另一种新的前端架构BFF下的Redux应该怎么管理，当后端API不再是资源的CRUD，而是面向业务的API操作时，Redux又应该怎么管理state。</p>
]]></content>
</entry>
<entry>
<title type="html"><![CDATA[React的思考（十）- 组织state和reducer（2）]]></title>
<link href="http://benweizhu.github.io/blog/2018/05/03/deep-thinking-in-react-10/"/>
<updated>2018-05-03T23:05:04+08:00</updated>
<id>http://benweizhu.github.io/blog/2018/05/03/deep-thinking-in-react-10</id>
<content type="html"><![CDATA[<h2>共享的数据</h2>
<p>前面提到应用的数据类型有三种：领域数据，应用状态，UI状态。</p>
<p>其中，UI状态看上去更多是存放在React组件里面。</p>
<p>那是不是领域数据和应用状态都应该归Redux管理呢？不一定，为什么呢？我是从这角度思考的：</p>
<p>Redux本身存储的是一个全局数据，即被共享的数据。那么，我们就会问，不会被共享的数据应该存在哪呢？</p>
<h4>存储在Redux里面</h4>
<p>首先，你当然仍然可以存储在Redux里面，那么当前情况会是，数据被存放在Redux state树形结构的某一片区域中，被某一个路由下的某一个组件所使用。你一定会问，这样做有什么好处？</p>
<p>1.单一的数据源，你很放心的将数据存在那里，打开Redux Debug Tool就能看到（心里长叹一口气，数据还在还在）</p>
<p>2.将同类型（比如：领域数据）的数据统一类聚在一起，方便你统一管理，让你拥有绝对的上帝视角</p>
<p>3.某一天，设计师说，在一个离原组件很遥远的位置加一个按钮，当点击时，要改变这个数据，此时你可以很轻松的发送一个action并写一个reducer的case操作它，就完成了（数据操作和组件的关系是低耦合的）</p>
<p>等等其他我没有想到的好处</p>
<h4>存储在Component里面</h4>
<p>如果我不存放在Redux中，应该放哪里？可以是组件里面。那么同样的问题，放在组件里面有什么好处？</p>
<p>1.Redux中存放的都是被共享的数据，相比存放所有的数据，redux的state结构会小一些</p>
<p>2.组件的数据和状态是自管理的，无论你把我放在哪，我都能坚强的活着</p>
<p>3.某一天，我被页面的多个位置用到，你不需要在Redux里面配多个数据空间和action以供我的分身使用，我自带装备，我为自己“带盐”</p>
<p>4.我还能配合高阶组件（用高阶函数做数据加载部分），让你的代码看上更加装逼</p>
<p>等等其他我没有想到的好处</p>
<p>如果博客里面可以发表情，此时我特别想发一个拍脑袋的表情，请问，看完这一段，我胡乱分析的结果，是不是感觉有些不知所措？我到底应该放在哪呢？</p>
<h2>官文FAQ的经验法则</h2>
<p>正如官方文档里面FAQ的结果</p>
<blockquote><p>问：必须将所有state都维护在Redux中吗？ 可以用React的setState()方法吗？</p>
<p>答：没有 “标准”。作为一名开发者，应该决定使用何种 state 来组装你的应用，每个 state 的生存范围是什么。在两者之间做好平衡，然后就去做吧。</p></blockquote>
<p>所以说，看情况（It depends）永远是正确的答案。</p>
<p>当然官方文档也给了一些将怎样的数据放入Redux的经验法则：</p>
<p>1.应用的其他部分是否关心这个数据？ <br/>
2.是否需要根据需要在原始数据的基础上创建衍生数据？ <br/>
3.相同的数据是否被用作驱动多个组件？ <br/>
4.能否将状态恢复到特定时间点（在时光旅行调试的时候）？ <br/>
5.是否要缓存数据（比如：数据存在的情况下直接去使用它而不是重复去请求他）？</p>
<p>我个人感觉这些经验法则是有些道理的，和我们前面讲解和分析的套路差不多，可以作为参考。</p>
<h2>总结</h2>
<p>首先，我个人认为一定不是将所有的数据都放在Redux里面，但是至于什么样的数据该放在哪里，那就需要看是什么样的使用场景？你需要询问自己几个问题（如上面说问），分析它的好处和坏处，是否满足你的需求，然后做出判断。</p>
<p>我们下一节，来看redux state的组织结构（shape），以及action，reducer如何配合state的更新。</p>
]]></content>
</entry>
<entry>
<title type="html"><![CDATA[React的思考（九）- 组织state和reducer（1）]]></title>
<link href="http://benweizhu.github.io/blog/2018/05/01/deep-thinking-in-react-9/"/>
<updated>2018-05-01T22:47:38+08:00</updated>
<id>http://benweizhu.github.io/blog/2018/05/01/deep-thinking-in-react-9</id>
<content type="html"><![CDATA[<h2>前提条件</h2>
<p>在进行深入探讨之前，我先确保大家的理解是一致的，因为这部分是客观的，而之后的内容是相对主观和有争议的。</p>
<p>1.当组件的某个操作dispatch了一个action，所有的reducer都会接收到：<a href="https://stackoverflow.com/questions/33590579/all-reducers-will-be-invoked-when-an-action-is-dispatched" title="All reducers will be invoked when an action is dispatched?">All reducers will be invoked when an action is dispatched?</a>，<a href="https://cn.redux.js.org/docs/recipes/reducers/UsingCombineReducers.html" title="combineReducers用法中也有讲到">combineReducers用法中也有讲到</a></p>
<p>2.当combineReducers发现有任意一个reducer返回了新的state，会通知所有和redux关联（connect）的组件准备更新，请检查自己是否要更新</p>
<p>3.每一个通过connect构建的组件，其mapStateToProps中的state，是combineReducers合并的state，也就是每个组件都能拿到所有reducer中的state（曾经有遇到过有人误以为是跟它action相关的reducer的state）</p>
<h2>Redux的上帝视角</h2>
<p>Redux的三大设计原则之一，单一数据源，定义了整个应用的state被储存在一棵object tree中，并且这个object tree只存在于唯一一个store中。这样一个顶层的状态树，会拥有一个全局的视角，掌握着整个应用的状态。</p>
<p>单一数据源的设计原则在许多程序设计的领域都是准确的，但仅仅用一个JavaScript对象来存储整个应用的状态，总会让人感觉某一天这个对象一定会特别臃肿，上帝实在太忙，要关注的东西太多。</p>
<p>很自然的，我们就会去思考，物尽其用，到底什么样的数据应该放在Redux中，什么样的数据应该放在别处来管理。</p>
<h2>应用拥有的数据和状态</h2>
<p>大多数应用会处理多种类型的数据和状态，通常可以分为以下三类：领域数据（Domain data），应用状态（App state），UI状态（UI state）</p>
<p>领域数据也就是业务相关数据，一般和你的后台业务系统数据相关联，是领域数据的数据来源，但他们不一定直接对等。</p>
<p>应用状态和UI状态有时候不容易分清，应用状态是描述应用无限循环的生命周期中的某一种存在（中间）状态，而这样一个应用状态可能会导致一个或者多个UI的状态变化。比如：用户登录，是一个应用状态，它可能导致导航栏的UI状态改变。</p>
<p>UI的状态，自然是描述UI的改变，但不一定是由应用状态的变化导致。比如：页面上tab的切换。</p>
<h2>应用数据和状态的存储位置和影响范围</h2>
<p>根据上面的应用数据和状态的分类，好像让我们对这样一个问题有些头绪。不过在你下任何判断之前，我们在从另外一个维度继续思考一下。</p>
<p>从前面我们就了解到，Redux存储的数据，在任何一个与之关联的组件中都能拿到，也就是说，Redux存储的是一个全局的数据。</p>
<p>反之，React本身也有一个state，而它所关注的只是组件本身以及它的子组件，兄弟和父组件它都不关心。</p>
<p>除了Redux和React，就没有别的位置保存数据了？当然不是，比如：cookie，local storage。这些也是保存数据的关键位置，毕竟当页面刷新后，Redux和React中的state就丢失了，还需要从后台重新加载。</p>
<h2>React组件的state存什么？</h2>
<p>现在还不是时候讨论Redux里面存什么，我们反向推理，用排除法，先看看React里面应该存在什么。</p>
<p>React中的state存在于组件当中，那我们就需要思考组件的特性，它能独立，也许还能自治，一般都高可重用，这是它的部分典型特性。基于它这些特性，也就决定了它的state也必须满足这些要求，组件的state是服务于组件本身的，这些state能够在不收外部干预下就自我管理，这些state当组件被用在任何位置时，都能适应。一个典型的例子：UI状态。</p>
<p>当然，上面是我对React组件state理解的一个抽象描述，能够一定程度下知道我的思考。还有什么原则，能够帮助决策什么样的state可以放在组件中，Dan Abramov在他的Twitter发了这样一张图片也具有不错的指导意义：</p>
<p><img src="https://camo.githubusercontent.com/5e85994aa142e7699548e2f5a1e74583229ebd10/68747470733a2f2f7062732e7477696d672e636f6d2f6d656469612f436d654273477a57384151705f61762e6a7067" width="500" alt="" /></p>
<p>1.如果这个数据可以从props中计算得到，那么就不应该放在state中 <br/>
2.如果这个数据在render方法中不被使用，那么就不应该放在state中</p>
<h2>总结</h2>
<p>今天先写到这里，后续还会继续讨论redux的state和reducer的设计思考。</p>
]]></content>
</entry>
<entry>
<title type="html"><![CDATA[React的思考（八）- Redux的Middleware（下）异步的世界]]></title>
<link href="http://benweizhu.github.io/blog/2018/04/27/deep-thinking-in-react-8/"/>
<updated>2018-04-27T13:38:01+08:00</updated>
<id>http://benweizhu.github.io/blog/2018/04/27/deep-thinking-in-react-8</id>
<content type="html"><![CDATA[<h2>有序而独立的同步世界</h2>
<p>没有异步的情况下，Redux配合React很容易理解的（Action->Reducer->CombineReducers->React-Redux->Component），简单回顾下：</p>
<p>1.在组件里面dispatch（发出）一个action对象（带上类型和数据） <br/>
2.action对象被传递到reducer的入口，reducer根据类型给到不同的switch分支，然后根据带入的数据操作state，返回新的state <br/>
3.redux发现有新的state，配合React-Redux，通知所有component，告诉组件请注意你自己要不要更新，然后各自判断各自更新</p>
<p>一个被传递到组件里的disptach</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
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</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kr">const</span> <span class="nx">mapDispatchToProps</span> <span class="o">=</span> <span class="nx">dispatch</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="k">return</span> <span class="p">{</span>
</span><span class='line'> <span class="nx">onAddTodo</span><span class="o">:</span> <span class="nx">text</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="nx">dispatch</span><span class="p">(</span><span class="nx">addTodo</span><span class="p">(</span><span class="nx">text</span><span class="p">))</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span><span class='line'><span class="kr">const</span> <span class="nx">addTodo</span> <span class="o">=</span> <span class="p">(</span><span class="nx">text</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="nx">type</span><span class="o">:</span> <span class="s1">'ADD_TODO'</span><span class="p">,</span>
</span><span class='line'> <span class="nx">text</span>
</span><span class='line'><span class="p">}</span>
</span><span class='line'>
</span><span class='line'><span class="c1">//在组件里面点击时触发</span>
</span><span class='line'><span class="nx">handleClickAdd</span><span class="p">()</span> <span class="p">{</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">onAddTodo</span><span class="p">(</span><span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">todoText</span><span class="p">)</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p>action被传递到reducer的入口</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
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</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kd">function</span> <span class="nx">todoApp</span><span class="p">(</span><span class="nx">state</span> <span class="o">=</span> <span class="nx">initialState</span><span class="p">,</span> <span class="nx">action</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="k">switch</span> <span class="p">(</span><span class="nx">action</span><span class="p">.</span><span class="nx">type</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="k">case</span> <span class="nx">ADD_TODO</span><span class="o">:</span><span class="c1">//命中这个switch case</span>
</span><span class='line'> <span class="k">return</span> <span class="nb">Object</span><span class="p">.</span><span class="nx">assign</span><span class="p">({},</span> <span class="nx">state</span><span class="p">,</span> <span class="p">{</span><span class="c1">//返回新的state</span>
</span><span class='line'> <span class="nx">todos</span><span class="o">:</span> <span class="p">[</span>
</span><span class='line'> <span class="p">...</span><span class="nx">state</span><span class="p">.</span><span class="nx">todos</span><span class="p">,</span>
</span><span class='line'> <span class="p">{</span>
</span><span class='line'> <span class="nx">text</span><span class="o">:</span> <span class="nx">action</span><span class="p">.</span><span class="nx">text</span><span class="p">,</span>
</span><span class='line'> <span class="nx">completed</span><span class="o">:</span> <span class="kc">false</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'> <span class="p">]</span>
</span><span class='line'> <span class="p">})</span>
</span><span class='line'> <span class="k">default</span><span class="o">:</span>
</span><span class='line'> <span class="k">return</span> <span class="nx">state</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p>组件被通知请查看你是否需要更新，connect发现todos变了，所以要更新这个connect嵌入的组件</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
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</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kr">const</span> <span class="nx">mapStateToProps</span> <span class="o">=</span> <span class="nx">state</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="k">return</span> <span class="p">{</span>
</span><span class='line'> <span class="nx">todos</span><span class="o">:</span> <span class="nx">getVisibleTodos</span><span class="p">(</span><span class="nx">state</span><span class="p">.</span><span class="nx">todos</span><span class="p">,</span> <span class="nx">state</span><span class="p">.</span><span class="nx">visibilityFilter</span><span class="p">)</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<h2>没有Redux的异步世界</h2>
<p>异步世界其实没什么可怕的（又不是异世界），看下面一个React里面用fetch实现的数据异步加载：</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
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</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kr">class</span> <span class="nx">ExampleComponent</span> <span class="kr">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
</span><span class='line'> <span class="nx">constructor</span><span class="p">(</span><span class="nx">props</span><span class="p">){</span>
</span><span class='line'> <span class="kr">super</span><span class="p">(</span><span class="nx">props</span><span class="p">);</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">state</span> <span class="o">=</span> <span class="p">{</span> <span class="nx">name</span><span class="o">:</span> <span class="s1">''</span><span class="p">,</span> <span class="nx">loading</span><span class="o">:</span> <span class="kc">false</span><span class="p">,</span> <span class="nx">errorMessage</span><span class="o">:</span> <span class="s1">''</span> <span class="p">};</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">_getRandomName</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">getRandomName</span><span class="p">.</span><span class="nx">bind</span><span class="p">(</span><span class="k">this</span><span class="p">);</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'>
</span><span class='line'> <span class="nx">render</span><span class="p">()</span> <span class="p">{</span>
</span><span class='line'> <span class="kr">const</span> <span class="p">{</span> <span class="nx">name</span><span class="p">,</span> <span class="nx">errorMessage</span> <span class="p">}</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">;</span>
</span><span class='line'> <span class="k">return</span><span class="p">(</span>
</span><span class='line'> <span class="o"><</span><span class="nx">div</span><span class="o">></span>
</span><span class='line'> <span class="o"><</span><span class="nx">h1</span><span class="o">></span><span class="p">{</span><span class="nx">name</span><span class="p">}</span><span class="o"><</span><span class="err">/h1></span>
</span><span class='line'> <span class="o"><</span><span class="nx">button</span> <span class="nx">onClick</span><span class="o">=</span><span class="p">{</span><span class="k">this</span><span class="p">.</span><span class="nx">_getRandomName</span><span class="p">}</span><span class="o">></span><span class="nx">PRESS</span> <span class="nx">ME</span><span class="o">!<</span><span class="err">/button></span>
</span><span class='line'> <span class="p">{</span> <span class="nx">errorMessage</span> <span class="o">&&</span> <span class="o"><</span><span class="nx">div</span><span class="o">></span><span class="p">{</span><span class="nx">errorMessage</span><span class="p">}</span><span class="o"><</span><span class="err">/div> }</span>
</span><span class='line'> <span class="o"><</span><span class="err">/div></span>
</span><span class='line'> <span class="p">);</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'>
</span><span class='line'> <span class="nx">getRandomName</span><span class="p">()</span> <span class="p">{</span> <span class="c1">// 重点是这么一段代码</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span><span class="nx">loading</span><span class="o">:</span> <span class="kc">true</span><span class="p">})</span>
</span><span class='line'> <span class="nx">fetch</span><span class="p">(</span><span class="s1">'https://randomuser.me/api/'</span><span class="p">)</span>
</span><span class='line'> <span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">response</span> <span class="o">=></span> <span class="nx">response</span><span class="p">.</span><span class="nx">json</span><span class="p">())</span>
</span><span class='line'> <span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">data</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="kr">const</span> <span class="nx">person</span> <span class="o">=</span> <span class="nx">data</span><span class="p">.</span><span class="nx">results</span><span class="p">[</span><span class="mi">0</span><span class="p">];</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span> <span class="nx">name</span><span class="o">:</span> <span class="err">`</span><span class="nx">$</span><span class="p">{</span><span class="nx">person</span><span class="p">.</span><span class="nx">name</span><span class="p">.</span><span class="nx">first</span><span class="p">}</span> <span class="nx">$</span><span class="p">{</span><span class="nx">person</span><span class="p">.</span><span class="nx">name</span><span class="p">.</span><span class="nx">last</span><span class="p">}</span><span class="err">`</span><span class="p">,</span> <span class="nx">loading</span><span class="o">:</span> <span class="kc">false</span> <span class="p">})</span>
</span><span class='line'> <span class="p">})</span>
</span><span class='line'> <span class="p">.</span><span class="k">catch</span><span class="p">(</span><span class="nx">reason</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span><span class="nx">errorMessage</span><span class="o">:</span><span class="s1">'get name failed'</span><span class="p">,</span> <span class="nx">loading</span><span class="o">:</span> <span class="kc">false</span><span class="p">})</span>
</span><span class='line'> <span class="p">});</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p>过程很简单，我们需要考虑三种页面状态：请求开始和进行中，请求成功，请求失败，然后分别设置组件的state。</p>
<h2>如果没有Redux异步中间件</h2>
<p>按照“没有Redux的异步世界”的思想，在Redux里面，我们仍然可以依葫芦画瓢的进行异步的Redux的操作。</p>
<p>首先，一个异步请求都需要dispatch至少三种action，对应至少三个不同的状态：</p>
<p>1.通知reducer请求开始 <br/>
2.通知reducer请求成功 <br/>
3.通知reducer请求失败</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="p">{</span> <span class="nx">type</span><span class="o">:</span> <span class="s1">'FETCH_POSTS_REQUEST'</span> <span class="p">}</span>
</span><span class='line'><span class="p">{</span> <span class="nx">type</span><span class="o">:</span> <span class="s1">'FETCH_POSTS_FAILURE'</span><span class="p">,</span> <span class="nx">error</span><span class="o">:</span> <span class="s1">'Oops'</span> <span class="p">}</span>
</span><span class='line'><span class="p">{</span> <span class="nx">type</span><span class="o">:</span> <span class="s1">'FETCH_POSTS_SUCCESS'</span><span class="p">,</span> <span class="nx">response</span><span class="o">:</span> <span class="p">{</span> <span class="nx">name</span><span class="o">:</span> <span class="s1">'Redux'</span><span class="p">}</span> <span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p>如果没有Redux异步中间件，那么你的做法和没有Redux时是类似的，你需要在mapDispatchToProps那传入三个disptach，将异步的fetch逻辑放在组件里面实现，Redux本身仍然是处理同步的state操作。</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
<span class='line-number'>13</span>
<span class='line-number'>14</span>
<span class='line-number'>15</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kr">class</span> <span class="nx">ExampleComponent</span> <span class="kr">extends</span> <span class="nx">React</span><span class="p">.</span><span class="nx">Component</span> <span class="p">{</span>
</span><span class='line'> <span class="p">...</span>
</span><span class='line'> <span class="nx">getRandomName</span><span class="p">()</span> <span class="p">{</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">fetchRequest</span><span class="p">(</span><span class="kc">true</span><span class="p">)</span>
</span><span class='line'> <span class="nx">fetch</span><span class="p">(</span><span class="s1">'https://randomuser.me/api/'</span><span class="p">)</span>
</span><span class='line'> <span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">response</span> <span class="o">=></span> <span class="nx">response</span><span class="p">.</span><span class="nx">json</span><span class="p">())</span>
</span><span class='line'> <span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">data</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="kr">const</span> <span class="nx">person</span> <span class="o">=</span> <span class="nx">data</span><span class="p">.</span><span class="nx">results</span><span class="p">[</span><span class="mi">0</span><span class="p">];</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">fetchSuccess</span><span class="p">(</span><span class="err">`</span><span class="nx">$</span><span class="p">{</span><span class="nx">person</span><span class="p">.</span><span class="nx">name</span><span class="p">.</span><span class="nx">first</span><span class="p">}</span> <span class="nx">$</span><span class="p">{</span><span class="nx">person</span><span class="p">.</span><span class="nx">name</span><span class="p">.</span><span class="nx">last</span><span class="p">}</span><span class="err">`</span><span class="p">,</span> <span class="kc">false</span><span class="p">)</span>
</span><span class='line'> <span class="p">})</span>
</span><span class='line'> <span class="p">.</span><span class="k">catch</span><span class="p">(</span><span class="nx">reason</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">fetchFailure</span><span class="p">(</span><span class="s1">'get name failed'</span><span class="p">,</span> <span class="kc">false</span><span class="p">)</span>
</span><span class='line'> <span class="p">});</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<h2>使用Redux-Thunk的不同点在哪？</h2>
<p>因为Redux官网推荐，我们就以Redux-Thunk为例。</p>
<p>Redux-Thunk刚刚引入的时候，往往容易让使用者有些感到混乱，一个原因是函数式编程的嵌套写法，第二个是和Redux之前dispatch函数做的事情不一样了。</p>
<p>其实，没有Redux-Thunk我们已经可以处理异步请求，只不过异步逻辑不在Redux里面，而是在组件里面，如果我们加入Redux-Thunk会有什么不同呢？</p>
<p>我在<a href="http://benweizhu.github.io/blog/2018/04/25/deep-thinking-in-react-7/" title="上面篇文章">上面篇文章</a>讲过，中间件的作用是在dispatch的附近做一些额外的操作，让Redux拥有不同的能力，Redux-Thunk中间件的能力，可以让action creater，不用返回一个action对象，而是一个函数，这个action创建的函数就成为一个thunk。</p>
<p>（关于Thunk函数的含义：编译器的”传名调用”实现，往往是将参数放到一个临时函数之中，再将这个临时函数传入函数体。这个临时函数就叫做Thunk函数。）</p>
<p>这个函数并不需要保持纯净，它还可以带有副作用，包括执行异步API请求。这个函数还可以dispatch action。</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
<span class='line-number'>13</span>
<span class='line-number'>14</span>
<span class='line-number'>15</span>
<span class='line-number'>16</span>
<span class='line-number'>17</span>
<span class='line-number'>18</span>
<span class='line-number'>19</span>
<span class='line-number'>20</span>
<span class='line-number'>21</span>
<span class='line-number'>22</span>
<span class='line-number'>23</span>
<span class='line-number'>24</span>
<span class='line-number'>25</span>
<span class='line-number'>26</span>
<span class='line-number'>27</span>
<span class='line-number'>28</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="c1">//actions.js</span>
</span><span class='line'><span class="kr">export</span> <span class="kd">function</span> <span class="nx">fetchRandmonName</span><span class="p">()</span> <span class="p">{</span>
</span><span class='line'> <span class="k">return</span> <span class="kd">function</span> <span class="p">(</span><span class="nx">dispatch</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="nx">dispatch</span><span class="p">(</span><span class="nx">fetchRequest</span><span class="p">(</span><span class="kc">true</span><span class="p">))</span>
</span><span class='line'> <span class="k">return</span> <span class="nx">fetch</span><span class="p">(</span><span class="s1">'https://randomuser.me/api/'</span><span class="p">)</span>
</span><span class='line'> <span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">response</span> <span class="o">=></span> <span class="nx">response</span><span class="p">.</span><span class="nx">json</span><span class="p">())</span>
</span><span class='line'> <span class="p">.</span><span class="nx">then</span><span class="p">(</span><span class="nx">data</span> <span class="o">=></span><span class="p">{</span>
</span><span class='line'> <span class="kr">const</span> <span class="nx">person</span> <span class="o">=</span> <span class="nx">data</span><span class="p">.</span><span class="nx">results</span><span class="p">[</span><span class="mi">0</span><span class="p">];</span>
</span><span class='line'> <span class="nx">dispatch</span><span class="p">(</span><span class="nx">fetchSuccess</span><span class="p">(</span><span class="err">`</span><span class="nx">$</span><span class="p">{</span><span class="nx">person</span><span class="p">.</span><span class="nx">name</span><span class="p">.</span><span class="nx">first</span><span class="p">}</span> <span class="nx">$</span><span class="p">{</span><span class="nx">person</span><span class="p">.</span><span class="nx">name</span><span class="p">.</span><span class="nx">last</span><span class="p">}</span><span class="err">`</span><span class="p">,</span> <span class="kc">false</span><span class="p">))</span>
</span><span class='line'> <span class="p">})</span>
</span><span class='line'> <span class="p">.</span><span class="k">catch</span><span class="p">(</span><span class="nx">reason</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="nx">dispatch</span><span class="p">(</span><span class="nx">fetchFailure</span><span class="p">(</span><span class="s1">'get name failed'</span><span class="p">,</span> <span class="kc">false</span><span class="p">))</span>
</span><span class='line'> <span class="p">})</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span><span class='line'>
</span><span class='line'><span class="kr">const</span> <span class="nx">mapDispatchToProps</span> <span class="o">=</span> <span class="nx">dispatch</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="k">return</span> <span class="p">{</span>
</span><span class='line'> <span class="nx">fetchRandmonName</span><span class="o">:</span> <span class="p">()</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="nx">dispatch</span><span class="p">(</span><span class="nx">fetchRandmonName</span><span class="p">())</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span><span class='line'>
</span><span class='line'><span class="c1">//ExampleComponent.js</span>
</span><span class='line'><span class="nx">getRandomName</span><span class="p">()</span> <span class="p">{</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">props</span><span class="p">.</span><span class="nx">fetchRandmonName</span><span class="p">();</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p>你看其实代码差不多，只不过，因为Redux-Thunk，<strong>你可以将异步的处理逻辑，从组件里面拿出来，将它放在一个和Redux其他代码更加的内聚的位置</strong>，也许是action的存放位置actions.js，而组件里面只需要dispatch一个thunk。</p>
<h2>总结</h2>
<p>关于Redux的异步世界，暂时更新到这里，Redux里面处理异步的中间件有好多，我就不一个个分析了，你肯定很关心这个，<a href="http://react-china.org/t/redux/8761" title="《Redux异步方案选型》">《Redux异步方案选型》</a></p>
]]></content>
</entry>
<entry>
<title type="html"><![CDATA[React的思考（七）- Redux的Middleware（上）- 中间件的概念]]></title>
<link href="http://benweizhu.github.io/blog/2018/04/25/deep-thinking-in-react-7/"/>
<updated>2018-04-25T21:14:18+08:00</updated>
<id>http://benweizhu.github.io/blog/2018/04/25/deep-thinking-in-react-7</id>
<content type="html"><![CDATA[<p>Redux的Middleware（中间件）是Redux中相对比较神秘的部分。</p>
<h2>怎么理解middleware这个概念呢？</h2>
<p>middle这个词很重要，它是指，这个件（ware），被放在（穿插）于某一个已存在操作的过程当中（middle）。</p>
<p>我这么平淡直白无水准的解释，你应该能get到吧，如果你熟悉Java Web开发，你可能第一时间会想到Java Servlet Filters，当然也许你比较年轻，你对Spring熟悉，你可能会立刻想到AOP（面向切面编程），它们可以完成日志，审计，authentication, authorization等。</p>
<p>那么，再往后面走，如果你使用过Express或者Koa等服务端框架，在这类框架中，middleware是指可以被嵌入在框架“接收请求到产生响应过程之中”的代码。例如，Express或者Koa的middleware可以完成添加CORS headers、记录日志、内容压缩等工作。</p>
<h2>Redux的中间件是做什么呢？</h2>
<p>回过头来，看Redux的中间件，同样，它肯定是指穿插在Redux的某一个过程当中，那么问题来了，这个ware可以在哪里穿插，或者哪里有穿插操作的需要呢？我们来逐一分析，以下内容，参考阮一峰的文章：</p>
<p>（1）Reducer：纯函数，只承担计算State的功能，不合适承担其他功能，也承担不了，因为理论上，纯函数不能进行读写操作。</p>
<p>（2）View：与State一一对应，可以看作State的视觉层，也不合适承担其他功能。</p>
<p>（3）Action：存放数据的对象，即消息的载体，只能被别人操作，自己不能进行任何操作。</p>
<p>其实，和其他服务端框架类似，它被嵌入到Redux“接收请求到产生响应过程之中”，位于action被发起之后，到达reducer之前，也就是store.dispatch()附近。</p>
<h2>举个例子：如何在dispatch的时候log state和action</h2>
<p>我在看官方文档的时候，看到一个非常有趣的概念，猴子补丁（monkey patching），大概的意思是指在运行时动态修改模块、类或函数，通常是添加功能或修正缺陷。</p>
<p>通过这种方式，可以在代码的运行过程中，给store.dispatch打补丁，增加额外的功能，比如log state和action，代码如下：</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kd">let</span> <span class="nx">next</span> <span class="o">=</span> <span class="nx">store</span><span class="p">.</span><span class="nx">dispatch</span><span class="p">;</span>
</span><span class='line'><span class="nx">store</span><span class="p">.</span><span class="nx">dispatch</span> <span class="o">=</span> <span class="kd">function</span> <span class="nx">dispatchAndLog</span><span class="p">(</span><span class="nx">action</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="s1">'dispatching'</span><span class="p">,</span> <span class="nx">action</span><span class="p">);</span>
</span><span class='line'> <span class="nx">next</span><span class="p">(</span><span class="nx">action</span><span class="p">);</span>
</span><span class='line'> <span class="nx">console</span><span class="p">.</span><span class="nx">log</span><span class="p">(</span><span class="s1">'next state'</span><span class="p">,</span> <span class="nx">store</span><span class="p">.</span><span class="nx">getState</span><span class="p">());</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p>通过上面这样一段代码，我们就在Redux原来的store.dispatch附近添加了我们自己代码，当我们再次调用store.dispatch，它就会打印log了，不过monkey patching本质上是一种hack，“将任意的方法替换成你想要的”。</p>
<p>如果，我们想给dispatch加另一个补丁，那就在它的前面，或者后面，在加上一段类似的代码呗。</p>
<h2>Redux applyMiddlewares()</h2>
<p>真实情况下，我们肯定不用这样写代码，那Redux提供了applyMiddlewares的方式，所以就不需要我们像上面那样写，那它是怎么做的呢？</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kr">const</span> <span class="nx">store</span> <span class="o">=</span> <span class="nx">createStore</span><span class="p">(</span>
</span><span class='line'> <span class="nx">reducer</span><span class="p">,</span>
</span><span class='line'> <span class="nx">applyMiddleware</span><span class="p">(</span><span class="nx">thunk</span><span class="p">,</span> <span class="nx">promise</span><span class="p">,</span> <span class="nx">logger</span><span class="p">)</span>
</span><span class='line'><span class="p">);</span>
</span></code></pre></td></tr></table></div></figure>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
<span class='line-number'>11</span>
<span class='line-number'>12</span>
<span class='line-number'>13</span>
<span class='line-number'>14</span>
<span class='line-number'>15</span>
<span class='line-number'>16</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kr">export</span> <span class="k">default</span> <span class="kd">function</span> <span class="nx">applyMiddleware</span><span class="p">(...</span><span class="nx">middlewares</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="k">return</span> <span class="p">(</span><span class="nx">createStore</span><span class="p">)</span> <span class="o">=></span> <span class="p">(</span><span class="nx">reducer</span><span class="p">,</span> <span class="nx">preloadedState</span><span class="p">,</span> <span class="nx">enhancer</span><span class="p">)</span> <span class="o">=></span> <span class="p">{</span>
</span><span class='line'> <span class="kd">var</span> <span class="nx">store</span> <span class="o">=</span> <span class="nx">createStore</span><span class="p">(</span><span class="nx">reducer</span><span class="p">,</span> <span class="nx">preloadedState</span><span class="p">,</span> <span class="nx">enhancer</span><span class="p">);</span>
</span><span class='line'> <span class="kd">var</span> <span class="nx">dispatch</span> <span class="o">=</span> <span class="nx">store</span><span class="p">.</span><span class="nx">dispatch</span><span class="p">;</span>
</span><span class='line'> <span class="kd">var</span> <span class="nx">chain</span> <span class="o">=</span> <span class="p">[];</span>
</span><span class='line'>
</span><span class='line'> <span class="kd">var</span> <span class="nx">middlewareAPI</span> <span class="o">=</span> <span class="p">{</span> <span class="c1">//注意这里：getState和dispatch都传入了middleware</span>
</span><span class='line'> <span class="nx">getState</span><span class="o">:</span> <span class="nx">store</span><span class="p">.</span><span class="nx">getState</span><span class="p">,</span>
</span><span class='line'> <span class="nx">dispatch</span><span class="o">:</span> <span class="p">(</span><span class="nx">action</span><span class="p">)</span> <span class="o">=></span> <span class="nx">dispatch</span><span class="p">(</span><span class="nx">action</span><span class="p">)</span>
</span><span class='line'> <span class="p">};</span>
</span><span class='line'> <span class="nx">chain</span> <span class="o">=</span> <span class="nx">middlewares</span><span class="p">.</span><span class="nx">map</span><span class="p">(</span><span class="nx">middleware</span> <span class="o">=></span> <span class="nx">middleware</span><span class="p">(</span><span class="nx">middlewareAPI</span><span class="p">));</span><span class="c1">//注意这里：这些middleware都是函数</span>
</span><span class='line'> <span class="nx">dispatch</span> <span class="o">=</span> <span class="nx">compose</span><span class="p">(...</span><span class="nx">chain</span><span class="p">)(</span><span class="nx">store</span><span class="p">.</span><span class="nx">dispatch</span><span class="p">);</span>
</span><span class='line'>
</span><span class='line'> <span class="k">return</span> <span class="p">{...</span><span class="nx">store</span><span class="p">,</span> <span class="nx">dispatch</span><span class="p">}</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p>所有中间件被放进了一个数组chain，通过compose，将多个中间件合并，从右到左执行。中间件可以拿到getState和dispatch这两个方法。</p>
<p>注：compose: 将多个函数合并成一个函数，从右到左执行。例如：</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="nx">R</span><span class="p">.</span><span class="nx">compose</span><span class="p">(</span><span class="nb">Math</span><span class="p">.</span><span class="nx">abs</span><span class="p">,</span> <span class="nx">R</span><span class="p">.</span><span class="nx">add</span><span class="p">(</span><span class="mi">1</span><span class="p">),</span> <span class="nx">R</span><span class="p">.</span><span class="nx">multiply</span><span class="p">(</span><span class="mi">2</span><span class="p">))(</span><span class="o">-</span><span class="mi">4</span><span class="p">)</span> <span class="c1">// 7</span>
</span></code></pre></td></tr></table></div></figure>
<p>-4 * 2 + 1 再求绝对值</p>
<h2>总结</h2>
<p>仔细看下来之后，中间件就没有那么神秘了，下一篇文章，我们来介绍下Redux的异步中间件，比如：Redux-Thunk，看懂这一个，其他的都差不多。</p>
]]></content>
</entry>
<entry>
<title type="html"><![CDATA[React的思考（六）- 不可变性]]></title>
<link href="http://benweizhu.github.io/blog/2018/04/23/deep-thinking-in-react-6/"/>
<updated>2018-04-23T22:29:51+08:00</updated>
<id>http://benweizhu.github.io/blog/2018/04/23/deep-thinking-in-react-6</id>
<content type="html"><![CDATA[<h2>为什么immutable在React中那么重要？</h2>
<p>我尝试用简（kuo）单（hao）易（li）懂（mian）的词解释</p>
<p>1.可以给你的React应用带来性能提升（不用深度对比） <br/>
2.简单的编程和调试体验，少出bug（不会操作共享对象） <br/>
3.数据更容易追踪，推导（保留previewState可以对比）</p>
<p>Talk is shit, show me your money.</p>
<p>光说还是差了点，虽然很通俗了，但是还是看下面一段代码，我用一个React初学者，特别容易犯的一个错误来说明这三个问题，请看题板：</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="nx">updateState</span><span class="p">(</span><span class="nx">event</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="kr">const</span> <span class="p">{</span><span class="nx">value</span><span class="p">}</span> <span class="o">=</span> <span class="nx">event</span><span class="p">.</span><span class="nx">target</span><span class="p">;</span>
</span><span class='line'> <span class="kd">let</span> <span class="nx">user</span> <span class="o">=</span> <span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">user</span><span class="p">;</span>
</span><span class='line'> <span class="nx">user</span><span class="p">.</span><span class="nx">name</span> <span class="o">=</span> <span class="nx">value</span><span class="p">;</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span><span class="nx">user</span><span class="p">});</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="nx">updateState</span><span class="p">(</span><span class="nx">event</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="kr">const</span> <span class="p">{</span><span class="nx">value</span><span class="p">}</span> <span class="o">=</span> <span class="nx">event</span><span class="p">.</span><span class="nx">target</span><span class="p">;</span>
</span><span class='line'> <span class="kd">let</span> <span class="nx">user</span> <span class="o">=</span> <span class="nb">Object</span><span class="p">.</span><span class="nx">assign</span><span class="p">({},</span> <span class="k">this</span><span class="p">.</span><span class="nx">state</span><span class="p">.</span><span class="nx">user</span><span class="p">,</span> <span class="p">{</span><span class="nx">name</span><span class="o">:</span> <span class="nx">value</span><span class="p">});</span>
</span><span class='line'> <span class="k">this</span><span class="p">.</span><span class="nx">setState</span><span class="p">({</span><span class="nx">user</span><span class="p">});</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p>请问，第一种写法有什么问题？你有20秒的时间思考并作答，对不起，由于我没法遮挡住答案，请自觉闭眼思考。</p>
<p>答案：就是上面说的三点，举个例子：shouldComponentUpdate(nextProps, nextState)，需要对比nextState和this.state的区别来决定是否渲染，但此时this.state已经不是以前的state了状态了，特别是PureComponent的shallow compare，直接导致组件不重新渲染，会出bug</p>
<h2>为什么immutable在Redux中那么重要？</h2>
<p>因为不用就会出bug，因为你就会问“为什么UI不更新”，因为Redux和React-Redux都使用了浅比较。</p>
<p>具体行为体现在两处，一个在按照Domain区分的combineReducers那，一个在使用数据被connect包裹的组件那：</p>
<p><strong>1.Redux的combineReducers方法浅比较它调用的reducer的引用是否发生变化</strong></p>
<p>比如:</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="nx">combineReducers</span><span class="p">({</span> <span class="nx">todos</span><span class="o">:</span> <span class="nx">myTodosReducer</span><span class="p">,</span> <span class="nx">counter</span><span class="o">:</span> <span class="nx">myCounterReducer</span> <span class="p">})</span>
</span></code></pre></td></tr></table></div></figure>
<p>combineReducers会遍历所有这些键值对，判断每一个reducer执行返回结果后的引用是否发生变化（所以是浅比较），如果其中一个变化了，就会设置一个标志位hasChanged为true，当遍历结束，如果hasChanged为true，则返回新的conbineReducers合并的总的state，否则返回旧的那个state（这个新旧的state在React-Redux中会用到）。</p>
<p>这就是为什么在reducer当中要使用Object.assign({},state,{..}}来返回一个新的state，如果你直接操作state，并返回它，combineReducers会认为它没有改变。</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kd">function</span> <span class="nx">myTodosReducer</span><span class="p">(</span><span class="nx">state</span> <span class="o">=</span> <span class="nx">initialState</span><span class="p">,</span> <span class="nx">action</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="k">switch</span> <span class="p">(</span><span class="nx">action</span><span class="p">.</span><span class="nx">type</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="k">case</span> <span class="nx">SET_VISIBILITY_FILTER</span><span class="o">:</span>
</span><span class='line'> <span class="k">return</span> <span class="nb">Object</span><span class="p">.</span><span class="nx">assign</span><span class="p">({},</span> <span class="nx">state</span><span class="p">,</span> <span class="p">{</span>
</span><span class='line'> <span class="nx">visibilityFilter</span><span class="o">:</span> <span class="nx">action</span><span class="p">.</span><span class="nx">filter</span>
</span><span class='line'> <span class="p">})</span>
</span><span class='line'> <span class="k">default</span><span class="o">:</span>
</span><span class='line'> <span class="k">return</span> <span class="nx">state</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p><strong>2.React-Redux的state和mapStateToProps</strong></p>
<p>React-Redux的connect方法生成的组件，会假设包装的组件是一个“纯”（pure）组件，即给定相同的props和state，这个组件会返回相同的结果。做出这样的假设后，React-Redux就只需检查根state对象或mapStateToProps的返回值是否改变。如果没变，包装的组件就无需重新渲染。</p>
<p>我们回忆一下，上面说道，combineReducers会决定是否返回新的根state，而每次调用React-Redux提供的connect函数时，它之前储存的根state对象的引用，会与当前传递给store的根state对象之间进行浅比较。如果相等，说明根state对象没有变化，也就无需重新渲染组件，甚至无需调用mapStateToProps。</p>
<p>如果不相等，则connect会调用mapStateToProps来，并查看最后传给组件的props是否被更新。同样，这里也是一个浅比较，它要对比的是这个对象的第一层引用是否变化。</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kd">function</span> <span class="nx">mapStateToProps</span><span class="p">(</span><span class="nx">state</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="k">return</span> <span class="p">{</span>
</span><span class='line'> <span class="nx">todos</span><span class="o">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">todos</span><span class="p">,</span> <span class="c1">// prop value</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span><span class='line'>
</span><span class='line'><span class="kr">export</span> <span class="k">default</span> <span class="nx">connect</span><span class="p">(</span><span class="nx">mapStateToProps</span><span class="p">)(</span><span class="nx">TodoApp</span><span class="p">)</span>
</span></code></pre></td></tr></table></div></figure>
<p>即，这个对象的todos是否变化。</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="p">{</span>
</span><span class='line'> <span class="nx">todos</span><span class="o">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">todos</span><span class="p">,</span> <span class="c1">// prop value</span>
</span><span class='line'><span class="p">}</span>
</span></code></pre></td></tr></table></div></figure>
<p>比如，如果在todos的reducer返回的state没有变化，那么这里的todos也就是没有变化，因此组件就不需要渲染。</p>
<p>mapStateToProps因为它特殊的作用，很容易出现一种反模式，我们需要注意，就是像下面这样写：</p>
<figure class='code'><figcaption><span></span></figcaption><div class="highlight"><table><tr><td class="gutter"><pre class="line-numbers"><span class='line-number'>1</span>
<span class='line-number'>2</span>
<span class='line-number'>3</span>
<span class='line-number'>4</span>
<span class='line-number'>5</span>
<span class='line-number'>6</span>
<span class='line-number'>7</span>
<span class='line-number'>8</span>
<span class='line-number'>9</span>
<span class='line-number'>10</span>
</pre></td><td class='code'><pre><code class='JavaScript'><span class='line'><span class="kd">function</span> <span class="nx">mapStateToProps</span><span class="p">(</span><span class="nx">state</span><span class="p">)</span> <span class="p">{</span>
</span><span class='line'> <span class="k">return</span> <span class="p">{</span>
</span><span class='line'> <span class="nx">todos</span><span class="o">:</span> <span class="p">{</span>
</span><span class='line'> <span class="nx">all</span><span class="o">:</span> <span class="nx">state</span><span class="p">.</span><span class="nx">todos</span><span class="p">,</span>
</span><span class='line'> <span class="nx">visibleTodos</span><span class="o">:</span> <span class="nx">getVisibleTodos</span><span class="p">(</span><span class="nx">state</span><span class="p">)</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'> <span class="p">}</span>
</span><span class='line'><span class="p">}</span>
</span><span class='line'>
</span><span class='line'><span class="kr">export</span> <span class="k">default</span> <span class="nx">connect</span><span class="p">(</span><span class="nx">mapStateToProps</span><span class="p">)(</span><span class="nx">TodoApp</span><span class="p">)</span>
</span></code></pre></td></tr></table></div></figure>
<p>todos对象永远拿到的都是新的对象{}，而不是直接由reducer里面返回的对象，所以组件一定会重新渲染，无论state是否变化。</p>
<p><strong>React-Redux这样设计是有道理的</strong></p>
<p>因为Redux本身的CombineReducers只会决定最根节点的state有没有变化，也就是这个{ todos: myTodosReducer, counter: myCounterReducer }里面的存不存在变化，然后决定返回新的或者旧的，而每一个connect的组件拿到这个新的根state，首先判断state有没有变化，然后判断这个变化和它想要的数据（一个或者多个reducer）有没有关，如果没有关系，当然就不用重新渲染。</p>
<h2>总结</h2>
<p>React和Redux之所以要求使用Immutable，原因的初衷是性能，避免深度对比。对于我们使用React和Redux的开发人员，除了关注性能，更是因为需要遵循React和Redux的编程规范来写出正确的代码。</p>
]]></content>
</entry>
<entry>
<title type="html"><![CDATA[React的思考（五）- Reconciliation]]></title>
<link href="http://benweizhu.github.io/blog/2018/04/22/deep-thinking-in-react-5/"/>
<updated>2018-04-22T12:33:42+08:00</updated>
<id>http://benweizhu.github.io/blog/2018/04/22/deep-thinking-in-react-5</id>
<content type="html"><![CDATA[<h3>Reconciliation的含义本身</h3>
<p>Reconciliation有的人翻译成“协调算法”，有的人翻译成“一致性对比”，在没有官方答案之前，我认为直译可能会比较准确，它的作用是React用来区分一棵节点树和另一棵节点树的算法，以确定哪些部分需要更改。</p>
<p>Reconciliation是通常被理解为“虚拟DOM”背后的算法。</p>
<p>简单描述就是：当您渲染React应用时，会生成描述该应用的节点树并将其保存在内存中。然后将该节点树刷新到渲染环境 - 例如，在浏览器应用程序的情况下，它会转换为一组DOM操作。当应用程序更新（比如，通过setState）时，会生成一棵新树。对比得到新树与前一棵树的区别，以计算需要更新渲染应用的操作。</p>
<h3>React 16+的Reconciler - React Fiber</h3>
<p>React团队在2016年7月公开发布React Fiber，React新的核心算法，React Fiber的目标是提高其对动画，布局和手势等领域的适用性。它的特征是增量渲染：能够将渲染工作分割成块并将其分散到多个帧中。</p>
<p>按照官方文档的说法，Fiber Reconciler的主要目标：</p>
<ul>
<li>能够将可中断的任务拆分成块。</li>
<li>能够对进程中的工作划分优先级、重新设定基址（Rebase）、恢复。</li>
<li>能够在父子之间来回反复，借此为React的Layout提供支持。</li>
<li>能够通过render()返回多个元素。</li>
<li>为错误边界提供了更好的支持。</li>
</ul>
<p>这个项目持续的2年之久，蕴含着过去多年来Facebook不断改进的工作成果。该架构可向后兼容，彻底重写了React的协调（Reconciliation）算法。他们还专门做了一个网站叫：<a href="http://isfiberreadyyet.com/">http://isfiberreadyyet.com/</a></p>
<p><img src="http://benweizhu.github.io/images/isfiberready.jpg" width="500" title="isfiberreadyyet" alt="Alt text" /> <br/>
27 September 2017</p>
<p>我们都知道DOM只是React可以渲染的渲染环境之一，另外一个就是React Native。（这就是为什么“虚拟DOM”有点用词不当）。</p>
<p>它可以支持如此多环境的原因是因为React的设计使是Reconciliation和渲染是分开的阶段。Reconciler执行计算树的哪些部分已经改变; 渲染器然后使用该信息实际更新呈现的应用。这种分离意味着React DOM和React Native可以在共享由React核心提供的相同Reconciler的同时使用它们自己的渲染器。</p>
<h3>React 15以及以前的Reconciler - Stack Reconciler</h3>
<p>Fiber在React 16中首次登场，发布时间是2017年9月26号，那么意味着，在这之前，有另外一套Reconciliation的算法。React现在把它命名为Stack Reconciler。它存在于React 15及更早版本的实现中。</p>
<p><strong>Stack Reconciler犯了一个单线程或者存在UI主线程环境下的“禁忌” - 用同步的方式来处理整个组件树。</strong></p>
<p>Virtual DOM diff会一次性处理整个组件树，重点在于，Stack Reconciler始终会一次性地同步处理整个组件树。因为整个过程都是在内存中完成，所以当组件树比较小的时候的，不会感觉到问题，但是，当组件树比较庞大的时候，就会出现卡顿（掉帧）的情况。</p>
<p><img src="http://benweizhu.github.io/images/Stack.jpg" width="500" title="Stack Reconciler" alt="Alt text" /> <br/>
单线程进入到栈中，要从栈从退出来，才能响应其他用户操作</p>
<p><img src="http://benweizhu.github.io/images/Fiber.jpg" width="500" title="Fiber Reconciler" alt="Alt text" /> <br/>
按照时间片段的方式执行</p>
<p>参考：<a href="https://www.youtube.com/watch?v=ZCuYPiUIONs" title="Lin Clark - A Cartoon Intro to Fiber - React Conf 2017">Lin Clark - A Cartoon Intro to Fiber - React Conf 2017</a></p>
<h3>会不会有影响（参考官方文档）</h3>
<p>首先，不变的地方是，diff节点或者说判断两个节点是否相同的方式没有变：</p>
<p>1.不同的元素类型</p>
<p>每当根元素具有不同类型时，React就会销毁旧的树并从头开始构建新树。从a到img ，或者从Article到Comment，从Button到div – 这些都将导致全部重新构建。</p>
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