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Vue3.0Reactivity之readonly
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<div class="post-body" itemprop="articleBody"><h1 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h1><p>最后一个<code>readonly</code>API了，这个API还是蛮容易理解的。</p>
<span id="more"></span>
<h1 id="readonly"><a href="#readonly" class="headerlink" title="readonly"></a><code>readonly</code></h1><p>老规矩，找到它的定义</p>
<figure class="highlight typescript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">export</span> <span class="keyword">function</span> <span class="keyword">readonly</span><T <span class="keyword">extends</span> <span class="built_in">object</span>>(</span><br><span class="line"> <span class="attr">target</span>: T</span><br><span class="line">): <span class="title class_">Readonly</span><<span class="title class_">UnwrapNestedRefs</span><T>> {</span><br><span class="line"> <span class="keyword">return</span> <span class="title function_">createReactiveObject</span>(</span><br><span class="line"> target,</span><br><span class="line"> rawToReadonly,</span><br><span class="line"> readonlyToRaw,</span><br><span class="line"> readonlyHandlers,</span><br><span class="line"> readonlyCollectionHandlers</span><br><span class="line"> )</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这里和我们之前看<code>reactive</code>的实现其实很像，都是主要由另一个函数<code>createReactiveObject</code>来传入参数进行实现</p>
<figure class="highlight typescript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">export</span> <span class="keyword">function</span> <span class="title function_">reactive</span>(<span class="params"><span class="attr">target</span>: <span class="built_in">object</span></span>) {</span><br><span class="line"> <span class="keyword">if</span> (readonlyToRaw.<span class="title function_">has</span>(target)) {</span><br><span class="line"> <span class="keyword">return</span> target</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="title function_">createReactiveObject</span>(</span><br><span class="line"> target,</span><br><span class="line"> rawToReactive,</span><br><span class="line"> reactiveToRaw,</span><br><span class="line"> mutableHandlers,</span><br><span class="line"> mutableCollectionHandlers</span><br><span class="line"> )</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>那么我们只要关注参数即可。</p>
<p>这里传入了两个WeakMap，不过这两个WeakMap是建立只读代理到原生对象之间的关系。还传入了两个handlers，这里我们直接找到<code>readonlyHandlers</code>。</p>
<figure class="highlight typescript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">export</span> <span class="keyword">const</span> <span class="attr">readonlyHandlers</span>: <span class="title class_">ProxyHandler</span><<span class="built_in">object</span>> = {</span><br><span class="line"> <span class="attr">get</span>: readonlyGet,</span><br><span class="line"> has,</span><br><span class="line"> ownKeys,</span><br><span class="line"> <span class="title function_">set</span>(<span class="params">target, key</span>) {</span><br><span class="line"> <span class="keyword">if</span> (__DEV__) {</span><br><span class="line"> <span class="variable language_">console</span>.<span class="title function_">warn</span>(</span><br><span class="line"> <span class="string">`Set operation on key "<span class="subst">${<span class="built_in">String</span>(key)}</span>" failed: target is readonly.`</span>,</span><br><span class="line"> target</span><br><span class="line"> )</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span></span><br><span class="line"> },</span><br><span class="line"> <span class="title function_">deleteProperty</span>(<span class="params">target, key</span>) {</span><br><span class="line"> <span class="keyword">if</span> (__DEV__) {</span><br><span class="line"> <span class="variable language_">console</span>.<span class="title function_">warn</span>(</span><br><span class="line"> <span class="string">`Delete operation on key "<span class="subst">${<span class="built_in">String</span>(key)}</span>" failed: target is readonly.`</span>,</span><br><span class="line"> target</span><br><span class="line"> )</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="literal">true</span></span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这里我们可以对比<code>reactive</code>使用到的<code>mutableHandlers</code></p>
<figure class="highlight typescript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">export</span> <span class="keyword">const</span> <span class="attr">mutableHandlers</span>: <span class="title class_">ProxyHandler</span><<span class="built_in">object</span>> = {</span><br><span class="line"> get,</span><br><span class="line"> set,</span><br><span class="line"> deleteProperty,</span><br><span class="line"> has,</span><br><span class="line"> ownKeys</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>发现不同的地方在<code>get</code>，<code>set</code>和<code>deleteProperty</code>参数上。</p>
<p>我们先看<code>set</code>，<code>deleteProperty</code>参数，它们直接就在这个对象里面实现。</p>
<p>对于只读的代理对象，当我们尝试对他设置值的时候，<code>set</code>就会在开发环境下打印一个警告，删除属性的操作也是如此。</p>
<p>接下来我们看这个<code>readonly</code>这个函数的实现</p>
<figure class="highlight typescript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// mutableHandlers用到的get</span></span><br><span class="line"><span class="keyword">const</span> get = <span class="comment">/*#__PURE__*/</span> <span class="title function_">createGetter</span>()</span><br><span class="line"><span class="comment">// readonlyHandlers用到的get</span></span><br><span class="line"><span class="keyword">const</span> readonlyGet = <span class="comment">/*#__PURE__*/</span> <span class="title function_">createGetter</span>(<span class="literal">true</span>)</span><br></pre></td></tr></table></figure>
<p>发现其实和<code>mutableHandlers</code>的实现都是通过<code>createGetter</code>这个函数，只不过传入的参数不同而已。</p>
<p>找到这个<code>createGetter</code>函数</p>
<figure class="highlight typescript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">function</span> <span class="title function_">createGetter</span>(<span class="params">isReadonly = <span class="literal">false</span>, shallow = <span class="literal">false</span></span>) {</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">function</span> <span class="title function_">get</span>(<span class="params"><span class="attr">target</span>: <span class="built_in">object</span>, <span class="attr">key</span>: <span class="built_in">string</span> | <span class="built_in">symbol</span>, <span class="attr">receiver</span>: <span class="built_in">object</span></span>) {</span><br><span class="line"> <span class="keyword">const</span> targetIsArray = <span class="title function_">isArray</span>(target)</span><br><span class="line"> <span class="keyword">if</span> (targetIsArray && <span class="title function_">hasOwn</span>(arrayInstrumentations, key)) {</span><br><span class="line"> <span class="keyword">return</span> <span class="title class_">Reflect</span>.<span class="title function_">get</span>(arrayInstrumentations, key, receiver)</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">const</span> res = <span class="title class_">Reflect</span>.<span class="title function_">get</span>(target, key, receiver)</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (<span class="title function_">isSymbol</span>(key) && builtInSymbols.<span class="title function_">has</span>(key)) {</span><br><span class="line"> <span class="keyword">return</span> res</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (shallow) {</span><br><span class="line"> <span class="comment">// 判断不是只读的就收集依赖。</span></span><br><span class="line"> !isReadonly && <span class="title function_">track</span>(target, <span class="title class_">TrackOpTypes</span>.<span class="property">GET</span>, key)</span><br><span class="line"> <span class="keyword">return</span> res</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (<span class="title function_">isRef</span>(res)) {</span><br><span class="line"> <span class="keyword">if</span> (targetIsArray) {</span><br><span class="line"> <span class="comment">// 判断不是只读的就收集依赖。</span></span><br><span class="line"> !isReadonly && <span class="title function_">track</span>(target, <span class="title class_">TrackOpTypes</span>.<span class="property">GET</span>, key)</span><br><span class="line"> <span class="keyword">return</span> res</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="comment">// ref unwrapping, only for Objects, not for Arrays.</span></span><br><span class="line"> <span class="keyword">return</span> res.<span class="property">value</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> </span><br><span class="line"> <span class="comment">// 判断不是只读的就收集依赖。</span></span><br><span class="line"> !isReadonly && <span class="title function_">track</span>(target, <span class="title class_">TrackOpTypes</span>.<span class="property">GET</span>, key)</span><br><span class="line"> <span class="keyword">return</span> <span class="title function_">isObject</span>(res)</span><br><span class="line"> ? isReadonly</span><br><span class="line"> ? <span class="comment">// need to lazy access readonly and reactive here to avoid</span></span><br><span class="line"> <span class="comment">// circular dependency</span></span><br><span class="line"> <span class="title function_">readonly</span>(res)</span><br><span class="line"> : <span class="title function_">reactive</span>(res)</span><br><span class="line"> : res</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>这个函数传入两个参数</p>
<ul>
<li><code>isReadOnly</code> 是否只读</li>
<li><code>shallow</code> 是否为浅代理</li>
</ul>
<p>所以其实我们可以发现有几种实现，比如<code>reactive</code>，<code>shallowReactive</code>，<code>readonly</code>，<code>shallowReadonly</code>，就是通过参数的真假组合出来的get所实现的。</p>
<p>这里我们不讲整个流程，我们发现当需要收集依赖，也就是执行<code>track</code>函数的时候，都会先做一个判断，判断传进来的<code>isReadonly</code>的真假。</p>
<p>也就是说，如果一个代理是只读的话，那么就没有必要对它执行依赖收集。因为它的<code>set</code>和<code>deleteProperty</code>没有触发它的依赖，不用触发，自然也就没必要收集。</p>
<p>至此，<code>readonly</code>的实现基本上已经明朗，和<code>reactive</code>的实现有相似之处，都返回了一个Proxy代理对象，不同的是对于<code>get</code>，<code>set</code>，<code>deleteProperty</code>所使用的函数不相同。</p>
<p>通过拦截<code>set</code>和<code>deleteProperty</code>来使得赋值和删除属性无效，也避免了无意义的依赖收集。</p>
<h1 id="后记"><a href="#后记" class="headerlink" title="后记"></a>后记</h1><p>今天重新地同步了vue-next的仓库，发现已经出到<code>beta.9</code>版本了，我也发现了一些代码做出了细微的改动，但是万变不离其宗，基本的原理还是一毛一样的。我看到的好像是增加了一些类型声明，还有一些函数的重构。</p>
<p>之后的话会写一些<code>reactivity</code>实现上的一些细节的理解。如果看得懂virtual-dom的话，到时也会写写。</p>
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