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<p>场景：事件频繁被触发，频繁执行DOM操作、资源加载等重复行为，导致UI停顿甚至浏览器崩溃。如 input 实时搜索、scrollview 滚动更新。</p>
<p>函数防抖和节流用于限制函数的执行。是优化高频率执行js代码的一种手段。</p>
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<h2 id="函数防抖"><a href="#函数防抖" class="headerlink" title="函数防抖"></a>函数防抖</h2><p>函数防抖就是让某个函数在上一次执行后，满足等待某个时间内不再触发此函数后再执行，而在这个等待时间内再次触发此函数，等待时间会重新计算。</p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> debounce = <span class="function"><span class="keyword">function</span>(<span class="params">idle,action</span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> last;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> ctx = <span class="keyword">this</span>,</span><br><span class="line"> args = <span class="built_in">arguments</span>; <span class="comment">// 存储参数</span></span><br><span class="line"> clearTimeout(last);</span><br><span class="line"> last = setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> action.apply(ctx,args);</span><br><span class="line"> },idle);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">// 返回函数连续调用时，空闲时间必须大于或等于idle，action才会执行。</span></span><br></pre></td></tr></table></figure>
<h3 id="实际例子"><a href="#实际例子" class="headerlink" title="实际例子"></a>实际例子</h3><p>当持续触发事件时，一定时间段内没有再触发事件，事件处理函数才会执行一次，如果设定的时间到来之前，又一次触发了事件，就重新开始延时。如下图，持续触发scroll事件时，并不执行handle函数，当1000毫秒内没有触发scroll事件时，才会延时触发scroll事件。</p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">debounce</span>(<span class="params">fn, wait</span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> timeout = <span class="literal">null</span>;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">if</span>(timeout !== <span class="literal">null</span>) </span><br><span class="line"> clearTimeout(timeout);</span><br><span class="line"> timeout = setTimeout(fn, wait);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="comment">// 处理函数</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">handle</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="built_in">Math</span>.random()); </span><br><span class="line">}</span><br><span class="line"><span class="comment">// 滚动事件</span></span><br><span class="line"><span class="built_in">window</span>.addEventListener(<span class="string">'scroll'</span>, debounce(handle, <span class="number">1000</span>));</span><br></pre></td></tr></table></figure>
<p>当持续触发scroll事件时，事件处理函数handle只在停止滚动1000毫秒之后才会调用一次，也就是说在持续触发scroll事件的过程中，事件处理函数handle一直没有执行。</p>
<p>函数防抖适用于连续调用函数，但只在延时后调用一次。定时器存在则销毁，第一次一定会创建并调用函数。</p>
<h2 id="函数节流"><a href="#函数节流" class="headerlink" title="函数节流"></a>函数节流</h2><p>每隔某个时间去执行某函数，避免函数的过多执行。</p>
<p>如果将水龙头拧紧直到水是以水滴的形式流出，那你会发现每隔一段时间，就会有一滴水流出。也就是说会先设定一个执行周期，当调用动作的时刻大于等于执行周期则执行该动作，然后进入下一个新周期。</p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> throttle = <span class="function"><span class="keyword">function</span>(<span class="params">delay,action</span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> last ;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> cur = +<span class="keyword">new</span> <span class="built_in">Date</span>(); <span class="comment">//返回毫秒数</span></span><br><span class="line"> <span class="keyword">if</span>(cur - last > = delay){</span><br><span class="line"> action.apply(<span class="keyword">this</span>,<span class="built_in">arguments</span>);</span><br><span class="line"> last = cur;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<h3 id="实际例子-1"><a href="#实际例子-1" class="headerlink" title="实际例子"></a>实际例子</h3><p>函数节流主要有两种实现方法：时间戳和定时器。</p>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> throttle = <span class="function"><span class="keyword">function</span>(<span class="params">func, delay</span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> prev = <span class="built_in">Date</span>.now();</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> context = <span class="keyword">this</span>;</span><br><span class="line"> <span class="keyword">var</span> args = <span class="built_in">arguments</span>;</span><br><span class="line"> <span class="keyword">var</span> now = <span class="built_in">Date</span>.now();</span><br><span class="line"> <span class="keyword">if</span> (now - prev >= delay) {</span><br><span class="line"> func.apply(context, args);</span><br><span class="line"> prev = <span class="built_in">Date</span>.now();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">handle</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="built_in">Math</span>.random());</span><br><span class="line">}</span><br><span class="line"><span class="built_in">window</span>.addEventListener(<span class="string">'scroll'</span>, throttle(handle, <span class="number">1000</span>));</span><br></pre></td></tr></table></figure>
<figure class="highlight javascript"><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> throttle = <span class="function"><span class="keyword">function</span>(<span class="params">func, delay</span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> timer = <span class="literal">null</span>;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> context = <span class="keyword">this</span>;</span><br><span class="line"> <span class="keyword">var</span> args = <span class="built_in">arguments</span>;</span><br><span class="line"> <span class="keyword">if</span> (!timer) {</span><br><span class="line"> timer = setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>{</span><br><span class="line"> func.apply(context, args);</span><br><span class="line"> timer = <span class="literal">null</span>;</span><br><span class="line"> }, delay);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">handle</span>(<span class="params"></span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="built_in">Math</span>.random());</span><br><span class="line">}</span><br><span class="line"><span class="built_in">window</span>.addEventListener(<span class="string">'scroll'</span>, throttle(handle, <span class="number">1000</span>));</span><br></pre></td></tr></table></figure>
<p>函数节流适用于间接性调用函数。定时器不存在则创建调用函数，定时销毁。</p>
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<p><span>文章作者:</span><a href="/" title="访问 陈宇（cosyer） 的个人博客">陈宇（cosyer）</a></p>
<p><span>发布时间:</span>2018年07月03日 - 22:07</p>
<p><span>最后更新:</span>2020年03月15日 - 00:03</p>
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