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677 lines
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HTML
677 lines
34 KiB
HTML
<!DOCTYPE HTML>
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<html lang="zh" class="sidebar-visible no-js light">
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<head>
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<!-- Book generated using https://github.com/wa-lang/mnbook -->
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<meta charset="UTF-8">
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<title>GIF动画 - Go语言圣经</title>
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</script>
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<nav id="sidebar" class="sidebar" aria-label="Table of contents">
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<div class="sidebar-scrollbox">
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<ol class="chapter">
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<li class="chapter-item expanded ">
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<a href="../index.html" >Go语言圣经</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../preface-zh.html" >译者序</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../preface.html" >前言</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1.html" ><strong aria-hidden="true">1.</strong> 入门</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-01.html" ><strong aria-hidden="true">1.1.</strong> Hello, World</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-02.html" ><strong aria-hidden="true">1.2.</strong> 命令行参数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-03.html" ><strong aria-hidden="true">1.3.</strong> 查找重复的行</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-04.html" class="active"><strong aria-hidden="true">1.4.</strong> GIF动画</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-05.html" ><strong aria-hidden="true">1.5.</strong> 获取URL</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-06.html" ><strong aria-hidden="true">1.6.</strong> 并发获取多个URL</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-07.html" ><strong aria-hidden="true">1.7.</strong> Web服务</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch1/ch1-08.html" ><strong aria-hidden="true">1.8.</strong> 本章要点</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2.html" ><strong aria-hidden="true">2.</strong> 程序结构</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-01.html" ><strong aria-hidden="true">2.1.</strong> 命名</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-02.html" ><strong aria-hidden="true">2.2.</strong> 声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-03.html" ><strong aria-hidden="true">2.3.</strong> 变量</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-04.html" ><strong aria-hidden="true">2.4.</strong> 赋值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-05.html" ><strong aria-hidden="true">2.5.</strong> 类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-06.html" ><strong aria-hidden="true">2.6.</strong> 包和文件</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch2/ch2-07.html" ><strong aria-hidden="true">2.7.</strong> 作用域</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3.html" ><strong aria-hidden="true">3.</strong> 基础数据类型</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-01.html" ><strong aria-hidden="true">3.1.</strong> 整型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-02.html" ><strong aria-hidden="true">3.2.</strong> 浮点数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-03.html" ><strong aria-hidden="true">3.3.</strong> 复数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-04.html" ><strong aria-hidden="true">3.4.</strong> 布尔型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-05.html" ><strong aria-hidden="true">3.5.</strong> 字符串</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch3/ch3-06.html" ><strong aria-hidden="true">3.6.</strong> 常量</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4.html" ><strong aria-hidden="true">4.</strong> 复合数据类型</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-01.html" ><strong aria-hidden="true">4.1.</strong> 数组</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-02.html" ><strong aria-hidden="true">4.2.</strong> Slice</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-03.html" ><strong aria-hidden="true">4.3.</strong> Map</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-04.html" ><strong aria-hidden="true">4.4.</strong> 结构体</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-05.html" ><strong aria-hidden="true">4.5.</strong> JSON</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch4/ch4-06.html" ><strong aria-hidden="true">4.6.</strong> 文本和HTML模板</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5.html" ><strong aria-hidden="true">5.</strong> 函数</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-01.html" ><strong aria-hidden="true">5.1.</strong> 函数声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-02.html" ><strong aria-hidden="true">5.2.</strong> 递归</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-03.html" ><strong aria-hidden="true">5.3.</strong> 多返回值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-04.html" ><strong aria-hidden="true">5.4.</strong> 错误</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-05.html" ><strong aria-hidden="true">5.5.</strong> 函数值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-06.html" ><strong aria-hidden="true">5.6.</strong> 匿名函数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-07.html" ><strong aria-hidden="true">5.7.</strong> 可变参数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-08.html" ><strong aria-hidden="true">5.8.</strong> Deferred函数</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-09.html" ><strong aria-hidden="true">5.9.</strong> Panic异常</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch5/ch5-10.html" ><strong aria-hidden="true">5.10.</strong> Recover捕获异常</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6.html" ><strong aria-hidden="true">6.</strong> 方法</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-01.html" ><strong aria-hidden="true">6.1.</strong> 方法声明</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-02.html" ><strong aria-hidden="true">6.2.</strong> 基于指针对象的方法</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-03.html" ><strong aria-hidden="true">6.3.</strong> 通过嵌入结构体来扩展类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-04.html" ><strong aria-hidden="true">6.4.</strong> 方法值和方法表达式</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-05.html" ><strong aria-hidden="true">6.5.</strong> 示例: Bit数组</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch6/ch6-06.html" ><strong aria-hidden="true">6.6.</strong> 封装</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7.html" ><strong aria-hidden="true">7.</strong> 接口</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-01.html" ><strong aria-hidden="true">7.1.</strong> 接口是合约</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-02.html" ><strong aria-hidden="true">7.2.</strong> 接口类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-03.html" ><strong aria-hidden="true">7.3.</strong> 实现接口的条件</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-04.html" ><strong aria-hidden="true">7.4.</strong> flag.Value接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-05.html" ><strong aria-hidden="true">7.5.</strong> 接口值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-06.html" ><strong aria-hidden="true">7.6.</strong> sort.Interface接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-07.html" ><strong aria-hidden="true">7.7.</strong> http.Handler接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-08.html" ><strong aria-hidden="true">7.8.</strong> error接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-09.html" ><strong aria-hidden="true">7.9.</strong> 示例: 表达式求值</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-10.html" ><strong aria-hidden="true">7.10.</strong> 类型断言</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-11.html" ><strong aria-hidden="true">7.11.</strong> 基于类型断言识别错误类型</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-12.html" ><strong aria-hidden="true">7.12.</strong> 通过类型断言查询接口</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-13.html" ><strong aria-hidden="true">7.13.</strong> 类型分支</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-14.html" ><strong aria-hidden="true">7.14.</strong> 示例: 基于标记的XML解码</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch7/ch7-15.html" ><strong aria-hidden="true">7.15.</strong> 补充几点</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8.html" ><strong aria-hidden="true">8.</strong> Goroutines和Channels</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-01.html" ><strong aria-hidden="true">8.1.</strong> Goroutines</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-02.html" ><strong aria-hidden="true">8.2.</strong> 示例: 并发的Clock服务</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-03.html" ><strong aria-hidden="true">8.3.</strong> 示例: 并发的Echo服务</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-04.html" ><strong aria-hidden="true">8.4.</strong> Channels</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-05.html" ><strong aria-hidden="true">8.5.</strong> 并发的循环</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-06.html" ><strong aria-hidden="true">8.6.</strong> 示例: 并发的Web爬虫</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-07.html" ><strong aria-hidden="true">8.7.</strong> 基于select的多路复用</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-08.html" ><strong aria-hidden="true">8.8.</strong> 示例: 并发的目录遍历</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-09.html" ><strong aria-hidden="true">8.9.</strong> 并发的退出</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch8/ch8-10.html" ><strong aria-hidden="true">8.10.</strong> 示例: 聊天服务</a>
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</li>
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</ol>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9.html" ><strong aria-hidden="true">9.</strong> 基于共享变量的并发</a>
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</li>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-01.html" ><strong aria-hidden="true">9.1.</strong> 竞争条件</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-02.html" ><strong aria-hidden="true">9.2.</strong> sync.Mutex互斥锁</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-03.html" ><strong aria-hidden="true">9.3.</strong> sync.RWMutex读写锁</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-04.html" ><strong aria-hidden="true">9.4.</strong> 内存同步</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-05.html" ><strong aria-hidden="true">9.5.</strong> sync.Once惰性初始化</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-06.html" ><strong aria-hidden="true">9.6.</strong> 竞争条件检测</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-07.html" ><strong aria-hidden="true">9.7.</strong> 示例: 并发的非阻塞缓存</a>
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</li>
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<li class="chapter-item expanded ">
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<a href="../ch9/ch9-08.html" ><strong aria-hidden="true">9.8.</strong> Goroutines和线程</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10.html" ><strong aria-hidden="true">10.</strong> 包和工具</a>
|
||
</li>
|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-01.html" ><strong aria-hidden="true">10.1.</strong> 包简介</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-02.html" ><strong aria-hidden="true">10.2.</strong> 导入路径</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-03.html" ><strong aria-hidden="true">10.3.</strong> 包声明</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-04.html" ><strong aria-hidden="true">10.4.</strong> 导入声明</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-05.html" ><strong aria-hidden="true">10.5.</strong> 包的匿名导入</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-06.html" ><strong aria-hidden="true">10.6.</strong> 包和命名</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch10/ch10-07.html" ><strong aria-hidden="true">10.7.</strong> 工具</a>
|
||
</li>
|
||
</ol>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11.html" ><strong aria-hidden="true">11.</strong> 测试</a>
|
||
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|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-01.html" ><strong aria-hidden="true">11.1.</strong> go test</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../ch11/ch11-02.html" ><strong aria-hidden="true">11.2.</strong> 测试函数</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch11/ch11-03.html" ><strong aria-hidden="true">11.3.</strong> 测试覆盖率</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../ch11/ch11-04.html" ><strong aria-hidden="true">11.4.</strong> 基准测试</a>
|
||
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|
||
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|
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<a href="../ch11/ch11-05.html" ><strong aria-hidden="true">11.5.</strong> 剖析</a>
|
||
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<a href="../ch11/ch11-06.html" ><strong aria-hidden="true">11.6.</strong> 示例函数</a>
|
||
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<a href="../ch12/ch12.html" ><strong aria-hidden="true">12.</strong> 反射</a>
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<ol class="section">
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<li class="chapter-item expanded ">
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<a href="../ch12/ch12-01.html" ><strong aria-hidden="true">12.1.</strong> 为何需要反射?</a>
|
||
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<a href="../ch12/ch12-02.html" ><strong aria-hidden="true">12.2.</strong> reflect.Type和reflect.Value</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-03.html" ><strong aria-hidden="true">12.3.</strong> Display递归打印</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch12/ch12-04.html" ><strong aria-hidden="true">12.4.</strong> 示例: 编码S表达式</a>
|
||
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|
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<li class="chapter-item expanded ">
|
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<a href="../ch12/ch12-05.html" ><strong aria-hidden="true">12.5.</strong> 通过reflect.Value修改值</a>
|
||
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|
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<li class="chapter-item expanded ">
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<a href="../ch12/ch12-06.html" ><strong aria-hidden="true">12.6.</strong> 示例: 解码S表达式</a>
|
||
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|
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<li class="chapter-item expanded ">
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<a href="../ch12/ch12-07.html" ><strong aria-hidden="true">12.7.</strong> 获取结构体字段标签</a>
|
||
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|
||
<li class="chapter-item expanded ">
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<a href="../ch12/ch12-08.html" ><strong aria-hidden="true">12.8.</strong> 显示一个类型的方法集</a>
|
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|
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<li class="chapter-item expanded ">
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<a href="../ch12/ch12-09.html" ><strong aria-hidden="true">12.9.</strong> 几点忠告</a>
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<a href="../ch13/ch13.html" ><strong aria-hidden="true">13.</strong> 底层编程</a>
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<ol class="section">
|
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<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-01.html" ><strong aria-hidden="true">13.1.</strong> unsafe.Sizeof, Alignof 和 Offsetof</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-02.html" ><strong aria-hidden="true">13.2.</strong> unsafe.Pointer</a>
|
||
</li>
|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-03.html" ><strong aria-hidden="true">13.3.</strong> 示例: 深度相等判断</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
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<a href="../ch13/ch13-04.html" ><strong aria-hidden="true">13.4.</strong> 通过cgo调用C代码</a>
|
||
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|
||
<li class="chapter-item expanded ">
|
||
<a href="../ch13/ch13-05.html" ><strong aria-hidden="true">13.5.</strong> 几点忠告</a>
|
||
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||
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|
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<a href="../appendix/appendix.html" ><strong aria-hidden="true">14.</strong> 附录</a>
|
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|
||
<ol class="section">
|
||
<li class="chapter-item expanded ">
|
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<a href="../appendix/appendix-a-errata.html" ><strong aria-hidden="true">14.1.</strong> 附录A：原文勘误</a>
|
||
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|
||
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|
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<a href="../appendix/appendix-b-author.html" ><strong aria-hidden="true">14.2.</strong> 附录B：作者译者</a>
|
||
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|
||
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|
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<a href="../appendix/appendix-c-cpoyright.html" ><strong aria-hidden="true">14.3.</strong> 附录C：译文授权</a>
|
||
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|
||
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|
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<a href="../appendix/appendix-d-translations.html" ><strong aria-hidden="true">14.4.</strong> 附录D：其它语言</a>
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<main>
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<ul dir="auto"><li><em>凹语言(Go实现, 面向WASM设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li><li><em>MnBook(Go语言实现的MD电子书构建工具): <a href="https://github.com/wa-lang/mnbook">https://github.com/wa-lang/mnbook</a></em></li></ul><hr>
|
||
|
||
<h2>1.4. GIF动画</h2>
|
||
<p>下面的程序会演示Go语言标准库里的image这个package的用法，我们会用这个包来生成一系列的bit-mapped图，然后将这些图片编码为一个GIF动画。我们生成的图形名字叫利萨如图形（Lissajous figures），这种效果是在1960年代的老电影里出现的一种视觉特效。它们是协振子在两个纬度上振动所产生的曲线，比如两个sin正弦波分别在x轴和y轴输入会产生的曲线。图1.1是这样的一个例子：</p>
|
||
<p><img src="../images/ch1-01.png" alt=""></p>
|
||
<p>译注：要看这个程序的结果，需要将标准输出重定向到一个GIF图像文件（使用 <code>./lissajous > output.gif</code> 命令）。下面是GIF图像动画效果：</p>
|
||
<p><img src="../images/ch1-01.gif" alt=""></p>
|
||
<p>这段代码里我们用了一些新的结构，包括const声明，struct结构体类型，复合声明。和我们举的其它的例子不太一样，这一个例子包含了浮点数运算。这些概念我们只在这里简单地说明一下，之后的章节会更详细地讲解。</p>
|
||
<p><!-- raw HTML omitted --><!-- raw HTML omitted -->gopl.io/ch1/lissajous<!-- raw HTML omitted --><!-- raw HTML omitted --></p>
|
||
<pre><code class="language-go">// Lissajous generates GIF animations of random Lissajous figures.
|
||
package main
|
||
|
||
import (
|
||
"image"
|
||
"image/color"
|
||
"image/gif"
|
||
"io"
|
||
"math"
|
||
"math/rand"
|
||
"os"
|
||
"time"
|
||
)
|
||
|
||
var palette = []color.Color{color.White, color.Black}
|
||
|
||
const (
|
||
whiteIndex = 0 // first color in palette
|
||
blackIndex = 1 // next color in palette
|
||
)
|
||
|
||
func main() {
|
||
// The sequence of images is deterministic unless we seed
|
||
// the pseudo-random number generator using the current time.
|
||
// Thanks to Randall McPherson for pointing out the omission.
|
||
rand.Seed(time.Now().UTC().UnixNano())
|
||
lissajous(os.Stdout)
|
||
}
|
||
|
||
func lissajous(out io.Writer) {
|
||
const (
|
||
cycles = 5 // number of complete x oscillator revolutions
|
||
res = 0.001 // angular resolution
|
||
size = 100 // image canvas covers [-size..+size]
|
||
nframes = 64 // number of animation frames
|
||
delay = 8 // delay between frames in 10ms units
|
||
)
|
||
|
||
freq := rand.Float64() * 3.0 // relative frequency of y oscillator
|
||
anim := gif.GIF{LoopCount: nframes}
|
||
phase := 0.0 // phase difference
|
||
for i := 0; i < nframes; i++ {
|
||
rect := image.Rect(0, 0, 2*size+1, 2*size+1)
|
||
img := image.NewPaletted(rect, palette)
|
||
for t := 0.0; t < cycles*2*math.Pi; t += res {
|
||
x := math.Sin(t)
|
||
y := math.Sin(t*freq + phase)
|
||
img.SetColorIndex(size+int(x*size+0.5), size+int(y*size+0.5),
|
||
blackIndex)
|
||
}
|
||
phase += 0.1
|
||
anim.Delay = append(anim.Delay, delay)
|
||
anim.Image = append(anim.Image, img)
|
||
}
|
||
gif.EncodeAll(out, &anim) // NOTE: ignoring encoding errors
|
||
}
|
||
|
||
</code></pre>
|
||
<p>当我们import了一个包路径包含有多个单词的package时，比如image/color（image和color两个单词），通常我们只需要用最后那个单词表示这个包就可以。所以当我们写color.White时，这个变量指向的是image/color包里的变量，同理gif.GIF是属于image/gif包里的变量。</p>
|
||
<p>这个程序里的常量声明给出了一系列的常量值，常量是指在程序编译后运行时始终都不会变化的值，比如圈数、帧数、延迟值。常量声明和变量声明一般都会出现在包级别，所以这些常量在整个包中都是可以共享的，或者你也可以把常量声明定义在函数体内部，那么这种常量就只能在函数体内用。目前常量声明的值必须是一个数字值、字符串或者一个固定的boolean值。</p>
|
||
<p>[]color.Color{...}和gif.GIF{...}这两个表达式就是我们说的复合声明（4.2和4.4.1节有说明）。这是实例化Go语言里的复合类型的一种写法。这里的前者生成的是一个slice切片，后者生成的是一个struct结构体。</p>
|
||
<p>gif.GIF是一个struct类型（参考4.4节）。struct是一组值或者叫字段的集合，不同的类型集合在一个struct可以让我们以一个统一的单元进行处理。anim是一个gif.GIF类型的struct变量。这种写法会生成一个struct变量，并且其内部变量LoopCount字段会被设置为nframes；而其它的字段会被设置为各自类型默认的零值。struct内部的变量可以以一个点（.）来进行访问，就像在最后两个赋值语句中显式地更新了anim这个struct的Delay和Image字段。</p>
|
||
<p>lissajous函数内部有两层嵌套的for循环。外层循环会循环64次，每一次都会生成一个单独的动画帧。它生成了一个包含两种颜色的201*201大小的图片，白色和黑色。所有像素点都会被默认设置为其零值（也就是调色板palette里的第0个值），这里我们设置的是白色。每次外层循环都会生成一张新图片，并将一些像素设置为黑色。其结果会append到之前结果之后。这里我们用到了append(参考4.2.1)内置函数，将结果append到anim中的帧列表末尾，并设置一个默认的80ms的延迟值。循环结束后所有的延迟值被编码进了GIF图片中，并将结果写入到输出流。out这个变量是io.Writer类型，这个类型支持把输出结果写到很多目标，很快我们就可以看到例子。</p>
|
||
<p>内层循环设置两个偏振值。x轴偏振使用sin函数。y轴偏振也是正弦波，但其相对x轴的偏振是一个0-3的随机值，初始偏振值是一个零值，随着动画的每一帧逐渐增加。循环会一直跑到x轴完成五次完整的循环。每一步它都会调用SetColorIndex来为(x,y)点来染黑色。</p>
|
||
<p>main函数调用lissajous函数，用它来向标准输出流打印信息，所以下面这个命令会像图1.1中产生一个GIF动画。</p>
|
||
<pre><code>$ go build gopl.io/ch1/lissajous
|
||
$ ./lissajous >out.gif
|
||
</code></pre>
|
||
<p><strong>练习 1.5：</strong> 修改前面的Lissajous程序里的调色板，由黑色改为绿色。我们可以用<code>color.RGBA{0xRR, 0xGG, 0xBB, 0xff}</code>来得到<code>#RRGGBB</code>这个色值，三个十六进制的字符串分别代表红、绿、蓝像素。</p>
|
||
<p><strong>练习 1.6：</strong> 修改Lissajous程序，修改其调色板来生成更丰富的颜色，然后修改SetColorIndex的第三个参数，看看显示结果吧。</p>
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