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<title>顾及不同类型值的 trait 对象 - Rust 程序设计语言 简体中文版</title>
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<ol class="chapter"><li class="chapter-item expanded affix "><a href="title-page.html">Rust 程序设计语言</a></li><li class="chapter-item expanded affix "><a href="foreword.html">前言</a></li><li class="chapter-item expanded affix "><a href="ch00-00-introduction.html">简介</a></li><li class="chapter-item expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> 入门指南</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> 安装</a></li><li class="chapter-item expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="chapter-item expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> 写个猜数字游戏</a></li><li class="chapter-item expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> 常见编程概念</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> 变量与可变性</a></li><li class="chapter-item expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> 数据类型</a></li><li class="chapter-item expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> 函数</a></li><li class="chapter-item expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> 注释</a></li><li class="chapter-item expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> 控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> 认识所有权</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> 什么是所有权？</a></li><li class="chapter-item expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> 引用与借用</a></li><li class="chapter-item expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slice 类型</a></li></ol></li><li class="chapter-item expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> 使用结构体组织相关联的数据</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> 结构体的定义和实例化</a></li><li class="chapter-item expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> 结构体示例程序</a></li><li class="chapter-item expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> 方法语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> 枚举和模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> 枚举的定义</a></li><li class="chapter-item expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> match 控制流结构</a></li><li class="chapter-item expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> if let 简洁控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> 使用包、Crate 和模块管理不断增长的项目</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> 包和 Crate</a></li><li class="chapter-item expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> 定义模块来控制作用域与私有性</a></li><li class="chapter-item expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> 引用模块项目的路径</a></li><li class="chapter-item expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> 使用 use 关键字将路径引入作用域</a></li><li class="chapter-item expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> 将模块拆分成多个文件</a></li></ol></li><li class="chapter-item expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> 常见集合</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> 使用 Vector 储存列表</a></li><li class="chapter-item expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> 使用字符串储存 UTF-8 编码的文本</a></li><li class="chapter-item expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> 使用 Hash Map 储存键值对</a></li></ol></li><li class="chapter-item expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> 错误处理</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> 用 panic! 处理不可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> 用 Result 处理可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> 要不要 panic!</a></li></ol></li><li class="chapter-item expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> 泛型、Trait 和生命周期</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> 泛型数据类型</a></li><li class="chapter-item expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Trait：定义共同行为</a></li><li class="chapter-item expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> 生命周期确保引用有效</a></li></ol></li><li class="chapter-item expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> 编写自动化测试</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> 如何编写测试</a></li><li class="chapter-item expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> 控制测试如何运行</a></li><li class="chapter-item expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> 测试的组织结构</a></li></ol></li><li class="chapter-item expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> 一个 I/O 项目：构建命令行程序</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> 接受命令行参数</a></li><li class="chapter-item expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> 读取文件</a></li><li class="chapter-item expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> 重构以改进模块化与错误处理</a></li><li class="chapter-item expanded "><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> 采用测试驱动开发完善库的功能</a></li><li class="chapter-item expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> 处理环境变量</a></li><li class="chapter-item expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> 将错误信息输出到标准错误而不是标准输出</a></li></ol></li><li class="chapter-item expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Rust 中的函数式语言功能：迭代器与闭包</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> 闭包：可以捕获其环境的匿名函数</a></li><li class="chapter-item expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> 使用迭代器处理元素序列</a></li><li class="chapter-item expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> 改进之前的 I/O 项目</a></li><li class="chapter-item expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> 性能比较：循环对迭代器</a></li></ol></li><li class="chapter-item expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> 更多关于 Cargo 和 Crates.io 的内容</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> 采用发布配置自定义构建</a></li><li class="chapter-item expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> 将 crate 发布到 Crates.io</a></li><li class="chapter-item expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo 工作空间</a></li><li class="chapter-item expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> 使用 cargo install 安装二进制文件</a></li><li class="chapter-item expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Cargo 自定义扩展命令</a></li></ol></li><li class="chapter-item expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> 智能指针</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> 使用 Box<T> 指向堆上数据</a></li><li class="chapter-item expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> 使用 Deref Trait 将智能指针当作常规引用处理</a></li><li class="chapter-item expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> 使用 Drop Trait 运行清理代码</a></li><li class="chapter-item expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T> 引用计数智能指针</a></li><li class="chapter-item expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> 与内部可变性模式</a></li><li class="chapter-item expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> 引用循环会导致内存泄漏</a></li></ol></li><li class="chapter-item expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> 无畏并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> 使用线程同时地运行代码</a></li><li class="chapter-item expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> 使用消息传递在线程间通信</a></li><li class="chapter-item expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> 共享状态并发</a></li><li class="chapter-item expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> 使用 Sync 与 Send Traits 的可扩展并发</a></li></ol></li><li class="chapter-item expanded "><a href="ch17-00-async-await.html"><strong aria-hidden="true">17.</strong> Async 和 await</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch17-01-futures-and-syntax.html"><strong aria-hidden="true">17.1.</strong> Futures 和 async 语法</a></li><li class="chapter-item expanded "><a href="ch17-02-concurrency-with-async.html"><strong aria-hidden="true">17.2.</strong> 并发与 async</a></li><li class="chapter-item expanded "><a href="ch17-03-more-futures.html"><strong aria-hidden="true">17.3.</strong> 使用任意数量的 futures</a></li><li class="chapter-item expanded "><a href="ch17-04-streams.html"><strong aria-hidden="true">17.4.</strong> 流（Streams）</a></li><li class="chapter-item expanded "><a href="ch17-05-traits-for-async.html"><strong aria-hidden="true">17.5.</strong> 深入理解 async 相关的 traits</a></li><li class="chapter-item expanded "><a href="ch17-06-futures-tasks-threads.html"><strong aria-hidden="true">17.6.</strong> Futures，任务（tasks）和线程（threads）</a></li></ol></li><li class="chapter-item expanded "><a href="ch18-00-oop.html"><strong aria-hidden="true">18.</strong> Rust 的面向对象编程特性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch18-01-what-is-oo.html"><strong aria-hidden="true">18.1.</strong> 面向对象语言的特点</a></li><li class="chapter-item expanded "><a href="ch18-02-trait-objects.html" class="active"><strong aria-hidden="true">18.2.</strong> 顾及不同类型值的 trait 对象</a></li><li class="chapter-item expanded "><a href="ch18-03-oo-design-patterns.html"><strong aria-hidden="true">18.3.</strong> 面向对象设计模式的实现</a></li></ol></li><li class="chapter-item expanded "><a href="ch19-00-patterns.html"><strong aria-hidden="true">19.</strong> 模式与模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch19-01-all-the-places-for-patterns.html"><strong aria-hidden="true">19.1.</strong> 所有可能会用到模式的位置</a></li><li class="chapter-item expanded "><a href="ch19-02-refutability.html"><strong aria-hidden="true">19.2.</strong> Refutability（可反驳性）: 模式是否会匹配失效</a></li><li class="chapter-item expanded "><a href="ch19-03-pattern-syntax.html"><strong aria-hidden="true">19.3.</strong> 模式语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch20-00-advanced-features.html"><strong aria-hidden="true">20.</strong> 高级特征</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch20-01-unsafe-rust.html"><strong aria-hidden="true">20.1.</strong> 不安全的 Rust</a></li><li class="chapter-item expanded "><a href="ch20-03-advanced-traits.html"><strong aria-hidden="true">20.2.</strong> 高级 trait</a></li><li class="chapter-item expanded "><a href="ch20-04-advanced-types.html"><strong aria-hidden="true">20.3.</strong> 高级类型</a></li><li class="chapter-item expanded "><a href="ch20-05-advanced-functions-and-closures.html"><strong aria-hidden="true">20.4.</strong> 高级函数与闭包</a></li><li class="chapter-item expanded "><a href="ch20-06-macros.html"><strong aria-hidden="true">20.5.</strong> 宏</a></li></ol></li><li class="chapter-item expanded "><a href="ch21-00-final-project-a-web-server.html"><strong aria-hidden="true">21.</strong> 最后的项目：构建多线程 web server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch21-01-single-threaded.html"><strong aria-hidden="true">21.1.</strong> 建立单线程 web server</a></li><li class="chapter-item expanded "><a href="ch21-02-multithreaded.html"><strong aria-hidden="true">21.2.</strong> 将单线程 server 变为多线程 server</a></li><li class="chapter-item expanded "><a href="ch21-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">21.3.</strong> 优雅停机与清理</a></li></ol></li><li class="chapter-item expanded "><a href="appendix-00.html"><strong aria-hidden="true">22.</strong> 附录</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">22.1.</strong> A - 关键字</a></li><li class="chapter-item expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">22.2.</strong> B - 运算符与符号</a></li><li class="chapter-item expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">22.3.</strong> C - 可派生的 trait</a></li><li class="chapter-item expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">22.4.</strong> D - 实用开发工具</a></li><li class="chapter-item expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">22.5.</strong> E - 版本</a></li><li class="chapter-item expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">22.6.</strong> F - 本书译本</a></li><li class="chapter-item expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">22.7.</strong> G - Rust 是如何开发的与 “Nightly Rust”</a></li></ol></li></ol>
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<h2 id="顾及不同类型值的-trait-对象"><a class="header" href="#顾及不同类型值的-trait-对象">顾及不同类型值的 trait 对象</a></h2>
|
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<blockquote>
|
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch18-02-trait-objects.md">ch18-02-trait-objects.md</a>
|
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<br>
|
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commit 96d4b0ec1c5e019b85604c33ceee68b3e2669d40</p>
|
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</blockquote>
|
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<p>在第八章中，我们谈到了 vector 只能存储同种类型元素的局限。示例 8-9 中提供了一个替代方案，通过定义 <code>SpreadsheetCell</code> 枚举，来储存整型、浮点型或文本类型的成员。这意味着，我们可以在每个单元中储存不同类型的数据，并仍能拥有一个代表一排单元的 vector。只要我们需存储的值由一组固定的类型组成，并且在代码编译时就知道具体会有哪些类型，那么这种使用枚举的办法是完全可行的。</p>
|
||
<p>然而有时我们希望库用户在特定情况下能够扩展有效的类型集合。为了展示如何实现这一点，这里将创建一个图形用户接口（Graphical User Interface，GUI）工具的例子，它通过遍历列表并调用每一个项目的 <code>draw</code> 方法来将其绘制到屏幕上 —— 此乃一个 GUI 工具的常见技术。我们将要创建一个叫做 <code>gui</code> 的库 crate，它含一个 GUI 库的结构。这个 GUI 库包含一些可供开发者使用的类型，比如 <code>Button</code> 或 <code>TextField</code>。在此之上，<code>gui</code> 的用户希望创建自定义的可以绘制于屏幕上的类型：比如，一个程序员可能会增加 <code>Image</code>，另一个可能会增加 <code>SelectBox</code>。</p>
|
||
<p>这个例子中并不会实现一个功能完善的 GUI 库，不过会展示其中各个部分是如何结合在一起的。编写库的时候，我们不可能知晓并定义所有其他程序员希望创建的类型。我们所知晓的是 <code>gui</code> 需要记录一系列不同类型的值，并需要能够对其中每一个值调用 <code>draw</code> 方法。这里无需知道调用 <code>draw</code> 方法时具体会发生什么，只要该值会有那个方法可供我们调用。</p>
|
||
<p>在拥有继承的语言中，可以定义一个名为 <code>Component</code> 的类，该类上有一个 <code>draw</code> 方法。其他的类比如 <code>Button</code>、<code>Image</code> 和 <code>SelectBox</code> 会从 <code>Component</code> 派生并因此继承 <code>draw</code> 方法。它们各自都可以覆盖 <code>draw</code> 方法来定义自己的行为，但是框架会把所有这些类型当作是 <code>Component</code> 的实例，并在其上调用 <code>draw</code>。不过 Rust 并没有继承，我们得另寻出路。</p>
|
||
<h3 id="定义通用行为的-trait"><a class="header" href="#定义通用行为的-trait">定义通用行为的 trait</a></h3>
|
||
<p>为了实现 <code>gui</code> 所期望的行为，让我们定义一个 <code>Draw</code> trait，其中包含名为 <code>draw</code> 的方法。接着可以定义一个存放 <strong>trait 对象</strong>（<em>trait object</em>）的 vector。trait 对象指向一个实现了我们指定 trait 的类型的实例，以及一个用于在运行时查找该类型的 trait 方法的表。我们通过指定某种指针来创建 trait 对象，例如 <code>&</code> 引用或 <code>Box<T></code> 智能指针，还有 <code>dyn</code> keyword，以及指定相关的 trait（第二十章 <a href="ch20-04-advanced-types.html#%E5%8A%A8%E6%80%81%E5%A4%A7%E5%B0%8F%E7%B1%BB%E5%9E%8B%E5%92%8C-sized-trait">“动态大小类型和 <code>Sized</code> trait”</a> 部分会介绍 trait 对象必须使用指针的原因）。我们可以使用 trait 对象代替泛型或具体类型。任何使用 trait 对象的位置，Rust 的类型系统会在编译时确保任何在此上下文中使用的值会实现其 trait 对象的 trait。如此便无需在编译时就知晓所有可能的类型。</p>
|
||
<p>之前提到过，Rust 刻意不将结构体与枚举称为 “对象”，以便与其他语言中的对象相区别。在结构体或枚举中，结构体字段中的数据和 <code>impl</code> 块中的行为是分开的，不同于其他语言中将数据和行为组合进一个称为对象的概念中。trait 对象将数据和行为两者相结合，从这种意义上说 <strong>则</strong> 其更类似其他语言中的对象。不过 trait 对象不同于传统的对象，因为不能向 trait 对象增加数据。trait 对象并不像其他语言中的对象那么通用：其（trait 对象）具体的作用是允许对通用行为进行抽象。</p>
|
||
<p>示例 17-3 展示了如何定义一个带有 <code>draw</code> 方法的 trait <code>Draw</code>：</p>
|
||
<p><span class="filename">文件名：src/lib.rs</span></p>
|
||
<pre><code class="language-rust noplayground">pub trait Draw {
|
||
fn draw(&self);
|
||
}</code></pre>
|
||
<p><span class="caption">示例 17-3：<code>Draw</code> trait 的定义</span></p>
|
||
<p>因为第十章已经讨论过如何定义 trait，其语法看起来应该比较眼熟。接下来就是新内容了：示例 17-4 定义了一个存放了名叫 <code>components</code> 的 vector 的结构体 <code>Screen</code>。这个 vector 的类型是 <code>Box<dyn Draw></code>，此为一个 trait 对象：它是 <code>Box</code> 中任何实现了 <code>Draw</code> trait 的类型的替身。</p>
|
||
<p><span class="filename">文件名：src/lib.rs</span></p>
|
||
<pre><code class="language-rust noplayground"><span class="boring">pub trait Draw {
|
||
</span><span class="boring"> fn draw(&self);
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>pub struct Screen {
|
||
pub components: Vec<Box<dyn Draw>>,
|
||
}</code></pre>
|
||
<p><span class="caption">示例 17-4: 一个 <code>Screen</code> 结构体的定义，它带有一个字段 <code>components</code>，其包含实现了 <code>Draw</code> trait 的 trait 对象的 vector</span></p>
|
||
<p>在 <code>Screen</code> 结构体上，我们将定义一个 <code>run</code> 方法，该方法会对其 <code>components</code> 上的每一个组件调用 <code>draw</code> 方法，如示例 17-5 所示：</p>
|
||
<p><span class="filename">文件名：src/lib.rs</span></p>
|
||
<pre><code class="language-rust noplayground"><span class="boring">pub trait Draw {
|
||
</span><span class="boring"> fn draw(&self);
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">pub struct Screen {
|
||
</span><span class="boring"> pub components: Vec<Box<dyn Draw>>,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>impl Screen {
|
||
pub fn run(&self) {
|
||
for component in self.components.iter() {
|
||
component.draw();
|
||
}
|
||
}
|
||
}</code></pre>
|
||
<p><span class="caption">示例 17-5：在 <code>Screen</code> 上实现一个 <code>run</code> 方法，该方法在每个 component 上调用 <code>draw</code> 方法</span></p>
|
||
<p>这与定义使用了带有 trait bound 的泛型类型参数的结构体不同。泛型类型参数一次只能替代一个具体类型，而 trait 对象则允许在运行时替代多种具体类型。例如，可以定义 <code>Screen</code> 结构体来使用泛型和 trait bound，如示例 17-6 所示：</p>
|
||
<p><span class="filename">文件名：src/lib.rs</span></p>
|
||
<pre><code class="language-rust noplayground"><span class="boring">pub trait Draw {
|
||
</span><span class="boring"> fn draw(&self);
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>pub struct Screen<T: Draw> {
|
||
pub components: Vec<T>,
|
||
}
|
||
|
||
impl<T> Screen<T>
|
||
where
|
||
T: Draw,
|
||
{
|
||
pub fn run(&self) {
|
||
for component in self.components.iter() {
|
||
component.draw();
|
||
}
|
||
}
|
||
}</code></pre>
|
||
<p><span class="caption">示例 17-6: 一种 <code>Screen</code> 结构体的替代实现，其 <code>run</code> 方法使用泛型和 trait bound</span></p>
|
||
<p>这限制了 <code>Screen</code> 实例必须拥有一个全是 <code>Button</code> 类型或者全是 <code>TextField</code> 类型的组件列表。如果只需要同质（相同类型）集合，则倾向于使用泛型和 trait bound，因为其定义会在编译时采用具体类型进行单态化。</p>
|
||
<p>另一方面，通过使用 trait 对象的方法，一个 <code>Screen</code> 实例可以存放一个既能包含 <code>Box<Button></code>，也能包含 <code>Box<TextField></code> 的 <code>Vec<T></code>。让我们看看它是如何工作的，接着会讲到其运行时性能影响。</p>
|
||
<h3 id="实现-trait"><a class="header" href="#实现-trait">实现 trait</a></h3>
|
||
<p>现在来增加一些实现了 <code>Draw</code> trait 的类型。我们将提供 <code>Button</code> 类型。再一次重申，真正实现 GUI 库超出了本书的范畴，所以 <code>draw</code> 方法体中不会有任何有意义的实现。为了想象一下这个实现看起来像什么，一个 <code>Button</code> 结构体可能会拥有 <code>width</code>、<code>height</code> 和 <code>label</code> 字段，如示例 17-7 所示：</p>
|
||
<p><span class="filename">文件名：src/lib.rs</span></p>
|
||
<pre><code class="language-rust noplayground"><span class="boring">pub trait Draw {
|
||
</span><span class="boring"> fn draw(&self);
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">pub struct Screen {
|
||
</span><span class="boring"> pub components: Vec<Box<dyn Draw>>,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl Screen {
|
||
</span><span class="boring"> pub fn run(&self) {
|
||
</span><span class="boring"> for component in self.components.iter() {
|
||
</span><span class="boring"> component.draw();
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>pub struct Button {
|
||
pub width: u32,
|
||
pub height: u32,
|
||
pub label: String,
|
||
}
|
||
|
||
impl Draw for Button {
|
||
fn draw(&self) {
|
||
// code to actually draw a button
|
||
}
|
||
}</code></pre>
|
||
<p><span class="caption">示例 17-7: 一个实现了 <code>Draw</code> trait 的 <code>Button</code> 结构体</span></p>
|
||
<p>在 <code>Button</code> 上的 <code>width</code>、<code>height</code> 和 <code>label</code> 字段会和其他组件不同，比如 <code>TextField</code> 可能有 <code>width</code>、<code>height</code>、<code>label</code> 以及 <code>placeholder</code> 字段。每一个我们希望能在屏幕上绘制的类型都会使用不同的代码来实现 <code>Draw</code> trait 的 <code>draw</code> 方法来定义如何绘制特定的类型，像这里的 <code>Button</code> 类型（如上提到的并不包含任何实际的 GUI 代码）。除了实现 <code>Draw</code> trait 之外，比如 <code>Button</code> 还可能有另一个包含按钮点击如何响应的方法的 <code>impl</code> 块。这类方法并不适用于像 <code>TextField</code> 这样的类型。</p>
|
||
<p>如果一些库的使用者决定实现一个包含 <code>width</code>、<code>height</code> 和 <code>options</code> 字段的结构体 <code>SelectBox</code>，并且也为其实现了 <code>Draw</code> trait，如示例 17-8 所示：</p>
|
||
<p><span class="filename">文件名：src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore">use gui::Draw;
|
||
|
||
struct SelectBox {
|
||
width: u32,
|
||
height: u32,
|
||
options: Vec<String>,
|
||
}
|
||
|
||
impl Draw for SelectBox {
|
||
fn draw(&self) {
|
||
// code to actually draw a select box
|
||
}
|
||
}
|
||
<span class="boring">
|
||
</span><span class="boring">fn main() {}</span></code></pre>
|
||
<p><span class="caption">示例 17-8: 另一个使用 <code>gui</code> 的 crate 中，在 <code>SelectBox</code> 结构体上实现 <code>Draw</code> trait</span></p>
|
||
<p>库使用者现在可以在他们的 <code>main</code> 函数中创建一个 <code>Screen</code> 实例。至此可以通过将 <code>SelectBox</code> 和 <code>Button</code> 放入 <code>Box<T></code> 转变为 trait 对象再放入 <code>Screen</code> 实例中。接着可以调用 <code>Screen</code> 的 <code>run</code> 方法，它会调用每个组件的 <code>draw</code> 方法。示例 17-9 展示了这个实现：</p>
|
||
<p><span class="filename">文件名：src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore"><span class="boring">use gui::Draw;
|
||
</span><span class="boring">
|
||
</span><span class="boring">struct SelectBox {
|
||
</span><span class="boring"> width: u32,
|
||
</span><span class="boring"> height: u32,
|
||
</span><span class="boring"> options: Vec<String>,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl Draw for SelectBox {
|
||
</span><span class="boring"> fn draw(&self) {
|
||
</span><span class="boring"> // code to actually draw a select box
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>use gui::{Button, Screen};
|
||
|
||
fn main() {
|
||
let screen = Screen {
|
||
components: vec![
|
||
Box::new(SelectBox {
|
||
width: 75,
|
||
height: 10,
|
||
options: vec![
|
||
String::from("Yes"),
|
||
String::from("Maybe"),
|
||
String::from("No"),
|
||
],
|
||
}),
|
||
Box::new(Button {
|
||
width: 50,
|
||
height: 10,
|
||
label: String::from("OK"),
|
||
}),
|
||
],
|
||
};
|
||
|
||
screen.run();
|
||
}</code></pre>
|
||
<p><span class="caption">示例 17-9: 使用 trait 对象来存储实现了相同 trait 的不同类型的值</span></p>
|
||
<p>当编写库的时候，我们不知道何人会在何时增加 <code>SelectBox</code> 类型，不过 <code>Screen</code> 的实现能够操作并绘制这个新类型，因为 <code>SelectBox</code> 实现了 <code>Draw</code> trait，这意味着它实现了 <code>draw</code> 方法。</p>
|
||
<p>这个概念 —— 只关心值所反映的信息而不是其具体类型 —— 类似于动态类型语言中称为 <strong>鸭子类型</strong>（<em>duck typing</em>）的概念：如果它走起来像一只鸭子，叫起来像一只鸭子，那么它就是一只鸭子！在示例 17-5 中 <code>Screen</code> 上的 <code>run</code> 实现中，<code>run</code> 并不需要知道各个组件的具体类型是什么。它并不检查组件是 <code>Button</code> 或者 <code>SelectBox</code> 的实例。通过指定 <code>Box<dyn Draw></code> 作为 <code>components</code> vector 中值的类型，我们就定义了 <code>Screen</code> 为需要可以在其上调用 <code>draw</code> 方法的值。</p>
|
||
<p>使用 trait 对象和 Rust 类型系统来进行类似鸭子类型操作的优势是无需在运行时检查一个值是否实现了特定方法或者担心在调用时因为值没有实现方法而产生错误。如果值没有实现 trait 对象所需的 trait 则 Rust 不会编译这些代码。</p>
|
||
<p>例如，示例 17-10 展示了当创建一个使用 <code>String</code> 做为其组件的 <code>Screen</code> 时发生的情况：</p>
|
||
<p><span class="filename">文件名：src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">use gui::Screen;
|
||
|
||
fn main() {
|
||
let screen = Screen {
|
||
components: vec![Box::new(String::from("Hi"))],
|
||
};
|
||
|
||
screen.run();
|
||
}</code></pre>
|
||
<p><span class="caption">示例 17-10: 尝试使用一种没有实现 trait 对象的 trait 的类型</span></p>
|
||
<p>我们会遇到这个错误，因为 <code>String</code> 没有实现 <code>rust_gui::Draw</code> trait：</p>
|
||
<pre><code class="language-console">$ cargo run
|
||
Compiling gui v0.1.0 (file:///projects/gui)
|
||
error[E0277]: the trait bound `String: Draw` is not satisfied
|
||
--> src/main.rs:5:26
|
||
|
|
||
5 | components: vec![Box::new(String::from("Hi"))],
|
||
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Draw` is not implemented for `String`
|
||
|
|
||
= help: the trait `Draw` is implemented for `Button`
|
||
= note: required for the cast from `Box<String>` to `Box<dyn Draw>`
|
||
|
||
For more information about this error, try `rustc --explain E0277`.
|
||
error: could not compile `gui` (bin "gui") due to 1 previous error
|
||
</code></pre>
|
||
<p>这告诉了我们，要么是我们传递了并不希望传递给 <code>Screen</code> 的类型并应该提供其他类型，要么应该在 <code>String</code> 上实现 <code>Draw</code> 以便 <code>Screen</code> 可以调用其上的 <code>draw</code>。</p>
|
||
<h3 id="trait-对象执行动态分发"><a class="header" href="#trait-对象执行动态分发">trait 对象执行动态分发</a></h3>
|
||
<p>回忆一下第十章 <a href="ch10-01-syntax.html#%E6%B3%9B%E5%9E%8B%E4%BB%A3%E7%A0%81%E7%9A%84%E6%80%A7%E8%83%BD">“泛型代码的性能”</a> 部分讨论过的，当对泛型使用 trait bound 时编译器所执行的单态化处理：编译器为每一个被泛型类型参数代替的具体类型生成了函数和方法的非泛型实现。单态化产生的代码在执行 <strong>静态分发</strong>（<em>static dispatch</em>）。静态分发发生于编译器在编译时就知晓调用了什么方法的时候。这与 <strong>动态分发</strong> （<em>dynamic dispatch</em>）相对，这时编译器在编译时无法知晓调用了什么方法。在动态分发的场景下，编译器会生成负责在运行时确定该调用什么方法的代码。</p>
|
||
<p>当使用 trait 对象时，Rust 必须使用动态分发。编译器无法知晓所有可能用于 trait 对象代码的类型，所以它也不知道应该调用哪个类型的哪个方法实现。为此，Rust 在运行时使用 trait 对象中的指针来知晓需要调用哪个方法。动态分发也阻止编译器有选择的内联方法代码，这会相应的禁用一些优化。尽管在编写示例 17-5 和可以支持示例 17-9 中的代码的过程中确实获得了额外的灵活性，但仍然需要权衡取舍。</p>
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