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< h1 id = "增加模块化和错误处理" > < a class = "header" href = "#增加模块化和错误处理" > 增加模块化和错误处理< / a > < / h1 >
< p > 但凡稍微没那么糟糕的程序，都应该具有代码模块化和错误处理，不然连玩具都谈不上。< / p >
< p > 梳理我们的代码和目标后，可以整理出大致四个改进点:< / p >
< ul >
< li > < strong > 单一且庞大的函数< / strong > 。对于 < code > minigrep< / code > 程序而言， < code > main< / code > 函数当前执行两个任务：解析命令行参数和读取文件。但随着代码的增加，< code > main< / code > 函数承载的功能也将快速增加。从软件工程角度来看，一个函数具有的功能越多，越是难以阅读和维护。因此最好的办法是将大的函数拆分成更小的功能单元。< / li >
< li > < strong > 配置变量散乱在各处< / strong > 。还有一点要考虑的是，当前 < code > main< / code > 函数中的变量都是独立存在的，这些变量很可能被整个程序所访问，在这个背景下，独立的变量越多，越是难以维护，因此我们还可以将这些用于配置的变量整合到一个结构体中。< / li >
< li > < strong > 细化错误提示< / strong > 。 目前的实现中，我们使用 < code > expect< / code > 方法来输出文件读取失败时的错误信息，这个没问题，但是无论任何情况下，都只输出 < code > Should have been able to read the file< / code > 这条错误提示信息，显然是有问题的，毕竟文件不存在、无权限等等都是可能的错误，一条大一统的消息无法给予用户更多的提示。< / li >
< li > < strong > 使用错误而不是异常< / strong > 。 假如用户不给任何命令行参数，那我们的程序显然会无情崩溃，原因很简单：< code > index out of bounds< / code > ，一个数组访问越界的 < code > panic< / code > ，但问题来了，用户能看懂吗？甚至于未来接手的维护者能看懂吗？因此需要增加合适的错误处理代码，来给予使用者详细、友善的提示。还有就是需要在一个统一的位置来处理所有错误，利人利己！< / li >
< / ul >
< h2 id = "分离-main-函数" > < a class = "header" href = "#分离-main-函数" > 分离 main 函数< / a > < / h2 >
< p > 关于如何处理庞大的 < code > main< / code > 函数， < / p >
< ul >
< li > 将程序分割为 < code > main.rs< / code > 和 < code > lib.rs< / code > ，并将程序的逻辑代码移动到后者内< / li >
< li > 命令行解析属于非常基础的功能，严格来说不算是逻辑代码的一部分，因此还可以放在 < code > main.rs< / code > 中< / li >
< / ul >
< p > 按照这个方案，将我们的代码重新梳理后，可以得出 < code > main< / code > 函数应该包含的功能:< / p >
< ul >
< li > 解析命令行参数< / li >
< li > 初始化其它配置< / li >
< li > 调用 < code > lib.rs< / code > 中的 < code > run< / code > 函数，以启动逻辑代码的运行< / li >
< li > 如果 < code > run< / code > 返回一个错误，需要对该错误进行处理< / li >
< / ul >
< p > 这个方案有一个很优雅的名字: 关注点分离(Separation of Concerns)。简而言之，< code > main.rs< / code > 负责启动程序，< code > lib.rs< / code > 负责逻辑代码的运行。从测试的角度而言，这种分离也非常合理： < code > lib.rs< / code > 中的主体逻辑代码可以得到简单且充分的测试，至于 < code > main.rs< / code > ？确实没办法针对其编写额外的测试代码，但是它的代码也很少啊，很容易就能保证它的正确性。< / p >
< blockquote >
< p > 关于如何在 Rust 中编写测试代码， ： < / p >
< / blockquote >
< h3 id = "分离命令行解析" > < a class = "header" href = "#分离命令行解析" > 分离命令行解析< / a > < / h3 >
< p > 根据之前的分析，我们需要将命令行解析的代码分离到一个单独的函数，然后将该函数放置在 < code > main.rs< / code > 中：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > // in main.rs
fn main() {
let args: Vec< String> = env::args().collect();
let (query, file_path) = parse_config(& args);
// --省略--
}
fn parse_config(args: & [String]) -> (& str, & str) {
let query = & args[1];
let file_path = & args[2];
(query, file_path)
}< / code > < / pre >
< p > 经过分离后，之前的设计目标完美达成，即精简了 < code > main< / code > 函数，又将配置相关的代码放在了 < code > main.rs< / code > 文件里。< / p >
< p > 看起来貌似是杀鸡用了牛刀，但是重构就是这样，一步一步，踏踏实实的前行，否则未来代码多一些后，你岂不是还要再重来一次重构？因此打好项目的基础是非常重要的！< / p >
< h3 id = "聚合配置变量" > < a class = "header" href = "#聚合配置变量" > 聚合配置变量< / a > < / h3 >
< p > 前文提到，配置变量并不适合分散的到处都是，因此使用一个结构体来统一存放是非常好的选择，这样修改后，后续的使用以及未来的代码维护都将更加简单明了。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let args: Vec< String> = env::args().collect();
let config = parse_config(& args);
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
let contents = fs::read_to_string(config.file_path)
.expect("Should have been able to read the file");
// --snip--
}
struct Config {
query: String,
file_path: String,
}
fn parse_config(args: & [String]) -> Config {
let query = args[1].clone();
let file_path = args[2].clone();
Config { query, file_path }
}< / code > < / pre >
< p > 值得注意的是，< code > Config< / code > 中存储的并不是 < code > & str< / code > 这样的引用类型，而是一个 < code > String< / code > 字符串，也就是 < code > Config< / code > 并没有去借用外部的字符串，而是拥有内部字符串的所有权。< code > clone< / code > 方法的使用也可以佐证这一点。大家可以尝试不用 < code > clone< / code > 方法，看看该如何解决相关的报错 :D< / p >
< blockquote >
< p > < code > clone< / code > 的得与失< / p >
< p > 在上面的代码中，除了使用 < code > clone< / code > ，还有其它办法来达成同样的目的，但 < code > clone< / code > 无疑是最简单的方法：直接完整的复制目标数据，无需被所有权、借用等问题所困扰，但是它也有其缺点，那就是有一定的性能损耗。< / p >
< p > 因此是否使用 < code > clone< / code > 更多是一种性能上的权衡，对于上面的使用而言，由于是配置的初始化，因此整个程序只需要执行一次，性能损耗几乎是可以忽略不计的。< / p >
< p > 总之，判断是否使用 < code > clone< / code > :< / p >
< ul >
< li > 是否严肃的项目，玩具项目直接用 < code > clone< / code > 就行，简单不好吗？< / li >
< li > 要看所在的代码路径是否是热点路径(hot path)，例如执行次数较多的显然就是热点路径，热点路径就值得去使用性能更好的实现方式< / li >
< / ul >
< / blockquote >
< p > 好了，言归正传，从 < code > C< / code > 语言过来的同学可能会觉得上面的代码已经很棒了，但是从 OO 语言角度来说，还差了那么一点意思。< / p >
< p > 下面我们试着来优化下，通过构造函数来初始化一个 < code > Config< / code > 实例，而不是直接通过函数返回实例，典型的，标准库中的 < code > String::new< / code > 函数就是一个范例。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let args: Vec< String> = env::args().collect();
let config = Config::new(& args);
// --snip--
}
// --snip--
impl Config {
fn new(args: & [String]) -> Config {
let query = args[1].clone();
let file_path = args[2].clone();
Config { query, file_path }
}
}< / code > < / pre >
< p > 修改后，类似 < code > String::new< / code > 的调用，我们可以通过 < code > Config::new< / code > 来创建一个实例，看起来代码是不是更有那味儿了 ： ） < / p >
< h2 id = "错误处理" > < a class = "header" href = "#错误处理" > 错误处理< / a > < / h2 >
< p > 回顾一下，如果用户不输入任何命令行参数，我们的程序会怎么样？< / p >
< pre > < code class = "language-shell" > $ cargo run
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.0s
Running `target/debug/minigrep`
thread 'main' panicked at 'index out of bounds: the len is 1 but the index is 1', src/main.rs:27:21
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
< / code > < / pre >
< p > 结果喜闻乐见，由于 < code > args< / code > 数组没有任何元素，因此通过索引访问时，会直接报出数组访问越界的 < code > panic< / code > 。< / p >
< p > 报错信息对于开发者会很明确，但是对于使用者而言，就相当难理解了，下面一起来解决它。< / p >
< h3 id = "改进报错信息" > < a class = "header" href = "#改进报错信息" > 改进报错信息< / a > < / h3 >
< p > 还记得在错误处理章节，我们提到过 < code > panic< / code > 的两种用法: 被动触发和主动调用嘛？上面代码的出现方式很明显是被动触发，这种报错信息是不可控的，下面我们先改成主动调用的方式:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // in main.rs
// --snip--
fn new(args: & [String]) -> Config {
if args.len() < 3 {
panic!("not enough arguments");
}
// --snip--
< span class = "boring" > }< / span > < / code > < / pre >
< p > 目的很明确，一旦传入的参数数组长度小于 3， ， < / p >
< pre > < code class = "language-shell" > $ cargo run
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.0s
Running `target/debug/minigrep`
thread 'main' panicked at 'not enough arguments', src/main.rs:26:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
< / code > < / pre >
< p > 不错，用户看到了更为明确的提示，但是还是有一大堆 < code > debug< / code > 输出，这些我们其实是不想让用户看到的。这么看来，想要输出对用户友好的信息, < code > panic< / code > 是不太适合的，它更适合告知开发者，哪里出现了问题。< / p >
< h3 id = "返回-result-来替代直接-panic" > < a class = "header" href = "#返回-result-来替代直接-panic" > 返回 Result 来替代直接 panic< / a > < / h3 >
< p > 那只能祭出之前学过的错误处理大法了，也就是返回一个 < code > Result< / code > ：成功时包含 < code > Config< / code > 实例，失败时包含一条错误信息。< / p >
< p > 有一点需要额外注意下，从代码惯例的角度出发，< code > new< / code > 往往不会失败，毕竟新建一个实例没道理失败，对不？因此修改为 < code > build< / code > 会更加合适。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl Config {
fn build(args: & [String]) -> Result< Config, & 'static str> {
if args.len() < 3 {
return Err("not enough arguments");
}
let query = args[1].clone();
let file_path = args[2].clone();
Ok(Config { query, file_path })
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这里的 < code > Result< / code > 可能包含一个 < code > Config< / code > 实例，也可能包含一条错误信息 < code > & static str< / code > ，不熟悉这种字符串类型的同学可以回头看看字符串章节，代码中的字符串字面量都是该类型，且拥有 < code > 'static< / code > 生命周期。< / p >
< h3 id = "处理返回的-result" > < a class = "header" href = "#处理返回的-result" > 处理返回的 Result< / a > < / h3 >
< p > 接下来就是在调用 < code > build< / code > 函数时，对返回的 < code > Result< / code > 进行处理了，目的就是给出准确且友好的报错提示, 为了让大家更好的回顾我们修改过的内容，这里给出整体代码:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::env;
use std::fs;
use std::process;
fn main() {
let args: Vec< String> = env::args().collect();
// 对 build 返回的 `Result` 进行处理
let config = Config::build(& args).unwrap_or_else(|err| {
println!("Problem parsing arguments: {err}");
process::exit(1);
});
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
let contents = fs::read_to_string(config.file_path)
.expect("Should have been able to read the file");
println!("With text:\n{contents}");
}
struct Config {
query: String,
file_path: String,
}
impl Config {
fn build(args: & [String]) -> Result< Config, & 'static str> {
if args.len() < 3 {
return Err("not enough arguments");
}
let query = args[1].clone();
let file_path = args[2].clone();
Ok(Config { query, file_path })
}
}< / code > < / pre >
< p > 上面代码有几点值得注意:< / p >
< ul >
< li > 当 < code > Result< / code > 包含错误时，我们不再调用 < code > panic< / code > 让程序崩溃，而是通过 < code > process::exit(1)< / code > 来终结进程，其中 < code > 1< / code > 是一个信号值(事实上非 0 值都可以)，通知调用我们程序的进程，程序是因为错误而退出的。< / li >
< li > < code > unwrap_or_else< / code > 是定义在 < code > Result< T,E> < / code > 上的常用方法，如果 < code > Result< / code > 是 < code > Ok< / code > ，那该方法就类似 < code > unwrap< / code > ：返回 < code > Ok< / code > 内部的值；如果是 < code > Err< / code > ，就调用< a href = "https://course.rs/advance/functional-programing/closure.html" > 闭包< / a > 中的自定义代码对错误进行进一步处理< / li >
< / ul >
< p > 综上可知，< code > config< / code > 变量的值是一个 < code > Config< / code > 实例，而 < code > unwrap_or_else< / code > 闭包中的 < code > err< / code > 参数，它的类型是 < code > 'static str< / code > ，值是 “not enough arguments” 那个字符串字面量。< / p >
< p > 运行后，可以看到以下输出：< / p >
< pre > < code class = "language-shell" > $ cargo run
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.48s
Running `target/debug/minigrep`
Problem parsing arguments: not enough arguments
< / code > < / pre >
< p > 终于，我们得到了自己想要的输出：既告知了用户为何报错，又消除了多余的 debug 信息，非常棒。可能有用户疑惑，< code > cargo run< / code > 底下还有一大堆 < code > debug< / code > 信息呢，实际上，这是 < code > cargo run< / code > 自带的，大家可以试试编译成二进制可执行文件后再调用，会是什么效果。< / p >
< h2 id = "分离主体逻辑" > < a class = "header" href = "#分离主体逻辑" > 分离主体逻辑< / a > < / h2 >
< p > 接下来可以继续精简 < code > main< / code > 函数，那就是将主体逻辑( 例如业务逻辑 )从 < code > main< / code > 中分离出去，这样 < code > main< / code > 函数就保留主流程调用，非常简洁。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > // in main.rs
fn main() {
let args: Vec< String> = env::args().collect();
let config = Config::build(& args).unwrap_or_else(|err| {
println!("Problem parsing arguments: {err}");
process::exit(1);
});
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
run(config);
}
fn run(config: Config) {
let contents = fs::read_to_string(config.file_path)
.expect("Should have been able to read the file");
println!("With text:\n{contents}");
}
// --snip--< / code > < / pre >
< p > 如上所示，< code > main< / code > 函数仅保留主流程各个环节的调用，一眼看过去非常简洁清晰。< / p >
< p > 继续之前，先请大家仔细看看 < code > run< / code > 函数，你们觉得还缺少什么？提示：参考 < code > build< / code > 函数的改进过程。< / p >
< h3 id = "使用--和特征对象来返回错误" > < a class = "header" href = "#使用--和特征对象来返回错误" > 使用 ? 和特征对象来返回错误< / a > < / h3 >
< p > 答案就是 < code > run< / code > 函数没有错误处理，因为在文章开头我们提到过，错误处理最好统一在一个地方完成，这样极其有利于后续的代码维护。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > //in main.rs
use std::error::Error;
// --snip--
fn run(config: Config) -> Result< (), Box< dyn Error> > {
let contents = fs::read_to_string(config.file_path)?;
println!("With text:\n{contents}");
Ok(())
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 值得注意的是这里的 < code > Result< (), Box< dyn Error> > < / code > 返回类型，首先我们的程序无需返回任何值，但是为了满足 < code > Result< T,E> < / code > 的要求，因此使用了 < code > Ok(())< / code > 返回一个单元类型 < code > ()< / code > 。< / p >
< p > 最重要的是 < code > Box< dyn Error> < / code > ， < code > Error< / code > 的特征对象(为了使用 < code > Error< / code > ，我们通过 < code > use std::error::Error;< / code > 进行了引入)，它表示函数返回一个类型，该类型实现了 < code > Error< / code > 特征，这样我们就无需指定具体的错误类型，否则你还需要查看 < code > fs::read_to_string< / code > 返回的错误类型，然后复制到我们的 < code > run< / code > 函数返回中，这么做一个是麻烦，最主要的是，一旦这么做，意味着我们无法在上层调用时统一处理错误，但是 < code > Box< dyn Error> < / code > 不同，其它函数也可以返回这个特征对象，然后调用者就可以使用统一的方式来处理不同函数返回的 < code > Box< dyn Error> < / code > 。< / p >
< p > 明白了 < code > Box< dyn Error> < / code > 的重要战略地位，接下来大家分析下，< code > fs::read_to_string< / code > 返回的具体错误类型是怎么被转化为 < code > Box< dyn Error> < / code > 的？其实原因在之前章节都有讲过，这里就不直接给出答案了，参见 < a href = "https://course.rs/basic/result-error/result.html#传播界的大明星-" > ?-传播界的大明星< / a > 。< / p >
< p > 运行代码看看效果：< / p >
< pre > < code class = "language-shell" > $ cargo run the poem.txt
Compiling minigrep v0.1.0 (file:///projects/minigrep)
warning: unused `Result` that must be used
--> src/main.rs:19:5
|
19 | run(config);
| ^^^^^^^^^^^^
|
= note: `#[warn(unused_must_use)]` on by default
= note: this `Result` may be an `Err` variant, which should be handled
warning: `minigrep` (bin "minigrep") generated 1 warning
Finished dev [unoptimized + debuginfo] target(s) in 0.71s
Running `target/debug/minigrep the poem.txt`
Searching for the
In file poem.txt
With text:
I'm nobody! Who are you?
Are you nobody, too?
Then there's a pair of us - don't tell!
They'd banish us, you know.
How dreary to be somebody!
How public, like a frog
To tell your name the livelong day
To an admiring bog!
< / code > < / pre >
< p > 没任何问题，不过 Rust 编译器也给出了善意的提示，那就是 < code > Result< / code > 并没有被使用，这可能意味着存在错误的潜在可能性。< / p >
< h3 id = "处理返回的错误" > < a class = "header" href = "#处理返回的错误" > 处理返回的错误< / a > < / h3 >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
// --snip--
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
if let Err(e) = run(config) {
println!("Application error: {e}");
process::exit(1);
}
}< / code > < / pre >
< p > 先回忆下在 < code > build< / code > 函数调用时，我们怎么处理错误的？然后与这里的方式做一下对比，是不是发现了一些区别？< / p >
< p > 没错 < code > if let< / code > 的使用让代码变得更简洁，可读性也更加好，原因是，我们并不关注 < code > run< / code > 返回的 < code > Ok< / code > 值，因此只需要用 < code > if let< / code > 去匹配是否存在错误即可。< / p >
< p > 好了，截止目前，代码看起来越来越美好了，距离我们的目标也只差一个：将主体逻辑代码分离到一个独立的文件 < code > lib.rs< / code > 中。< / p >
< h2 id = "分离逻辑代码到库包中" > < a class = "header" href = "#分离逻辑代码到库包中" > 分离逻辑代码到库包中< / a > < / h2 >
< blockquote >
< p > 对于 Rust 的代码组织( 包和模块 )还不熟悉的同学，强烈建议回头温习下< a href = "https://course.rs/basic/crate-module/intro.html" > 这一章< / a > 。< / p >
< / blockquote >
< p > 首先，创建一个 < code > src/lib.rs< / code > 文件，然后将所有的非 < code > main< / code > 函数都移动到其中。代码大概类似：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::error::Error;
use std::fs;
pub struct Config {
pub query: String,
pub file_path: String,
}
impl Config {
pub fn build(args: & [String]) -> Result< Config, & 'static str> {
// --snip--
}
}
pub fn run(config: Config) -> Result< (), Box< dyn Error> > {
// --snip--
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 为了内容的简洁性，这里忽略了具体的实现，下一步就是在 < code > main.rs< / code > 中引入 < code > lib.rs< / code > 中定义的 < code > Config< / code > 类型。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::env;
use std::process;
use minigrep::Config;
fn main() {
// --snip--
let args: Vec< String> = env::args().collect();
let config = Config::build(& args).unwrap_or_else(|err| {
println!("Problem parsing arguments: {err}");
process::exit(1);
});
println!("Searching for {}", config.query);
println!("In file {}", config.file_path);
if let Err(e) = minigrep::run(config) {
// --snip--
println!("Application error: {e}");
process::exit(1);
}
}< / code > < / pre >
< p > 很明显，这里的 < code > mingrep::run< / code > 的调用，以及 < code > Config< / code > 的引入，跟使用其它第三方包已经没有任何区别，也意味着我们成功的将逻辑代码放置到一个独立的库包中，其它包只要引入和调用就行。< / p >
< p > 呼，一顿书写猛如虎，回头一看。。。这么长的篇幅就写了这么点简单的代码？？只能说，我也希望像很多国内的大学教材一样，只要列出定理和解题方法，然后留下足够的习题，就万事大吉了，但是咱们不行。< / p >
< p > 接下来，到了最喜(令)闻(人)乐(讨)见(厌)的环节：写测试代码，一起来开心吧。< / p >
< / main >
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