rust(十二)-命令行程序

• 接收命令行参数

• 读取文件

• 重构：改进模块和错误处理

• 使用TDD（测试驱动开发）开发库功能

• 使用环境变量

• 将错误消息写入标准错误而不是标准输出

• 做一个grep工具

  • cargo new project

    • cargo run **** ***.txt ``` use std::env;

      fn main() { let args: Vec = env::args().collect();

        let query = &args[1];
        let filename = &args[2];
      
        println!("Searching for {}", query);
        println!("In file {}", filename);
      

      }

  • 每个函数只做一件事情

• 二进制程序关注点分离的指导性原则

  • 将程序拆分为main.rs和lib.rs，将业务逻辑放入lib.rs
  • 当命令行解析逻辑较少时，将它放在main.rs也行
  • 当命令行解析逻辑变复杂时，需要将它从main.rs提取到lib.rs

• 经过上述拆分，留在main的功能有：

  • 使用参数值调用命令行解析逻辑
  • 进行其它配置
  • 调用lib.rs中的run函数
  • 处理run函数可能出现的错误

• 错误处理

  use std::env;
    use std::fs;
  
    fn main() {
        let args: Vec<String> = env::args().collect();
  
        let config = parse_config(&args);
  
        println!("Searching for {}", config.query);
        println!("In file {}", config.filename);
  
        let contents = fs::read_to_string(config.filename)
            .expect("Something went wrong reading the file");
  
        // --snip--
  
        println!("With text:\n{}", contents);
    }
  
    struct Config {
        query: String,
        filename: String,
    }
  
    fn parse_config(args: &[String]) -> Config {
        let query = args[1].clone();
        let filename = args[2].clone();
  
        Config { query, filename }
    }
  

• 将业务逻辑移至lib.rs

• 测试驱动开发 TDD（Test-Driven Development）

  • 编写一个会失败的测试，运行该测试，确保它是按照预期的原因失败
  • 编写或修改刚好足够的代码，让新测试通过
  • 重构刚刚添加或修改的代码，确保测试会始终通过
  • 返回步骤1，继续

use std::env;
// --snip--

use std::error::Error;
use std::fs;

pub struct Config {
    pub query: String,
    pub filename: String,
    pub ignore_case: bool,
}

impl Config {
    pub fn new(args: &[String]) -> Result<Config, &'static str> {
        if args.len() < 3 {
            return Err("not enough arguments");
        }

        let query = args[1].clone();
        let filename = args[2].clone();

        let ignore_case = env::var("IGNORE_CASE").is_ok();

        Ok(Config {
            query,
            filename,
            ignore_case,
        })
    }
}

pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
    let contents = fs::read_to_string(config.filename)?;

    let results = if config.ignore_case {
        search_case_insensitive(&config.query, &contents)
    } else {
        search(&config.query, &contents)
    };

    for line in results {
        println!("{}", line);
    }

    Ok(())
}

pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
    let mut results = Vec::new();

    for line in contents.lines() {
        if line.contains(query) {
            results.push(line);
        }
    }

    results
}

pub fn search_case_insensitive<'a>(
    query: &str,
    contents: &'a str,
) -> Vec<&'a str> {
    let query = query.to_lowercase();
    let mut results = Vec::new();

    for line in contents.lines() {
        if line.to_lowercase().contains(&query) {
            results.push(line);
        }
    }

    results
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn case_sensitive() {
        let query = "duct";
        let contents = "\
Rust:
safe, fast, productive.
Pick three.
Duct tape.";

        assert_eq!(vec!["safe, fast, productive."], search(query, contents));
    }

    #[test]
    fn case_insensitive() {
        let query = "rUsT";
        let contents = "\
Rust:
safe, fast, productive.
Pick three.
Trust me.";

        assert_eq!(
            vec!["Rust:", "Trust me."],
            search_case_insensitive(query, contents)
        );
    }
}

• 将错误信息输出输出到标准错误
• 标准输出 vs 标准错误
  • 标准输出： stdout
    • println!
  • 标准错误：stderr
    • eprintln!