< 返回我的博客

JasonkayZK 发表于 2022-11-29 22:49

Tags:Cargo,Features

通过一个例子学习Cargo Features

前言

本文只是通过一个例子来讲解如何使用 Cargo Features,并非讲解 Cargo Features 的全部特性以及底层实现;

同时,在阅读本文之前,请先阅读:

对 Cargo Features 有一定的基础,再来学习本文;

实现一个含有feature的lib

和上面 条件编译 Features 的内容保持一致,在例子中会定义这么几个 feature:

[features]
bmp = []
png = []
ico = ["bmp", "png"]
webp = []

bmp、png、ico、webp,并且 ico 依赖 bmp 和 png 特性;

创建 feature-demo crate:

cargo new feature-demo --lib

修改 Cargo.toml,增加 feature 定义:

feature-demo/Cargo.toml

+ [features]

+ default = []

+ full = [
+     "bmp",
+     "png",
+     "ico",
+     "webp"
+ ]

+ bmp = []
+ png = []
+ ico = ["bmp", "png"]
+ webp = []

同时实现各个 feature:

// feature-demo/src/lib.rs
pub mod bmp;
pub mod ico;
pub mod png;
pub mod webp;

// feature-demo/src/bmp.rs
#[cfg(feature = "bmp")]
pub fn process_bmp() {
    println!("Processing bmp");
}

// feature-demo/src/png.rs
#[cfg(feature = "png")]
pub fn process_png() {
    println!("Processing png");
}

// feature-demo/src/webp.rs
#[cfg(feature = "webp")]
pub fn process_webp() {
    println!("Processing webp");
}

// feature-demo/src/ico.rs
use crate::bmp::process_bmp;
use crate::png::process_png;

#[cfg(all(feature = "png", feature = "bmp"))]
#[cfg(feature = "ico")]
pub fn process_ico() {
    println!("Before process_ico: ");
    process_bmp();
    process_png();
    println!("Processing ico")
}

各个文件中的内容如上所示,其中 #[cfg(...)] 的意思是,只有定义了这个 feature 才编译下面的代码;

关于 cfg:

  • https://doc.rust-lang.org/stable/reference/conditional-compilation.html#the-cfg-attribute

同时,由于 ico 特性依赖 bmp、png 特性,因此需要先使用了 png、bmp 特性,同时使用了 ico 特性才会编译相应的代码;

至此,我们带有 feature 的 lib 编写完成;

使用一个含有feature的lib

下面来使用我们刚刚编写的含有 feature 的 lib;

首先引入这个 crate:

Cargo.toml

[dependencies]
feature-demo = { path = "./feature-demo", features = ["bmp", "ico"] }

这里引入了 bmp 和 ico 特性,但是由于 ico 特性依赖了 png 特性,因此实际上我们也是可以使用 png 特性的!

在 main 中使用这个 lib:

src/main.rs

use feature_demo::bmp::process_bmp;
use feature_demo::ico::process_ico;
use feature_demo::png::process_png;

fn main() {
    process_bmp();

    process_png();

    process_ico();
}

执行后输出:

Processing bmp
Processing png
Before process_ico: 
Processing bmp
Processing png
Processing ico

但是,由于我们没有引入 webp 特性,因此这部分代码实际上不会被编译,并且我们也无法使用!

例如,添加下面的代码,会造成编译失败:

use feature_demo::bmp::process_bmp;
use feature_demo::ico::process_ico;
use feature_demo::png::process_png;
+ use feature_demo::webp::process_webp;

fn main() {
    process_bmp();

    process_png();

    process_ico();
+
+    process_webp();
}

在添加了相关的 feature 之后,编译成功:

- feature-demo = { path = "./feature-demo", features = ["bmp", "ico"] }
+ feature-demo = { path = "./feature-demo", features = ["bmp", "ico", "webp"] }

附录

源代码:

  • https://github.com/JasonkayZK/rust-learn/tree/cargo-features

文章参考:

  • https://course.rs/cargo/reference/features/intro.html

评论区

写评论

还没有评论

1 共 0 条评论, 1 页