12. 面向对象篇¶

let mut post = Post::new();
post.add_text("今天学了 Rust");
assert_eq!(post.content(), "");          // 草稿不显示
post.request_review();
assert_eq!(post.content(), "");          // 审核中不显示
post.approve();
assert_eq!(post.content(), "今天学了 Rust");   // 已发布才显示

pub struct Post {
    // state 用 Box<dyn State> 存"当前状态"（trait 对象，运行期才知道具体状态）
    // Option 包装：转换状态时 take() 拿走旧状态，允许临时为空
    state: Option<Box<dyn State>>,
    content: String,
}

// 三个具体状态：空结构体，只承载"身份"
struct Draft;
struct PendingReview;
struct Published;

// 状态 trait：定义所有状态共有的行为
trait State {
    // self: Box<Self>：按值接收"装箱的自身"，消耗旧状态并返回新状态
    fn request_review(self: Box<Self>) -> Box<dyn State>;
    fn approve(self: Box<Self>) -> Box<dyn State>;
    // 默认实现：默认不显示内容，只有 Published 覆盖它
    fn content<'a>(&self, _post: &'a Post) -> &'a str {
        ""
    }
}

impl State for Draft {
    fn request_review(self: Box<Self>) -> Box<dyn State> {
        Box::new(PendingReview)   // 草稿 → 审核中
    }

    fn approve(self: Box<Self>) -> Box<dyn State> {
        self   // 草稿不能直接通过，保持原样
    }
}

impl State for PendingReview {
    fn request_review(self: Box<Self>) -> Box<dyn State> {
        self   // 审核中不能再提交
    }

    fn approve(self: Box<Self>) -> Box<dyn State> {
        Box::new(Published)   // 审核通过 → 已发布
    }
}

impl State for Published {
    fn request_review(self: Box<Self>) -> Box<dyn State> {
        self
    }

    fn approve(self: Box<Self>) -> Box<dyn State> {
        self
    }

    // 已发布才返回真正的正文
    fn content<'a>(&self, post: &'a Post) -> &'a str {
        &post.content
    }
}

impl Post {
    pub fn new() -> Self {
        Self {
            state: Some(Box::new(Draft)),   // 初始状态：草稿
            content: String::new(),
        }
    }

    pub fn add_text(&mut self, text: &str) {
        self.content.push_str(text);
    }

    // 委托给当前状态决定是否显示内容
    pub fn content(&self) -> &str {
        self.state.as_ref().unwrap().content(self)
    }

    pub fn request_review(&mut self) {
        // take() 拿走旧状态 → 调用其方法得到新状态
        if let Some(s) = self.state.take() {
            self.state = Some(s.request_review());
        }
    }

    pub fn approve(&mut self) {
        if let Some(s) = self.state.take() {
            self.state = Some(s.approve());
        }
    }
}

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

    #[test]
    fn draft_not_shown() {
        let mut post = Post::new();
        post.add_text("hello");
        assert_eq!(post.content(), "");
    }

    #[test]
    fn full_workflow() {
        let mut post = Post::new();
        post.add_text("hello");
        post.request_review();
        assert_eq!(post.content(), "");
        post.approve();
        assert_eq!(post.content(), "hello");
    }
}

use blog::Post;

fn main() {
    let mut post = Post::new();
    post.add_text("今天学了 Rust的状态模式");

    println!("草稿阶段，内容：{}", post.content());

    post.request_review();
    println!("审核中，内容：{}", post.content());

    post.approve();
    println!("已批准，内容：{}", post.content());
}