17.3-1.rs

fn main() {
    let mut post = Post::new();

    post.add_text("This is a draft tweet");
    println!("printable content: {}", post.content());

    post.request_review();
    println!("printable content: {}", post.content());

    post.approve();
    println!("printable content: {}", post.content());

    post.reject();
    println!("printable content: {}", post.content());

    println!("fixing");
    post.add_text("This is a betterer");
    println!("printable content: {}", post.content());

    post.request_review();
    println!("printable content: {}", post.content());

    post.approve();
    println!("printable content: {}", post.content());

    post.approve();
    println!("printable content: {}", post.content());

    post.reject();
    println!("printable content: {}", post.content());
}

struct Post {
    state: Option<Box<dyn State>>,
    content: String,
}

impl Post {
    fn new() -> Self {
        Post {
            state: Some(Box::new(Draft {})),
            content: String::new(),
        }
    }

    fn add_text(&mut self, text: &str) {
        self.state.as_ref().unwrap().add_text(self, text);
    }
    fn _add_text(&mut self, text: &str) {
        self.content.push_str(text);
    }

    fn content(&self) -> &str {
        self.state.as_ref().unwrap().content(&self)
    }

    fn request_review(&mut self) {
        println!("requesting review");

        if let Some(s) = self.state.take() {
            self.state = Some(s.request_review())
        }
    }
        fn reject(&mut self) {
        println!("rejecting!");

        if let Some(s) = self.state.take() {
            self.state = Some(s.reject())
        }
    }

    fn approve(&mut self) {
        println!("approving");

        if let Some(s) = self.state.take() {
            self.state = Some(s.approve())
        }
    }
}

trait State {
    fn content<'a>(&self, _post: &'a Post) -> &'a str {
        ""
    }
    fn add_text(&self, _post: &mut Post, _text: &str) -> () {
        ()
    }
    fn request_review(self: Box<Self>) -> Box<dyn State>;
    fn reject(self: Box<Self>) -> Box<dyn State>;
    fn approve(self: Box<Self>) -> Box<dyn State>;
}

struct Draft {}

impl State for Draft {
    fn add_text(&self, post: &mut Post, text: &str) -> () {
        post._add_text(text);
    }
    fn request_review(self: Box<Self>) -> Box<dyn State> {
        Box::new(PendingReview::new())
    }
    fn reject(self: Box<Self>) -> Box<dyn State> {
        self
    }
    fn approve(self: Box<Self>) -> Box<dyn State> {
        self
    }
}

struct PendingReview {
    approvals: usize,
}

impl PendingReview {
    fn new() -> Self {
        PendingReview { approvals: 0 }
    }
}

impl State for PendingReview {
    fn request_review(self: Box<Self>) -> Box<dyn State> {
        self
    }
    fn reject(self: Box<Self>) -> Box<dyn State> {
        Box::new(Draft {})
    }
    fn approve(mut self: Box<Self>) -> Box<dyn State> {
        self.approvals += 1;
        if self.approvals >= 2 {
            Box::new(Published {})
        } else {
            self
        }
    }
}

struct Published {}

impl State for Published {
    fn content<'a>(&self, post: &'a Post) -> &'a str {
        &post.content
    }
    fn request_review(self: Box<Self>) -> Box<dyn State> {
        self
    }
    fn reject(self: Box<Self>) -> Box<dyn State> {
        self
    }
    fn approve(self: Box<Self>) -> Box<dyn State> {
        self
    }
}