//! State is a behavioral design pattern that lets an object alter its behavior when its internal state changes.//! It appears as if the object changed its class.//! We’ll implement a blog post workflow//! 1. A blog post starts as an empty draft.//! 2. When the draft is done, a review of the post is requested.//! 3. When the post is approved, it gets published.//! 4. Only published blog posts return content to print, so unapproved posts can’t accidentally be published.trait State {    fn request_review(self: Box<Self>) -> Box<dyn State>;    fn approve(self: Box<Self>) -> Box<dyn State>;    fn content<'a>(&self, _post: &'a Post) -> &'a str {        ""    }}struct Draft;impl State for Draft {    fn request_review(self: Box<Draft>) -> Box<dyn State> {        Box::new(PendingReview {})    }    fn approve(self: Box<Draft>) -> Box<dyn State> {        self    }}struct PendingReview;impl State for PendingReview {    fn request_review(self: Box<PendingReview>) -> Box<dyn State> {        self    }    fn approve(self: Box<PendingReview>) -> Box<dyn State> {        Box::new(Published {})    }}struct Published;impl State for Published {    fn request_review(self: Box<Published>) -> Box<dyn State> {        self    }    fn approve(self: Box<Published>) -> Box<dyn State> {        self    }    fn content<'a>(&self, post: &'a Post) -> &'a str {        &post.content    }}struct Post {    state: Option<Box<dyn State>>,    content: String,}impl Post {    fn new() -> Post {        Post {            state: Some(Box::new(Draft {})),            content: String::new(),        }    }    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) {        if let Some(s) = self.state.take() {            self.state = Some(s.request_review())        }    }    fn approve(&mut self) {        if let Some(s) = self.state.take() {            self.state = Some(s.approve())        }    }}fn main() {    let mut post = Post::new();    let text = "State is a behavioral design pattern.";    post.add_text(text);    assert_eq!("", post.content());    post.request_review();    assert_eq!("", post.content());    post.approve();    assert_eq!(text, post.content());    println!("post content: {}", post.content());}