concepts: Dynamic trait objects

[learn-rust.git] / concepts / src / lib.rs

diff --git a/concepts/src/lib.rs b/concepts/src/lib.rs
index b7e5eb8f1df11116f8d94fd31fb1afe29491d087..ca0624142dea4d751e4506184bf84508de7e0696 100644 (file)
--- a/concepts/src/lib.rs
+++ b/concepts/src/lib.rs
@@ -108,3 +108,86 @@ impl Iterator for Counter5 {
         }
     }
 }
+
+pub struct Post {
+    state: Option<Box<dyn State>>,
+    content: String,
+}
+
+impl Post {
+    pub fn new() -> Post {
+        Post {
+            state: Some(Box::new(Draft {})),
+            content: String::new(),
+        }
+    }
+
+    pub fn add_text(&mut self, text: &str) {
+        self.content.push_str(text);
+    }
+
+    pub fn content(&self) -> &str {
+        // as_ref() converts Option<Box<State>> to Option<&Box<State>>
+        // state can never be None, all state transitions return a new one
+        self.state.as_ref().unwrap().content(self)
+    }
+
+    pub fn request_review(&mut self) {
+        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());
+        }
+    }
+}
+
+trait State {
+    fn request_review(self: Box::<Self>) -> Box<dyn State>;
+    fn approve(self: Box::<Self>) -> Box<dyn State>;
+
+    #[allow(unused_variables)]
+    fn content<'a>(&self, post: &'a Post) -> &'a str {
+        ""
+    }
+}
+
+struct Draft {}
+impl State for Draft {
+    fn request_review(self: Box::<Self>) -> Box<dyn State> {
+        Box::new(PendingReview {})
+    }
+
+    fn approve(self: Box::<Self>) -> Box<dyn State> {
+        self
+    }
+}
+
+struct PendingReview {}
+impl State for PendingReview {
+    fn request_review(self: Box::<Self>) -> Box<dyn State> {
+        self
+    }
+
+    fn approve(self: Box::<Self>) -> Box<dyn State> {
+        Box::new(Published {})
+    }
+}
+
+struct Published {}
+impl State for Published {
+    fn request_review(self: Box::<Self>) -> Box<dyn State> {
+        self
+    }
+
+    fn approve(self: Box::<Self>) -> Box<dyn State> {
+        Box::new(Published {})
+    }
+
+    fn content<'a>(&self, post: &'a Post) -> &'a str {
+        &post.content
+    }
+}