Implementing an Object-Oriented Design Pattern - The Rust Programming Language

## Implementing an Object-Oriented Design Pattern

The state pattern is an object-oriented design pattern. The crux of the pattern is that a value has some internal state, which is represented by a set of state objects, and the value’s behavior changes based on the internal state. The state objects share functionality: in Rust, of course, we use structs and traits rather than objects and inheritance. Each state object is responsible for its own behavior and for governing when it should change into another state. The value that holds a state object knows nothing about the different behavior of the states or when to transition between states.

Using the state pattern means when the business requirements of the program change, we won’t need to change the code of the value holding the state or the code that uses the value. We’ll only need to update the code inside one of the state objects to change its rules or perhaps add more state objects. Let’s look at an example of the state design pattern and how to use it in Rust.

We’ll implement a blog post workflow in an incremental way. The blog’s final functionality will look like this:

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.

Any other changes attempted on a post should have no effect. For example, if we try to approve a draft blog post before we’ve requested a review, the post should remain an unpublished draft.

Listing 17-11 shows this workflow in code form: this is an example usage of the API we’ll implement in a library crate named `blog`. This won’t compile yet because we haven’t implemented the `blog` crate yet.

Filename: src/main.rs

    extern crate blog;
    use blog::Post;

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

        post.add_text("I ate a salad for lunch today");
        assert_eq!("", post.content());

        post.request_review();
        assert_eq!("", post.content());

        post.approve();
        assert_eq!("I ate a salad for lunch today", post.content());
    }

Listing 17-11: Code that demonstrates the desired behavior we want our `blog` crate to have

We want to allow the user to create a new draft blog post with `Post::new`. Then we want to allow text to be added to the blog post while it’s in the draft state. If we try to get the post’s content immediately, before approval, nothing should happen because the post is still a draft. We’ve added `assert_eq!` in the code for demonstration purposes. An excellent unit test for this would be to assert that a draft blog post returns an empty string from the `content` method, but we’re not going to write tests for this example.

Next, we want to enable a request for a review of the post, and we want `content` to return an empty string while waiting for the review. When the post receives approval, it should get published, meaning the text of the post will be returned when `content` is called.

Notice that the only type we’re interacting with from the crate is the `Post` type. This type will use the state pattern and will hold a value that will be one of three state objects representing the various states a post can be in—draft, waiting for review, or published. Changing from one state to another will be managed internally within the `Post` type. The states change in response to the methods called by our library’s users on the `Post` instance, but they don’t have to manage the state changes directly. Also, users can’t make a mistake with the states, like publishing a post before it’s reviewed.

### Defining `Post` and Creating a New Instance in the Draft State

Let’s get started on the implementation of the library! We know we need a public `Post` struct that holds some content, so we’ll start with the definition of the struct and an associated public `new` function to create an instance of `Post`, as shown in Listing 17-12. We’ll also make a private `State` trait. Then `Post` will hold a trait object of `Box<dyn State>` inside an `Option<T>` in a private field named `state`. You’ll see why the `Option<T>` is necessary in a bit.

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    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(),
            }
        }
    }

    trait State {}

    struct Draft {}

    impl State for Draft {}
    #}

Listing 17-12: Definition of a `Post` struct and a `new` function that creates a new `Post` instance, a `State` trait, and a `Draft` struct

The `State` trait defines the behavior shared by different post states, and the `Draft`, `PendingReview`, and `Published` states will all implement the `State` trait. For now, the trait doesn’t have any methods, and we’ll start by defining just the `Draft` state because that is the state we want a post to start in.

When we create a new `Post`, we set its `state` field to a `Some` value that holds a `Box`. This `Box` points to a new instance of the `Draft` struct. This ensures whenever we create a new instance of `Post`, it will start out as a draft. Because the `state` field of `Post` is private, there is no way to create a `Post` in any other state! In the `Post::new` function, we set the `content` field to a new, empty `String`.

### Storing the Text of the Post Content

Listing 17-11 showed that we want to be able to call a method named `add_text` and pass it a `&str` that is then added to the text content of the blog post. We implement this as a method rather than exposing the `content` field as `pub`. This means we can implement a method later that will control how the `content` field’s data is read. The `add_text` method is pretty straightforward, so let’s add the implementation in Listing 17-13 to the `impl Post` block:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub struct Post {
    #     content: String,
    # }
    #
    impl Post {
        // --snip--
        pub fn add_text(&mut self, text: &str) {
            self.content.push_str(text);
        }
    }
    #}

Listing 17-13: Implementing the `add_text` method to add text to a post’s `content`

The `add_text` method takes a mutable reference to `self`, because we’re changing the `Post` instance that we’re calling `add_text` on. We then call `push_str` on the `String` in `content` and pass the `text` argument to add to the saved `content`. This behavior doesn’t depend on the state the post is in, so it’s not part of the state pattern. The `add_text` method doesn’t interact with the `state` field at all, but it is part of the behavior we want to support.

### Ensuring the Content of a Draft Post Is Empty

Even after we’ve called `add_text` and added some content to our post, we still want the `content` method to return an empty string slice because the post is still in the draft state, as shown on line 8 of Listing 17-11. For now, let’s implement the `content` method with the simplest thing that will fulfill this requirement: always returning an empty string slice. We’ll change this later once we implement the ability to change a post’s state so it can be published. So far, posts can only be in the draft state, so the post content should always be empty. Listing 17-14 shows this placeholder implementation:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub struct Post {
    #     content: String,
    # }
    #
    impl Post {
        // --snip--
        pub fn content(&self) -> &str {
            ""
        }
    }
    #}

Listing 17-14: Adding a placeholder implementation for the `content` method on `Post` that always returns an empty string slice

With this added `content` method, everything in Listing 17-11 up to line 8 works as intended.

### Requesting a Review of the Post Changes Its State

Next, we need to add functionality to request a review of a post, which should change its state from `Draft` to `PendingReview`. Listing 17-15 shows this code:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub struct Post {
    #     state: Option<Box<dyn State>>,
    #     content: String,
    # }
    #
    impl Post {
        // --snip--
        pub fn request_review(&mut self) {
            if let Some(s) = self.state.take() {
                self.state = Some(s.request_review())
            }
        }
    }

    trait State {
        fn request_review(self: Box<Self>) -> Box<dyn State>;
    }

    struct Draft {}

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

    struct PendingReview {}

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

Listing 17-15: Implementing `request_review` methods on `Post` and the `State` trait

We give `Post` a public method named `request_review` that will take a mutable reference to `self`. Then we call an internal `request_review` method on the current state of `Post`, and this second `request_review` method consumes the current state and returns a new state.

We’ve added the `request_review` method to the `State` trait; all types that implement the trait will now need to implement the `request_review` method. Note that rather than having `self`, `&self`, or `&mut self` as the first parameter of the method, we have `self: Box<Self>`. This syntax means the method is only valid when called on a `Box` holding the type. This syntax takes ownership of `Box<Self>`, invalidating the old state so the state value of the `Post` can transform into a new state.

To consume the old state, the `request_review` method needs to take ownership of the state value. This is where the `Option` in the `state` field of `Post` comes in: we call the `take` method to take the `Some` value out of the `state` field and leave a `None` in its place, because Rust doesn’t let us have unpopulated fields in structs. This lets us move the `state` value out of `Post` rather than borrowing it. Then we’ll set the post’s `state` value to the result of this operation.

We need to set `state` to `None` temporarily rather than setting it directly with code like `self.state = self.state.request_review();` to get ownership of the `state` value. This ensures `Post` can’t use the old `state` value after we’ve transformed it into a new state.

The `request_review` method on `Draft` needs to return a new, boxed instance of a new `PendingReview` struct, which represents the state when a post is waiting for a review. The `PendingReview` struct also implements the `request_review` method but doesn’t do any transformations. Rather, it returns itself, because when we request a review on a post already in the `PendingReview` state, it should stay in the `PendingReview` state.

Now we can start seeing the advantages of the state pattern: the `request_review` method on `Post` is the same no matter its `state` value. Each state is responsible for its own rules.

We’ll leave the `content` method on `Post` as is, returning an empty string slice. We can now have a `Post` in the `PendingReview` state as well as in the `Draft` state, but we want the same behavior in the `PendingReview` state. Listing 17-11 now works up to line 11!

### Adding the `approve` Method that Changes the Behavior of `content`

The `approve` method will be similar to the `request_review` method: it will set `state` to the value that the current state says it should have when that state is approved, as shown in Listing 17-16:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub struct Post {
    #     state: Option<Box<dyn State>>,
    #     content: String,
    # }
    #
    impl Post {
        // --snip--
        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>;
    }

    struct Draft {}

    impl State for Draft {
    #     fn request_review(self: Box<Self>) -> Box<dyn State> {
    #         Box::new(PendingReview {})
    #     }
    #
        // --snip--
        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
    #     }
    #
        // --snip--
        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> {
            self
        }
    }
    #}

Listing 17-16: Implementing the `approve` method on `Post` and the `State` trait

We add the `approve` method to the `State` trait and add a new struct that implements `State`, the `Published` state.

Similar to `request_review`, if we call the `approve` method on a `Draft`, it will have no effect because it will return `self`. When we call `approve` on `PendingReview`, it returns a new, boxed instance of the `Published` struct. The `Published` struct implements the `State` trait, and for both the `request_review` method and the `approve` method, it returns itself, because the post should stay in the `Published` state in those cases.

Now we need to update the `content` method on `Post`: if the state is `Published`, we want to return the value in the post’s `content` field; otherwise, we want to return an empty string slice, as shown in Listing 17-17:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # trait State {
    #     fn content<'a>(&self, post: &'a Post) -> &'a str;
    # }
    # pub struct Post {
    #     state: Option<Box<dyn State>>,
    #     content: String,
    # }
    #
    impl Post {
        // --snip--
        pub fn content(&self) -> &str {
            self.state.as_ref().unwrap().content(&self)
        }
        // --snip--
    }
    #}

Listing 17-17: Updating the `content` method on `Post` to delegate to a `content` method on `State`

Because the goal is to keep all these rules inside the structs that implement `State`, we call a `content` method on the value in `state` and pass the post instance (that is, `self`) as an argument. Then we return the value that is returned from using the `content` method on the `state` value.

We call the `as_ref` method on the `Option` because we want a reference to the value inside the `Option` rather than ownership of the value. Because `state` is an `Option<Box<dyn State>>`, when we call `as_ref`, an `Option<&Box<dyn State>>` is returned. If we didn’t call `as_ref`, we would get an error because we can’t move `state` out of the borrowed `&self` of the function parameter.

We then call the `unwrap` method, which we know will never panic, because we know the methods on `Post` ensure that `state` will always contain a `Some` value when those methods are done. This is one of the cases we talked about in the “Cases When You Have More Information Than the Compiler” section of Chapter 9 when we know that a `None` value is never possible, even though the compiler isn’t able to understand that.

At this point, when we call `content` on the `&Box<dyn State>`, deref coercion will take effect on the `&` and the `Box` so the `content` method will ultimately be called on the type that implements the `State` trait. That means we need to add `content` to the `State` trait definition, and that is where we’ll put the logic for what content to return depending on which state we have, as shown in Listing 17-18:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub struct Post {
    #     content: String
    # }
    trait State {
        // --snip--
        fn content<'a>(&self, post: &'a Post) -> &'a str {
            ""
        }
    }

    // --snip--
    struct Published {}

    impl State for Published {
        // --snip--
        fn content<'a>(&self, post: &'a Post) -> &'a str {
            &post.content
        }
    }
    #}

Listing 17-18: Adding the `content` method to the `State` trait

We add a default implementation for the `content` method that returns an empty string slice. That means we don’t need to implement `content` on the `Draft` and `PendingReview` structs. The `Published` struct will override the `content` method and return the value in `post.content`.

Note that we need lifetime annotations on this method, as we discussed in Chapter 10. We’re taking a reference to a `post` as an argument and returning a reference to part of that `post`, so the lifetime of the returned reference is related to the lifetime of the `post` argument.

And we’re done—all of Listing 17-11 now works! We’ve implemented the state pattern with the rules of the blog post workflow. The logic related to the rules lives in the state objects rather than being scattered throughout `Post`.

### Trade-offs of the State Pattern

We’ve shown that Rust is capable of implementing the object-oriented state pattern to encapsulate the different kinds of behavior a post should have in each state. The methods on `Post` know nothing about the various behaviors. The way we organized the code, we have to look in only one place to know the different ways a published post can behave: the implementation of the `State` trait on the `Published` struct.

If we were to create an alternative implementation that didn’t use the state pattern, we might instead use `match` expressions in the methods on `Post` or even in the `main` code that checks the state of the post and changes behavior in those places. That would mean we would have to look in several places to understand all the implications of a post being in the published state! This would only increase the more states we added: each of those `match` expressions would need another arm.

With the state pattern, the `Post` methods and the places we use `Post` don’t need `match` expressions, and to add a new state, we would only need to add a new struct and implement the trait methods on that one struct.

The implementation using the state pattern is easy to extend to add more functionality. To see the simplicity of maintaining code that uses the state pattern, try a few of these suggestions:

- Add a `reject` method that changes the post’s state from `PendingReview` back to `Draft`.
- Require two calls to `approve` before the state can be changed to `Published`.
- Allow users to add text content only when a post is in the `Draft` state. Hint: have the state object responsible for what might change about the content but not responsible for modifying the `Post`.

One downside of the state pattern is that, because the states implement the transitions between states, some of the states are coupled to each other. If we add another state between `PendingReview` and `Published`, such as `Scheduled`, we would have to change the code in `PendingReview` to transition to `Scheduled` instead. It would be less work if `PendingReview` didn’t need to change with the addition of a new state, but that would mean switching to another design pattern.

Another downside is that we’ve duplicated some logic. To eliminate some of the duplication, we might try to make default implementations for the `request_review` and `approve` methods on the `State` trait that return `self`; however, this would violate object safety, because the trait doesn’t know what the concrete `self` will be exactly. We want to be able to use `State` as a trait object, so we need its methods to be object safe.

Other duplication includes the similar implementations of the `request_review` and `approve` methods on `Post`. Both methods delegate to the implementation of the same method on the value in the `state` field of `Option` and set the new value of the `state` field to the result. If we had a lot of methods on `Post` that followed this pattern, we might consider defining a macro to eliminate the repetition (see Appendix D for more on macros).

By implementing the state pattern exactly as it’s defined for object-oriented languages, we’re not taking as full advantage of Rust’s strengths as we could. Let’s look at some changes we can make to the `blog` crate that can make invalid states and transitions into compile time errors.

#### Encoding States and Behavior as Types

We’ll show you how to rethink the state pattern to get a different set of trade-offs. Rather than encapsulating the states and transitions completely so outside code has no knowledge of them, we’ll encode the states into different types. Consequently, Rust’s type checking system will prevent attempts to use draft posts where only published posts are allowed by issuing a compiler error.

Let’s consider the first part of `main` in Listing 17-11:

Filename: src/main.rs

    # extern crate blog;
    # use blog::Post;

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

        post.add_text("I ate a salad for lunch today");
        assert_eq!("", post.content());
    }

We still enable the creation of new posts in the draft state using `Post::new` and the ability to add text to the post’s content. But instead of having a `content` method on a draft post that returns an empty string, we’ll make it so draft posts don’t have the `content` method at all. That way, if we try to get a draft post’s content, we’ll get a compiler error telling us the method doesn’t exist. As a result, it will be impossible for us to accidentally display draft post content in production, because that code won’t even compile. Listing 17-19 shows the definition of a `Post` struct and a `DraftPost` struct, as well as methods on each:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    pub struct Post {
        content: String,
    }

    pub struct DraftPost {
        content: String,
    }

    impl Post {
        pub fn new() -> DraftPost {
            DraftPost {
                content: String::new(),
            }
        }

        pub fn content(&self) -> &str {
            &self.content
        }
    }

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

Listing 17-19: A `Post` with a `content` method and a `DraftPost` without a `content` method

Both the `Post` and `DraftPost` structs have a private `content` field that stores the blog post text. The structs no longer have the `state` field because we’re moving the encoding of the state to the types of the structs. The `Post` struct will represent a published post, and it has a `content` method that returns the `content`.

We still have a `Post::new` function, but instead of returning an instance of `Post`, it returns an instance of `DraftPost`. Because `content` is private and there aren’t any functions that return `Post`, it’s not possible to create an instance of `Post` right now.

The `DraftPost` struct has an `add_text` method, so we can add text to `content` as before, but note that `DraftPost` does not have a `content` method defined! So now the program ensures all posts start as draft posts, and draft posts don’t have their content available for display. Any attempt to get around these constraints will result in a compiler error.

#### Implementing Transitions as Transformations into Different Types

So how do we get a published post? We want to enforce the rule that a draft post has to be reviewed and approved before it can be published. A post in the pending review state should still not display any content. Let’s implement these constraints by adding another struct, `PendingReviewPost`, defining the `request_review` method on `DraftPost` to return a `PendingReviewPost`, and defining an `approve` method on `PendingReviewPost` to return a `Post`, as shown in Listing 17-20:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub struct Post {
    #     content: String,
    # }
    #
    # pub struct DraftPost {
    #     content: String,
    # }
    #
    impl DraftPost {
        // --snip--

        pub fn request_review(self) -> PendingReviewPost {
            PendingReviewPost {
                content: self.content,
            }
        }
    }

    pub struct PendingReviewPost {
        content: String,
    }

    impl PendingReviewPost {
        pub fn approve(self) -> Post {
            Post {
                content: self.content,
            }
        }
    }
    #}

Listing 17-20: A `PendingReviewPost` that gets created by calling `request_review` on `DraftPost` and an `approve` method that turns a `PendingReviewPost` into a published `Post`

The `request_review` and `approve` methods take ownership of `self`, thus consuming the `DraftPost` and `PendingReviewPost` instances and transforming them into a `PendingReviewPost` and a published `Post`, respectively. This way, we won’t have any lingering `DraftPost` instances after we’ve called `request_review` on them, and so forth. The `PendingReviewPost` struct doesn’t have a `content` method defined on it, so attempting to read its content results in a compiler error, as with `DraftPost`. Because the only way to get a published `Post` instance that does have a `content` method defined is to call the `approve` method on a `PendingReviewPost`, and the only way to get a `PendingReviewPost` is to call the `request_review` method on a `DraftPost`, we’ve now encoded the blog post workflow into the type system.

But we also have to make some small changes to `main`. The `request_review` and `approve` methods return new instances rather than modifying the struct they’re called on, so we need to add more `let post =` shadowing assignments to save the returned instances. We also can’t have the assertions about the draft and pending review post’s contents be empty strings, nor do we need them: we can’t compile code that tries to use the content of posts in those states any longer. The updated code in `main` is shown in Listing 17-21:

Filename: src/main.rs

    extern crate blog;
    use blog::Post;

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

        post.add_text("I ate a salad for lunch today");

        let post = post.request_review();

        let post = post.approve();

        assert_eq!("I ate a salad for lunch today", post.content());
    }

Listing 17-21: Modifications to `main` to use the new implementation of the blog post workflow

The changes we needed to make to `main` to reassign `post` mean that this implementation doesn’t quite follow the object-oriented state pattern anymore: the transformations between the states are no longer encapsulated entirely within the `Post` implementation. However, our gain is that invalid states are now impossible because of the type system and the type checking that happens at compile time! This ensures that certain bugs, such as display of the content of an unpublished post, will be discovered before they make it to production.

Try the tasks suggested for additional requirements that we mentioned at the start of this section on the `blog` crate as it is after Listing 17-20 to see what you think about the design of this version of the code. Note that some of the tasks might be completed already in this design.

We’ve seen that even though Rust is capable of implementing object-oriented design patterns, other patterns, such as encoding state into the type system, are also available in Rust. These patterns have different trade-offs. Although you might be very familiar with object-oriented patterns, rethinking the problem to take advantage of Rust’s features can provide benefits, such as preventing some bugs at compile time. Object-oriented patterns won’t always be the best solution in Rust due to certain features, like ownership, that object-oriented languages don’t have.

## Summary

No matter whether or not you think Rust is an object-oriented language after reading this chapter, you now know that you can use trait objects to get some object-oriented features in Rust. Dynamic dispatch can give your code some flexibility in exchange for a bit of runtime performance. You can use this flexibility to implement object-oriented patterns that can help your code’s maintainability. Rust also has other features, like ownership, that object-oriented languages don’t have. An object-oriented pattern won’t always be the best way to take advantage of Rust’s strengths, but is an available option.

Next, we’ll look at patterns, which are another of Rust’s features that enable lots of flexibility. We’ve looked at them briefly throughout the book but haven’t seen their full capability yet. Let’s go!