Object-Oriented Design Pattern Implementations - The Rust Programming Language

## Object-Oriented Design Pattern Implementation

Let's look at an example of the state design pattern and how to use it in Rust. The state pattern is when a value has some internal state, and the value's behavior changes based on the internal state. The internal state is represented by a set of objects that inherit shared functionality (we'll use structs and traits since Rust doesn't have objects and inheritance). Each state object is responsible for its own behavior and the rules for when it should change into another state. The value that holds one of these state objects doesn't know anything about the different behavior of the states or when to transition between states. In the future when requirements 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.

In order to explore this idea, we're going to implement a blog post workflow in an incremental way. The workflow that we want our blog posts to follow, once we're done with the implementation, is:

1. A blog post starts as an empty draft.
2. Once the draft is done, we request a review of the post.
3. Once the post is approved, it gets published.
4. Only published blog posts return content to print so that we can't accidentally print the text of a post that hasn't been approved.

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 For example, if we try to approve a draft blog post before we've requested a review, the post should stay an unpublished draft.

Listing 17-11 shows this workflow in code form. This is an example usage of the API we're going to implement in a library crate named `blog`:

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 be able to create a new draft blog post with `Post::new`. Then, we want to add some text to the blog post while we're in the draft state. If we try to print out the post's content immediately, though, we shouldn't get any text, since the post is still a draft. We've added an `assert_eq!` here for demonstration purposes. Asserting that a draft blog post returns an empty string from the `content` method would make an excellent unit test in our library, but we're not going to write tests for this example.

Next, we want to be able to request a review of our post, and `content` should still return an empty string while waiting for a review. Lastly, when we approve the blog post, it should get published, which means the text we added will be returned when we call `content`.

Notice that the only type we're interacting with from the crate is the `Post` type. The various states a post can be in (draft, waiting for review, published) are managed internally to the `Post` type. The states change due to the methods we call on the `Post` instance, but we don't have to manage the state changes directly. This also means we won't make a mistake with the states, like forgetting to request a review before publishing.

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

Let's get started on the implementation of the library! We know we want to have a public `Post` struct that holds some content, so let's 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're also going to have a private trait `State`. `Post` will hold a trait object of `Box<State>` inside an `Option` in a private field named `state`. We'll see why the `Option` is necessary in a bit. The `State` trait defines all the behavior different post states share, and the `Draft`, `PendingReview`, and `Published` states will all implement the `State` trait. For now, the trait does not have any methods, and we're going to start by defining just the `Draft` state since that's the state we want to start in:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    pub struct Post {
        state: Option<Box<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 that implements `State`

When we create a new `Post`, we set its `state` field to a `Some` value holding a `Box` pointing to a new instance of the `Draft` struct. This ensures whenever we create a new instance of `Post`, it'll start out as a draft. Because the `state` field of `Post` is private, there's no way to create a `Post` in any other state!

### Storing the Text of the Post Content

In the `Post::new` function, we set the `content` field to a new, empty `String`. In Listing 17-11, we showed that we want to be able to call a method named `add_text` and pass a `&str` to it to add that text to the content of the blog post. We're choosing to implement this as a method rather than exposing the `content` field as `pub` because we want to be able to control how the `content` field's data is read by implementing a method later. The `add_text` method is pretty straightforward though, 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`

`add_text` takes a mutable reference to `self`, since 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 isn't part of the state pattern since its behavior doesn't depend on the state that the post is in. The `add_text` method doesn't interact with the `state` field at all, but it is part of the behavior we want to support.

### Content of a Draft Post is Empty

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 since 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're going to change this later once we implement the ability to change a post's state to be published. With what we have so far, though, posts can only be in the draft state, which means 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 we intend.

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

Next up is requesting a review of a post, which should change its state from `Draft` to `PendingReview`. We want `Post` to have a public method named `request_review` that will take a mutable reference to `self`. Then we're going to call an internal `request_review` method on the state that we're holding, and this second `request_review` method will consume the current state and return a new state. In order to be able to consume the old state, the second `request_review` method needs to take ownership of the state value. This is where the `Option` comes in: we're going to `take` the `Some` value out of the `state` field and leave a `None` in its place since Rust doesn't let us have unpopulated fields in structs. Then we'll set the post's `state` value to the result of this operation. Listing 17-15 shows this code:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub struct Post {
    #     state: Option<Box<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<State>;
    }

    struct Draft {}

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

    struct PendingReview {}

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

    #}

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

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>`, which is what we want because we're transforming the old state into a new state, and we want the old state to no longer be valid.

The implementation for the `request_review` method on `Draft` is to return a new, boxed instance of the `PendingReview` struct, which is a new type we've introduced that represents the state when a post is waiting for a review. The `PendingReview` struct also implements the `request_review` method, but it doesn't do any transformations. It returns itself since requesting a review on a post that's already in the `PendingReview` state 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 what its `state` value is. Each state is responsible for its own rules.

We're going to leave the `content` method on `Post` as it is, returning an empty string slice. We can now have a `Post` in the `PendingReview` state, not just the `Draft` state, but we want the same behavior in the `PendingReview` state. Listing 17-11 now works up until line 11!

### Approving a Post Changes the Behavior of `content`

The `approve` method on `Post` will be similar to that of the `request_review` method: it will set the `state` to the value that the current state says it should have when that state is approved. We'll need to add the `approve` method to the `State` trait, and we'll add a new struct that implements `State`, the `Published` state. Listing 17-16 shows the new code:

Filename: src/lib.rs

    # #![allow(unused_variables)]
    #fn main() {
    # pub struct Post {
    #     state: Option<Box<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<State>;
        fn approve(self: Box<Self>) -> Box<State>;
    }

    struct Draft {}

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

    struct PendingReview {}

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

    struct Published {}

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

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

    #}

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

Similarly to `request_review`, if we call the `approve` method on a `Draft`, it will have no effect since 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 since the post should stay in the `Published` state in those cases.

Now for updating the `content` method on `Post`: we want to return the value in the post's `content` field if its state is `Published`, otherwise we want to return an empty string slice. Because the goal is to keep all the rules like this in the structs that implement `State`, we're going to call a `content` method on the value in `state` and pass the post instance (that is, `self`) as an argument. Then we'll return the value returned from the `content` method on the `state` value 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<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`

We're calling the `as_ref` method on the `Option` because we want a reference to the value inside the `Option`. We're then calling the `unwrap` method, which we know will never panic because all the methods on `Post` ensure that the `state` value will have a `Some` value in it when those methods are done. This is one of the cases we talked about in Chapter 12 where we know that a `None` value is never possible even though the compiler isn't able to understand that.

The `content` method on the `State` trait is where the logic for what content to return will be. We're going to add a default implementation for the `content` method that returns an empty string slice. That lets us not need to implement `content` on the `Draft` and `PendingReview` structs. The `Published` struct will override the `content` method and will return the value in `post.content`, 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

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

### Tradeoffs of the State Pattern

We've shown that Rust is capable of implementing the object-oriented state pattern in order to encapsulate the different kinds of behavior that a post should have that depends on the state that the post is in. The methods on `Post` don't know anything about the different kinds of behavior. The way this code is organized, we have one place to look in order to find out all the different ways that a published post behaves: the implementation of the `State` trait on the `Published` struct.

An alternative implementation that didn't use the state pattern might have `match` statements in the methods on `Post` or even in the code that uses `Post` (`main` in our case) that checks what the state of the post is and changes behavior in those places instead. That would mean we'd have a lot of places to look in order to understand all the implications of a post being in the published state! This would get worse the more states we added: each of those `match` statements would need another arm. With the state pattern, the `Post` methods and the places we use `Post` don't need `match` statements and adding a new state only involves adding a new `struct` and implementing the trait methods on that one struct.

This implementation is easy to extend to add more functionality. Here are some changes you can try making to the code in this section to see for yourself what it's like to maintain code using this pattern over time:

- Only allow adding text content when a post is in the `Draft` state
- Add a `reject` method that changes the post's state from `PendingReview` back to `Draft`
- Require two calls to `approve` before changing the state to `Published`

A downside of the state pattern is that since the states implement the transitions between the 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 nicer if `PendingReview` wouldn't need to change because of the addition of a new state, but that would mean switching to another design pattern.

There are a few bits of duplicated logic that are a downside of this implementation in Rust. It would be nice if we could make default implementations for the `request_review` and `approve` methods on the `State` trait that return `self`, but this would violate object safety since 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.

The other duplication that would be nice to get rid of is the similar implementations of the `request_review` and `approve` methods on `Post`. They both delegate to the implementation of the same method on the value in the `Option` in the `state` field, 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 E on macros).

A downside of implementing this object-oriented pattern exactly as it's defined for object-oriented languages is that we're not taking advantage of Rust's strengths as much as we could be. Let's take a look at some changes we can make to this code that can make invalid states and transitions into compile time errors.

#### Encoding States and Behavior as Types

We're going to show how to rethink the state pattern a bit in order to get a different set of tradeoffs. Rather than encapsulating the states and transitions completely so that outside code has no knowledge of them, we're going to encode the states into different types. When the states are types, Rust's type checking will make any attempt to use a draft post where we should only use published posts into a compiler error.

Let's consider the first part of `main` from Listing 17-11:

Filename: src/main.rs

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

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

We still want to create a new post in the draft state using `Post::new`, and we still want to be able 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're going to make it so that 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 that the method doesn't exist. This will make it impossible for us to accidentally display draft post content in production, since that code won't even compile. Listing 17-19 shows the definition of a `Post` struct, a `DraftPost` struct, and 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 since we're moving the encoding of the state to the types of the structs. `Post` 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`. It's not possible to create an instance of `Post` right now since `content` is private and there aren't any functions that return `Post`. `DraftPost` has an `add_text` method defined on it so that we can add text to `content` as before, but note that `DraftPost` does not have a `content` method defined! So we've enforced that 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 be a compiler error.

#### Implementing Transitions as Transformations into Different Types

So how do we get a published post then? The rule we want to enforce is 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 `DraftPost` instances lingering around after we've called `request_review` on them, and so forth. `PendingReviewPost` doesn't have a `content` method defined on it, so attempting to read its content is a compiler error like it is 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.

This does mean we have to make some small changes to `main`. Because `request_review` and `approve` return new instances rather than modifying the struct they're called on, 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 being empty string anymore, 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

Having to change `main` to reassign `post` is what makes this implementation not quite following the object-oriented state pattern anymore: the transformations between the states are no longer encapsulated entirely within the `Post` implementation. However, we've gained the property of having invalid states be impossible because of the type system and type checking that happens at compile time! This ensures that certain bugs, such as displaying the content of an unpublished post, will be discovered before they make it to production.

Try the tasks suggested that add additional requirements that we mentioned at the start of this section to see how working with this version of the code feels.

Even though Rust is capable of implementing object-oriented design patterns, there are other patterns like encoding state into the type system that are available in Rust. These patterns have different tradeoffs than the object-oriented patterns do. While you may be very familiar with object-oriented patterns, rethinking the problem in order to take advantage of Rust's features can give benefits like preventing some bugs at compile-time. Object-oriented patterns won't always be the best solution in Rust, since Rust has features like ownership that object-oriented languages don't have.

## Summary

No matter whether you think Rust is an object-oriented language or not after reading this chapter, you've now seen that trait objects are a way to get some object-oriented features in Rust. Dynamic dispatch can give your code some flexibility in exchange for a bit of runtime performance. This flexibility can be used to implement object-oriented patterns that can help with the maintainability of your code. Rust also has different features, like ownership, than object-oriented languages. An object-oriented pattern won't always be the best way to take advantage of Rust's strengths.

Next, let's look at another feature of Rust that enables lots of flexibility: patterns. We've looked at them briefly throughout the book, but haven't seen everything they're capable of yet. Let's go!