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:
- A blog post starts as an empty draft.
- When the draft is done, a review of the post is requested.
- When the post is approved, it gets published.
- 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 For example, if we try to approve a draft blog post before we’ve requested a review, the post should remain an unpublished draft.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
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
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
pub struct Post {
state: Option<Box<dyn State>>,
content: String,
}
impl Post {
// --snip--
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);
}
}
trait State {}
struct Draft {}
impl State for Draft {}
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 7 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
pub struct Post {
state: Option<Box<dyn State>>,
content: String,
}
impl Post {
// --snip--
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 {
""
}
}
trait State {}
struct Draft {}
impl State for Draft {}
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 7
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
pub struct Post {
state: Option<Box<dyn State>>,
content: String,
}
impl Post {
// --snip--
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 {
""
}
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 10!
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
pub struct Post {
state: Option<Box<dyn State>>,
content: String,
}
impl Post {
// --snip--
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 {
""
}
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>;
}
struct Draft {}
impl State for Draft {
// --snip--
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 {
// --snip--
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> {
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
pub struct Post {
state: Option<Box<dyn State>>,
content: String,
}
impl Post {
// --snip--
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 {
self.state.as_ref().unwrap().content(self)
}
// --snip--
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>;
}
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> {
self
}
}
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 In Which 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
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 {
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 {
// --snip--
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 {
""
}
}
// --snip--
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 {
// --snip--
fn request_review(self: Box<Self>) -> Box<dyn State> {
self
}
fn approve(self: Box<Self>) -> Box<dyn State> {
self
}
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
rejectmethod that changes the post’s state fromPendingReviewback toDraft. - Require two calls to
approvebefore the state can be changed toPublished. - Allow users to add text content only when a post is in the
Draftstate. Hint: have the state object responsible for what might change about the content but not responsible for modifying thePost.
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 the “Macros” section in Chapter 19).
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
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());
}
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
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
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 {
// --snip--
pub fn add_text(&mut self, text: &str) {
self.content.push_str(text);
}
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
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!