RustBrock/Implementing OO Design Pattern.md

# Implementing an Object-Oriented Design Pattern
The *state pattern* is an object-oriented design pattern.

The crux of the pattern is that we define a set of states a value can have internally. 

The states are represented by a set of *state objects* and the value's behavior changes based on its state.

We are going to work through an example of a blog post struct that has aa filed to hold its state, this will be a state object form the set "draft", "review", or "published".

The sate objects share functionality. In Rust 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.


The advantage of using this is that 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 will only need to update the code inside one of the state objects to change its rules or perhaps add more state objects.

We are going to implement the state pattern in a more tradition object-oriented way.

Next we will use an approach that is a bit more natural in Rust.

Lets start with incrementally implementing a blog post workflow using the state pattern.

The final functionality will look like this:
1. A blog post starts as an empty draft
2. When the draft is complete, a review of the post is requested
3. When the post is approved, it will be published
4. Only published blog posts return content to print, so unapproved posts can't be accidentally be published
Any other changes attempted on  a post should have no effect.

An example of this is if we try to approve a draft blog post before we have requested a review, the post should remain an unpublished draft.

In this example, it shows the workflow in code form.

This is an example usage of the API, we will implement in a library crate named `blog`.

This will not compile yet because we haven't implemented the `blog` crate.

```rust
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 want to allow the user to create a new draft blog post with `Post::new`.

We also want to allow text to be added to be blog post.

If we try to get the post's content immediately, before approval, we shouldn't get any text because the post is still a draft.

We added `assert_eq!` for demonstration purposes.

An excellent unit test for this would be to assert that a draft post returns an empty string from the `content` method, but we are 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.

Then when the post receives approval, it should get published, meaning the text of the post will be returned when `content` is called.

Note that the only type we are interacting with from the crate s 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 change in response to the methods called by our library users on the `Post` instance.

They don't have to manage the state changes directly.

Users can't make a mistake with the states, like publishing a post before it is reviewed.

## Defining `Post` and Creating a New Instance in the Draft State
First we need a public `Post` struct that holds some content.

so we will start with the definition of the struct and the associated public `new` function to create an instance of `Post`.

This is shown below.

We will also make a private `State` trait that will define the behavior that all state objects for a `Post` must have.

`Post` will hold a trait object of `Box<dyn State>` inside an `Option<T>` in a private field named `state` to hold the state object.

You will see why the `Option<T>` is necessary.
```rust
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 {}
```

The `State` trait defines the behavior shared by different post states.

The state objects are `Draft`, `PendingReview` and `Published`, and they will all implement the `State` trait.

For now the trait doesn't have any methods.

We will 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 aa new instance of the `Draft` struct.

This ensures whenever we create a new instance of `Post`, it will start out as a draft.

Due to the state field of `Post` being private, there is no way to create a `Psot` 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
Previously we saw that we wanted to be able to call a method named `add_test` and pass it a `&str` that is then added as the text content of the blog post.

We implemented this as a method, rather than exposing the `content` field as `pub`, so that later we can implement a method that will control how the `content` field's data is read.

The `add_text` method is fairly straightforward, so lets add the implementation below to the `impl Post` block.
```rust
impl Post {
    // --snip--
    pub fn add_text(&mut self, text: &str) {
        self.content.push_str(text);
    }
}
```
The `add_text` method takes a mutable reference to `self`.

Because we changed the `Post` instance that we are calling `add_text` on.

We then can 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 is 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 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.

For now we will implement the `content` method with the simplest thing that will fulfill this requirement.

Always returning an empty string slice.

We will change this late once we implement the ability to change a post's state so that it can be published.

Posts so far can only be in the draft state., so the post content should always be empty.

Here is a placeholder implementation:
```rust
impl Post {
    // --snip--
    pub fn content(&self) -> &str {
        ""
    }
}
```

## 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`

Here is the code that shows this
```rust
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
    }
}
```
Here `request_review` is a public method on the `Post` struct, this will take a mutable reference to `self`.

Then we call an internal `request_review` method on the current state of `Post`.

The second `request_review` method consumes the current state and returns a new state.

We add the `request_review` method to the `State` trait.

Now all types that implement the trait will now need to implement the `request_review` method.

Note, 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.

In order 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` filed 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 not allowing us to have unpopulated fields in structs.

This lets us move the `state` value out of `Post` rather than borrowing it.

Then we will 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 something like `self.state = self.state.request_review();` to get ownership of the `state` value.

This ensures that `Post` can't use the old `state` value after we transformed it into a new state.


The `request_review` method on `Draft` returns a new boxed instance of a new `PendingReview` struct.

This 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.

It instead returns itself, because when we request a review on a post already in the `PendingReview` state, it should stay in the `PendingReview` state.

Now the advantages of the state pattern are staring to be seen: the `request_review` method on `Post` is the same no matter its `state` value.

Each state is responsible for its own rules.

We 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.

## Adding `approve` to Change 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.

Here is the new code
```rust
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 {
    // --snip--
    fn approve(self: Box<Self>) -> Box<dyn State> {
        self
    }
}

struct PendingReview {}

impl State for PendingReview {
    // --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
    }
}
```
Here we added the `spprove` method to the `State` trait and add a new struct that implements `State`, the `Published` state.

Similar to how `request_review` on `PendingReview` works, if we call the `approve` method on a `Draft`, it will have no effect because `approve` 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.

We now need a way to update the `content` method on `Post`.

We want the value returned from `content` to depend on the current state of `Post`, so we are going to have the `Post` delegate to `cotnent` method defined on its `state`.

Here is the code for this
```rust
impl Post {
    // --snip--
    pub fn content(&self) -> &str {
        self.state.as_ref().unwrap().content(self)
    }
    // --snip--
}
```
The goal is to keep all the 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.

As 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.

Then we call the `unwrap` method, we know this will never panic.

We know the methods on `Post` ensure that `state` will always contain a `Some` value when those methods are done.

This is a case where we have more information than the compiler (previously discussed in [Ch 9]()) when we know that a `None` value is never possible, even though the compiler isn't able to understand that.

Now 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.

This means we need to add `content` to the `State` trait definition, and that is where we will put the logic for what content to return depending on which state we have.

Here is that addition
```rust
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
    }
}
```
Here we added a default implementation for the `content` method that returns an empty string slice.

This means we don't need to implement `cotent` 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 a lifetime annotation on this method.

Here we are 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.

We have finally 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`.

Final Code:
```rust
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
    }
}
```
### Why Not An Enum?
You may wonder why we didn't use an `enum` with the different possible post states as variants.

This is a possible solution, you have to try it and compare the end results to see which is preferred.

One disadvantage of using an enum is every place that checks the value of the enum will need a `match` expression or similar to handle every possible variant.

This could get more repetitive than this trait object solution.

## Trade-offs of the State Pattern
Here we have 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 in which code is organized, we have to look only in 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` expression in the `Post` or even in the `main` code.

This would check for the state of the post and changes behavior ion those places.

That means 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 highlight 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 transitioned 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 have dupliced some logic.

In order to eliminate some of the duplication, we may try to make default implementations for the `request_review` and `approve` methods on the `State` trait that return `self`

However, this would not be dyn compatible.

This is 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 dyn compatible.

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 may consider defining a macro to eliminate the repetition (This will be discussed in Ch20).

By implementing the state pattern exactly as it is defined for object-oriented languages, we are not taking full advantage of Rust's strengths as we could.

Now lets look at some changes to make the `blog` crate that can make invalid states and transitions into compile time errors.

## Encoding States and Behavior as Types