CODESAMPLE

Memento - Rust

Memento Rust

The Memento pattern captures and externalizes the internal state of an object without violating encapsulation. This allows restoring an object to a previous state. Here, a Gist struct holds the text content, and the GistMemento stores a snapshot of that content at a specific time. The Gist provides a method to create a memento, and GistMemento only allows access to its state by the original Gist object. This protects the state integrity. The implementation is idiomatic Rust as it leverages ownership, borrowing, and structs to naturally enforce the required access control and immutability where appropriate.

// Memento Interface
trait Memento {
    fn get_state(&self) -> String;
}

// Originator
struct Gist {
    content: String,
}

impl Gist {
    fn new(initial_content: String) -> Self {
        Gist { content: initial_content }
    }

    fn get_content(&self) -> &String {
        &self.content
    }

    fn set_content(&mut self, new_content: String) {
        self.content = new_content;
    }

    // Creates a memento containing the current state.
    fn create_memento(&self) -> GistMemento {
        GistMemento { state: self.content.clone() }
    }

    // Restores the state from a memento.  Only the originator can do this.
    fn restore_from_memento(&mut self, memento: &GistMemento) {
        self.content = memento.get_state().clone();
    }
}

// Memento Implementation
struct GistMemento {
    state: String,
}

impl GistMemento {
    fn get_state(&self) -> String {
        self.state.clone()
    }
}

// Caretaker (Optional - manages mementos)
struct GistHistory {
    mementos: Vec<GistMemento>,
}

impl GistHistory {
    fn new() -> Self {
        GistHistory { mementos: Vec::new() }
    }

    fn add_memento(&mut self, memento: GistMemento) {
        self.mementos.push(memento);
    }

    fn get_latest_memento(&self) -> Option<&GistMemento> {
        self.mementos.last()
    }
}

fn main() {
    let mut gist = Gist::new("Initial content.".to_string());
    let mut history = GistHistory::new();

    println!("Initial content: {}", gist.get_content());

    gist.set_content("First edit.".to_string());
    history.add_memento(gist.create_memento());
    println!("First edit: {}", gist.get_content());

    gist.set_content("Second edit.".to_string());
    history.add_memento(gist.create_memento());
    println!("Second edit: {}", gist.get_content());

    // Rollback to the first edit
    if let Some(memento) = history.get_latest_memento() {
        gist.restore_from_memento(memento);
        println!("Rolled back to: {}", gist.get_content());
    }
}