Chain of Responsibility em Rust

O Chain of Responsibility é um padrão de projeto comportamental que permite passar a solicitação ao longo da cadeia de handlers em potencial até que um deles lide com a solicitação.

O padrão permite que vários objetos tratem a solicitação sem acoplar a classe remetente às classes concretas dos destinatários. A cadeia pode ser composta dinamicamente em tempo de execução com qualquer handler que siga uma interface de handler padrão.

Saiba mais sobre o Chain of Responsibility

Conceptual Example

The example demonstrates processing a patient through a chain of departments. The chain of responsibility is constructed as follows:

Patient -> Reception -> Doctor -> Medical -> Cashier

The chain is constructed using Box pointers, which means dynamic dispatch in runtime. Why? It seems quite difficult to narrow down implementation to a strict compile-time typing using generics: in order to construct a type of a full chain Rust needs full knowledge of the “next of the next” link in the chain. Thus, it would look like this:

let mut reception = Reception::<Doctor::<Medical::<Cashier>>>::new(doctor); // 😱

Instead, Box allows chaining in any combination:

let mut reception = Reception::new(doctor); // 👍

let mut reception = Reception::new(cashier); // 🕵️‍♀️

patient.rs: Request

#[derive(Default)]
pub struct Patient {
    pub name: String,
    pub registration_done: bool,
    pub doctor_check_up_done: bool,
    pub medicine_done: bool,
    pub payment_done: bool,
}

department.rs: Handlers

mod cashier;
mod doctor;
mod medical;
mod reception;

pub use cashier::Cashier;
pub use doctor::Doctor;
pub use medical::Medical;
pub use reception::Reception;

use crate::patient::Patient;

/// A single role of objects that make up a chain.
/// A typical trait implementation must have `handle` and `next` methods,
/// while `execute` is implemented by default and contains a proper chaining
/// logic.
pub trait Department {
    fn execute(&mut self, patient: &mut Patient) {
        self.handle(patient);

        if let Some(next) = &mut self.next() {
            next.execute(patient);
        }
    }

    fn handle(&mut self, patient: &mut Patient);
    fn next(&mut self) -> &mut Option<Box<dyn Department>>;
}

/// Helps to wrap an object into a boxed type.
pub fn into_next(department: impl Department + Sized + 'static) -> Option<Box<dyn Department>> {
    Some(Box::new(department))
}

department/cashier.rs

use super::{Department, Patient};

#[derive(Default)]
pub struct Cashier {
    next: Option<Box<dyn Department>>,
}

impl Department for Cashier {
    fn handle(&mut self, patient: &mut Patient) {
        if patient.payment_done {
            println!("Payment done");
        } else {
            println!("Cashier getting money from a patient {}", patient.name);
            patient.payment_done = true;
        }
    }

    fn next(&mut self) -> &mut Option<Box<dyn Department>> {
        &mut self.next
    }
}

department/doctor.rs

use super::{into_next, Department, Patient};

pub struct Doctor {
    next: Option<Box<dyn Department>>,
}

impl Doctor {
    pub fn new(next: impl Department + 'static) -> Self {
        Self {
            next: into_next(next),
        }
    }
}

impl Department for Doctor {
    fn handle(&mut self, patient: &mut Patient) {
        if patient.doctor_check_up_done {
            println!("A doctor checkup is already done");
        } else {
            println!("Doctor checking a patient {}", patient.name);
            patient.doctor_check_up_done = true;
        }
    }

    fn next(&mut self) -> &mut Option<Box<dyn Department>> {
        &mut self.next
    }
}

department/medical.rs

use super::{into_next, Department, Patient};

pub struct Medical {
    next: Option<Box<dyn Department>>,
}

impl Medical {
    pub fn new(next: impl Department + 'static) -> Self {
        Self {
            next: into_next(next),
        }
    }
}

impl Department for Medical {
    fn handle(&mut self, patient: &mut Patient) {
        if patient.medicine_done {
            println!("Medicine is already given to a patient");
        } else {
            println!("Medical giving medicine to a patient {}", patient.name);
            patient.medicine_done = true;
        }
    }

    fn next(&mut self) -> &mut Option<Box<dyn Department>> {
        &mut self.next
    }
}

department/reception.rs

use super::{into_next, Department, Patient};

#[derive(Default)]
pub struct Reception {
    next: Option<Box<dyn Department>>,
}

impl Reception {
    pub fn new(next: impl Department + 'static) -> Self {
        Self {
            next: into_next(next),
        }
    }
}

impl Department for Reception {
    fn handle(&mut self, patient: &mut Patient) {
        if patient.registration_done {
            println!("Patient registration is already done");
        } else {
            println!("Reception registering a patient {}", patient.name);
            patient.registration_done = true;
        }
    }

    fn next(&mut self) -> &mut Option<Box<dyn Department>> {
        &mut self.next
    }
}

main.rs: Client code

mod department;
mod patient;

use department::{Cashier, Department, Doctor, Medical, Reception};
use patient::Patient;

fn main() {
    let cashier = Cashier::default();
    let medical = Medical::new(cashier);
    let doctor = Doctor::new(medical);
    let mut reception = Reception::new(doctor);

    let mut patient = Patient {
        name: "John".into(),
        ..Patient::default()
    };

    // Reception handles a patient passing him to the next link in the chain.
    // Reception -> Doctor -> Medical -> Cashier.
    reception.execute(&mut patient);

    println!("\nThe patient has been already handled:\n");

    reception.execute(&mut patient);
}

Output

Reception registering a patient John
Doctor checking a patient John
Medical giving medicine to a patient John
Cashier getting money from a patient John

The patient has been already handled:

Patient registration is already done
A doctor checkup is already done
Medicine is already given to a patient
Payment done