chain_of_responsibility.rs

//! Chain of Responsibility is a behavioral design pattern that lets you pass requests along a chain of handlers.
//! Upon receiving a request, each handler decides either to process the request or to pass it to the next handler in the chain.

// The Handler trait declares a method for building the chain of
// handlers. It also declares a method for executing a request.
trait Handler<'a> {
    fn set_next(&mut self, next: &'a dyn Handler<'a>) -> &mut dyn Handler<'a>;
    fn handle(&self, request: &str);
}

struct AHandler<'a> {
    name: String,
    next: Option<&'a dyn Handler<'a>>,
}
impl<'a> AHandler<'a> {
    fn new(name: String) -> AHandler<'a> {
        AHandler { name, next: None }
    }
}
impl<'a> Handler<'a> for AHandler<'a> {
    fn set_next(&mut self, next: &'a dyn Handler<'a>) -> &mut dyn Handler<'a> {
        self.next = Some(next);
        self
    }
    fn handle(&self, request: &str) {
        println!("{} handle the request: {}", self.name, request);
        if let Some(v) = &self.next {
            v.handle(request);
        }
    }
}

struct BHandler<'a> {
    next: Option<&'a dyn Handler<'a>>,
}
impl<'a> BHandler<'a> {
    fn new() -> BHandler<'a> {
        BHandler { next: None }
    }
}
impl<'a> Handler<'a> for BHandler<'a> {
    fn set_next(&mut self, next: &'a dyn Handler<'a>) -> &mut dyn Handler<'a> {
        self.next = Some(next);
        self
    }
    fn handle(&self, request: &str) {
        println!("BHandler handle the request: {}", request);
        if let Some(v) = &self.next {
            v.handle(request);
        }
    }
}

struct Client;
impl<'a> Client {
    fn handle<T: Handler<'a>>(h: &T) {
        h.handle("do something...")
    }
}

fn main() {
    let a1 = AHandler::new("dog".to_string());
    Client::handle(&a1);

    println!();
    let mut b = BHandler::new();
    let mut a2 = AHandler::new("cat".to_string());
    b.set_next(&a1);
    // or
    // let h = b.set_next(&a1);
    //a2.set_next(h);
    a2.set_next(&b);
    Client::handle(&a2);
}