Rust 访问者模式讲解和代码示例

use crate::{TwoValuesArray, TwoValuesStruct}; /// Visitor can visit one type, do conversions, and output another type. /// /// It's not like all visitors must return a new type, it's just an example /// that demonstrates the technique. pub trait Visitor { type Value; /// Visits a vector of integers and outputs a desired type. fn visit_vec(&self, v: Vec<i32>) -> Self::Value; } /// Visitor implementation for a struct of two values. impl Visitor for TwoValuesStruct { type Value = TwoValuesStruct; fn visit_vec(&self, v: Vec<i32>) -> Self::Value { TwoValuesStruct { a: v[0], b: v[1] } } } /// Visitor implementation for a struct of values array. impl Visitor for TwoValuesArray { type Value = TwoValuesArray; fn visit_vec(&self, v: Vec<i32>) -> Self::Value { let mut ab = [0i32; 2]; ab[0] = v[0]; ab[1] = v[1]; TwoValuesArray { ab } } } 

#![allow(unused)] mod visitor; use visitor::Visitor; /// A struct of two integer values. /// /// It's going to be an output of `Visitor` trait which is defined for the type /// in `visitor.rs`. #[derive(Default, Debug)] pub struct TwoValuesStruct { a: i32, b: i32, } /// A struct of values array. /// /// It's going to be an output of `Visitor` trait which is defined for the type /// in `visitor.rs`. #[derive(Default, Debug)] pub struct TwoValuesArray { ab: [i32; 2], } /// `Deserializer` trait defines methods that can parse either a string or /// a vector, it accepts a visitor which knows how to construct a new object /// of a desired type (in our case, `TwoValuesArray` and `TwoValuesStruct`). trait Deserializer<V: Visitor> { fn create(visitor: V) -> Self; fn parse_str(&self, input: &str) -> Result<V::Value, &'static str> { Err("parse_str is unimplemented") } fn parse_vec(&self, input: Vec<i32>) -> Result<V::Value, &'static str> { Err("parse_vec is unimplemented") } } struct StringDeserializer<V: Visitor> { visitor: V, } impl<V: Visitor> Deserializer<V> for StringDeserializer<V> { fn create(visitor: V) -> Self { Self { visitor } } fn parse_str(&self, input: &str) -> Result<V::Value, &'static str> { // In this case, in order to apply a visitor, a deserializer should do // some preparation. The visitor does its stuff, but it doesn't do everything. let input_vec = input .split_ascii_whitespace() .map(|x| x.parse().unwrap()) .collect(); Ok(self.visitor.visit_vec(input_vec)) } } struct VecDeserializer<V: Visitor> { visitor: V, } impl<V: Visitor> Deserializer<V> for VecDeserializer<V> { fn create(visitor: V) -> Self { Self { visitor } } fn parse_vec(&self, input: Vec<i32>) -> Result<V::Value, &'static str> { Ok(self.visitor.visit_vec(input)) } } fn main() { let deserializer = StringDeserializer::create(TwoValuesStruct::default()); let result = deserializer.parse_str("123 456"); println!("{:?}", result); let deserializer = VecDeserializer::create(TwoValuesStruct::default()); let result = deserializer.parse_vec(vec![123, 456]); println!("{:?}", result); let deserializer = VecDeserializer::create(TwoValuesArray::default()); let result = deserializer.parse_vec(vec![123, 456]); println!("{:?}", result); println!( "Error: {}", deserializer.parse_str("123 456").err().unwrap() ) } 

Ok(TwoValuesStruct { a: 123, b: 456 }) Ok(TwoValuesStruct { a: 123, b: 456 }) Ok(TwoValuesArray { ab: [123, 456] }) Error: parse_str unimplemented