rust - 如何为我不拥有的类型实现 serde 并让它支持复合/wrapper/collection 类型

Question

这个问题类似
How do I implement a trait I don't own for a type I don't own?
我为 Date 编写了一个序列化程序，使用文档中描述的机制，我的模块包装了一个序列化函数

pub mod my_date_format {
    use chrono::{Date, NaiveDate, Utc};
    use serde::{self, Deserialize, Deserializer, Serializer};

    const SERIALIZE_FORMAT: &'static str = "%Y-%m-%d";

    pub fn serialize<S>(date: &Date<Utc>, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        let s = format!("{}", date.format(SERIALIZE_FORMAT));
        serializer.serialize_str(&s)
    }

    pub fn deserialize<'de, D>(deserializer: D) -> Result<Date<Utc>, D::Error>
    where
        D: Deserializer<'de>,
    {
        let s = String::deserialize(deserializer)?;
        NaiveDate::parse_from_str(s.as_str(), SERIALIZE_FORMAT)
            .map_err(serde::de::Error::custom)
            .map(|x| {
                let now = Utc::now();
                let date: Date<Utc> = Date::from_utc(x, now.offset().clone());
                date
            })
    }
}

那么我可以这样做:

struct MyStruct {
    #[serde(with = "my_date_format")]
    pub start: Date<Utc>,
}

问题是如果我将序列化的东西包装在其他类型(它们本身是可序列化的)中，我会收到错误:

#[serde(with = "my_date_format")]
pub dates: Vec<Date<Utc> // this won't work now since my function doesn't serialize vectors
pub maybe_date: Option<Date<Utc>>> // won't work
pub box_date: Box<Date<Utc>> // won't work...

在使用我自己的序列化程序时如何获得提供的实现？
https://docs.serde.rs/serde/ser/index.html#implementations-of-serialize-provided-by-serde

Answer 1

最直接的方法，是作为 question you linked谈论，即创建一个新类型，换行 Date<Utc> ，并实现 Serialize 和 Deserialize 对于那种类型。

#[derive(PartialOrd, Ord, PartialEq, Eq, Clone, Debug)]
struct FormattedDate(Date<Utc>);

impl Serialize for FormattedDate {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        // If you implement `Deref`, then you don't need to add `.0`
        let s = format!("{}", self.0.format(SERIALIZE_FORMAT));
        serializer.serialize_str(&s)
    }
}

impl<'de> Deserialize<'de> for FormattedDate {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        let s = String::deserialize(deserializer)?;
        NaiveDate::parse_from_str(s.as_str(), SERIALIZE_FORMAT)
            .map_err(serde::de::Error::custom)
            .map(|x| {
                let now = Utc::now();
                let date: Date<Utc> = Date::from_utc(x, now.offset().clone());
                Self(date)
                // or
                // date.into()
            })
    }
}

为了让生活更轻松，您可以实现 Deref 和 DerefMut 然后使用 FormattedDate透明地表现得好像您正在使用 Date<Utc>直接地。

use std::ops::{Deref, DerefMut};

impl Deref for FormattedDate {
    type Target = Date<Utc>;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl DerefMut for FormattedDate {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}

同样你可以实现 From 和 Into , 这样您就可以轻松地在 FormattedDate 之间转换和 Date<Utc> .

impl From<Date<Utc>> for FormattedDate {
    fn from(date: Date<Utc>) -> Self {
        Self(date)
    }
}

impl Into<Date<Utc>> for FormattedDate {
    fn into(self) -> Date<Utc> {
        self.0
    }
}

现在，您提供的所有示例都易于使用:

#[derive(Serialize, Deserialize, Debug)]
struct MyStruct {
    date: FormattedDate,
    dates: Vec<FormattedDate>,
    opt_date: Option<FormattedDate>,
    boxed_date: Box<FormattedDate>,
}

fn main() {
    let s = MyStruct {
        date: Utc::now().date().into(),
        dates: std::iter::repeat(Utc::now().date().into()).take(4).collect(),
        opt_date: Some(Utc::now().date().into()),
        boxed_date: Box::new(Utc::now().date().into()),
    };

    let json = serde_json::to_string_pretty(&s).unwrap();
    println!("{}", json);
}

哪个输出:

{
  "date": "2020-12-13",       
  "dates": [
    "2020-12-13",
    "2020-12-13",
    "2020-12-13",
    "2020-12-13"
  ],
  "opt_date": "2020-12-13",   
  "boxed_date": "2020-12-13"  
}