rust - 如何忽略序列的部分项

Question

而不是将字段定义为可选字段，而是像下面这样收集None和Some:

extern crate serde;
extern crate serde_json;

use serde::Deserialize;

#[derive(Debug, Deserialize, PartialEq)]
struct Bar {
    a: u32,
    b: Option<u32>,
}

#[derive(Debug, Deserialize, PartialEq)]
struct Foo {
    vec: Vec<Bar>,
}

fn main() {
    let data = r#"{ "vec": [ { "a": 1 }, { "a": 2, "b": 3 } ] }"#;
    assert_eq!(
        serde_json::from_str::<Foo>(data).unwrap(),
        Foo {
            vec: vec![Bar { a: 1, b: None }, Bar { a: 2, b: Some(3) }]
        }
    );
}

最好只收集完全定义的元素，这样Bar结构可以定义为struct Bar { a: u32, b: u32 }，而serde_json::from_str只会返回Foo { vec: [ Bar { a: 2, b: 3 } ] }。

如何实现这种行为？这是我尝试创建自定义Deserialize实现的尝试，但未成功。

extern crate serde;
extern crate serde_json;

use core::fmt;
use serde::{
    de::{SeqAccess, Visitor},
    Deserialize, Deserializer,
};

#[derive(Debug, Deserialize)]
struct Bar {
    a: i32,
    b: i32,
    c: i32,
}

#[derive(Debug)]
struct VecOpt(Vec<Bar>);

impl<'de> Deserialize<'de> for VecOpt {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        struct ArrayVecVisitor;

        impl<'de> Visitor<'de> for ArrayVecVisitor {
            type Value = VecOpt;

            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                write!(formatter, "VecOpt")
            }

            fn visit_seq<SA>(self, mut seq: SA) -> Result<Self::Value, SA::Error>
            where
                SA: SeqAccess<'de>,
            {
                let mut values = Vec::new();

                loop {
                    match seq.next_element::<Bar>() {
                        Ok(Some(x)) => values.push(x),
                        Ok(None) => break,
                        // If error, the input reader won't move to the next element of the sequence and the following `seq.next_element` will
                        // simply try to read contents of the current element. In this case, `"c": 5 }, ... `
                        Err(_) => {}
                    }
                }

                Ok(VecOpt(values))
            }
        }

        deserializer.deserialize_seq(ArrayVecVisitor)
    }
}

#[derive(Debug, Deserialize)]
struct Foo {
    vec: VecOpt,
}

fn main() {
    let data = r#"{ "vec": [ { "a": 1 "b": 2, "c": 3 }, { "b": 4, "c": 5 }, { "a": 6 "b": 7, "c": 8 } ] }"#;
    dbg!(serde_json::from_str::<Foo>(data).unwrap());
}

Answer 1

您的代码在大多数情况下都起作用，但main中的JSON文字无效，该字符串错过了几个逗号，使其绊倒了。问题在于Err(_)分支不分青红皂白地吞噬了所有错误。它应该只消除了缺少的字段错误:

fn visit_seq<SA>(self, mut seq: SA) -> Result<Self::Value, SA::Error>
where
    SA: SeqAccess<'de>,
{
    let mut values = Vec::new();

    loop {
        match seq.next_element::<Bar>() {
            Ok(Some(x)) => values.push(x),
            Ok(None) => break,
            Err(e) => {
                if !e.to_string().starts_with("missing field") {
                    return Err(e);
                }
            }
        }
    }

    Ok(VecOpt(values))
}

TBH尽管确实有效，但我真的不喜欢这种解决方案。 Option非常适合在此处建模潜在的缺失字段。