无法使用Rustc-serialize处理JSON中的可选字段

Question

我正在尝试使用rustc_serialize将JSON反序列化为Rust结构.问题是某些JSON具有一些可选字段,即可能存在也可能不存在.遇到第一个缺席字段的那一刻,解码器似乎纾困并且不考虑后续字段,即使它们存在.有办法克服这个问题吗？

这是代码:

extern crate rustc_serialize;

#[derive(Debug)]
struct B {
    some_field_0: Option<u64>,
    some_field_1: Option<String>,
}

impl rustc_serialize::Decodable for B {
    fn decode<D: rustc_serialize::Decoder>(d: &mut D) -> Result<Self, D::Error> {
        Ok(B {
            some_field_0: d.read_struct_field("some_field_0", 0, |d| rustc_serialize::Decodable::decode(d)).ok(),
            some_field_1: d.read_struct_field("some_field_1", 0, |d| rustc_serialize::Decodable::decode(d)).ok(),
        })
    }
}

fn main() {
    {
        println!("--------------------------------\n1st run - all field present\n--------------------------------");
        let json_str = "{\"some_field_0\": 1234, \"some_field_1\": \"There\"}".to_string();
        let obj_b: B = rustc_serialize::json::decode(&json_str).unwrap();

        println!("\nJSON: {}\nDecoded: {:?}", json_str, obj_b);
    }

    {
        println!("\n\n--------------------------------\n2nd run - \"some_field_1\" absent\n---------------------------------");
        let json_str = "{\"some_field_0\": 1234}".to_string();
        let obj_b: B = rustc_serialize::json::decode(&json_str).unwrap();

        println!("\nJSON: {}\nDecoded: {:?}", json_str, obj_b);
    }

    {
        println!("\n\n--------------------------------\n3rd run - \"some_field_0\" absent\n---------------------------------");
        let json_str = "{\"some_field_1\": \"There\"}".to_string();
        let obj_b: B = rustc_serialize::json::decode(&json_str).unwrap();

        println!("\nJSON: {}\nDecoded: {:?}", json_str, obj_b);
    }
}

这是输出:

--------------------------------
1st run - all field present
--------------------------------

JSON: {"some_field_0": 1234, "some_field_1": "There"}
Decoded: B { some_field_0: Some(1234), some_field_1: Some("There") }


--------------------------------
2nd run - "some_field_1" absent
---------------------------------

JSON: {"some_field_0": 1234}
Decoded: B { some_field_0: Some(1234), some_field_1: None }


--------------------------------
3rd run - "some_field_0" absent
---------------------------------

JSON: {"some_field_1": "There"}
Decoded: B { some_field_0: None, some_field_1: None }

请注意,第三次运行会产生意外结果.当解码器未能找到some_field_0它在所有后续令牌上失败时,即使some_field_1存在.

Answer 1

你的Decodable实施有问题.使用自动生成的实现工作:

#[derive(Debug, RustcDecodable)]
struct B {
    some_field_1: Option<String>,
    some_field_0: Option<u64>,
}

JSON: {"some_field_1": "There"}
Decoded: B { some_field_1: Some("There"), some_field_0: None }

如果可以的话,使用生成的实现是正确的.如果你不能,这是正确的实现:

impl rustc_serialize::Decodable for B {
    fn decode<D: rustc_serialize::Decoder>(d: &mut D) -> Result<Self, D::Error> {
        Ok(B {
            some_field_0: try!(d.read_struct_field("some_field_0", 0, |d| rustc_serialize::Decodable::decode(d))),
            some_field_1: try!(d.read_struct_field("some_field_1", 0, |d| rustc_serialize::Decodable::decode(d))),
        })
    }
}

重要的变化是使用try!.解码可能会失败.通过使用ok,你说的是解码失败实际上是成功的,虽然成功解码了None.