这是这样做的一种方法：
#[macro_use]
extern crate serde_derive; // 1.0.117
extern crate serde; // 1.0.117
extern crate serde_json; // 1.0.59
#[derive(Serialize, Deserialize, Debug)]
pub struct MyError {
error: String,
}
#[derive(Serialize, Deserialize, Debug)]
pub struct MyAge {
age: i32,
name: String,
}
#[derive(Debug)]
enum AgeOrError {
Age(MyAge),
Error(MyError),
}
impl serde::Serialize for AgeOrError {
fn serialize<S: serde::Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
match self {
&AgeOrError::Age(ref my_age) => serializer.serialize_some(my_age),
&AgeOrError::Error(ref my_error) => serializer.serialize_some(my_error),
}
}
}
enum AgeOrErrorField {
Age,
Name,
Error,
}
impl<'de> serde::Deserialize<'de> for AgeOrErrorField {
fn deserialize<D>(deserializer: D) -> Result<AgeOrErrorField, D::Error>
where
D: serde::Deserializer<'de>,
{
struct AgeOrErrorFieldVisitor;
impl<'de> serde::de::Visitor<'de> for AgeOrErrorFieldVisitor {
type Value = AgeOrErrorField;
fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
write!(formatter, "age or error")
}
fn visit_str<E>(self, value: &str) -> Result<AgeOrErrorField, E>
where
E: serde::de::Error,
{
Ok(match value {
"age" => AgeOrErrorField::Age,
"name" => AgeOrErrorField::Name,
"error" => AgeOrErrorField::Error,
_ => panic!("Unexpected field name: {}", value),
})
}
}
deserializer.deserialize_any(AgeOrErrorFieldVisitor)
}
}
impl<'de> serde::Deserialize<'de> for AgeOrError {
fn deserialize<D>(deserializer: D) -> Result<AgeOrError, D::Error>
where
D: serde::Deserializer<'de>,
{
deserializer.deserialize_map(AgeOrErrorVisitor)
}
}
struct AgeOrErrorVisitor;
impl<'de> serde::de::Visitor<'de> for AgeOrErrorVisitor {
type Value = AgeOrError;
fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
write!(formatter, "age or error")
}
fn visit_map<A>(self, mut map: A) -> Result<AgeOrError, A::Error>
where
A: serde::de::MapAccess<'de>,
{
let mut age: Option<i32> = None;
let mut name: Option<String> = None;
let mut error: Option<String> = None;
loop {
match map.next_key()? {
Some(AgeOrErrorField::Age) => age = map.next_value()?,
Some(AgeOrErrorField::Name) => name = map.next_value()?,
Some(AgeOrErrorField::Error) => error = map.next_value()?,
None => break,
}
}
if let Some(error) = error {
Ok(AgeOrError::Error(MyError { error: error }))
} else {
Ok(AgeOrError::Age(MyAge {
age: age.expect("!age"),
name: name.expect("!name"),
}))
}
}
}
fn get_results(ages: &[i32]) -> Vec<AgeOrError> {
let mut results = Vec::with_capacity(ages.len());
for &age in ages.iter() {
if age < 100 && age > 0 {
results.push(AgeOrError::Age(MyAge {
age: age,
name: String::from("The dude"),
}));
} else {
results.push(AgeOrError::Error(MyError {
error: format!("{} is invalid age", age),
}));
}
}
results
}
fn main() {
let v = get_results(&[1, -6, 7]);
let serialized = serde_json::to_string(&v).expect("Can't serialize");
println!("serialized: {}", serialized);
let deserialized: Vec<AgeOrError> =
serde_json::from_str(&serialized).expect("Can't deserialize");
println!("deserialized: {:?}", deserialized);
}
请注意，在反序列化中我们不能重用自动生成的反序列化器，因为：
- 反序列化是将字段流式传输给我们，我们无法查看字符串化的 JSON 表示并猜测它是什么；
- 我们无权访问
serde::de::VisitorSerde 生成的实现。
我也做了一个捷径并panic避免了错误。在生产代码中，您希望返回正确的 Serde 错误。
另一种解决方案是创建一个所有字段可选的合并结构，如下所示：
#[macro_use]
extern crate serde_derive; // 1.0.70
extern crate serde; // 1.0.70
extern crate serde_json; // 1.0.24
#[derive(Debug)]
pub struct MyError {
error: String,
}
#[derive(Debug)]
pub struct MyAge {
age: i32,
name: String,
}
#[derive(Serialize, Deserialize, Debug)]
pub struct MyAgeOrError {
#[serde(skip_serializing_if = "Option::is_none")]
age: Option<i32>,
#[serde(skip_serializing_if = "Option::is_none")]
name: Option<String>,
#[serde(skip_serializing_if = "Option::is_none")]
error: Option<String>,
}
impl MyAgeOrError {
fn from_age(age: MyAge) -> MyAgeOrError {
MyAgeOrError {
age: Some(age.age),
name: Some(age.name),
error: None,
}
}
fn from_error(error: MyError) -> MyAgeOrError {
MyAgeOrError {
age: None,
name: None,
error: Some(error.error),
}
}
}
fn get_results(ages: &[i32]) -> Vec<MyAgeOrError> {
let mut results = Vec::with_capacity(ages.len());
for &age in ages.iter() {
if age < 100 && age > 0 {
results.push(MyAgeOrError::from_age(MyAge {
age: age,
name: String::from("The dude"),
}));
} else {
results.push(MyAgeOrError::from_error(MyError {
error: format!("{} is invalid age", age),
}));
}
}
results
}
fn main() {
let v = get_results(&[1, -6, 7]);
let serialized = serde_json::to_string(&v).expect("Can't serialize");
println!("serialized: {}", serialized);
let deserialized: Vec<MyAgeOrError> =
serde_json::from_str(&serialized).expect("Can't deserialize");
println!("deserialized: {:?}", deserialized);
}
我会为这个做担保，因为它允许 Rust 结构（例如MyAgeOrError）与您的 JSON 的布局相匹配。这样，JSON 布局就会记录在 Rust 代码中。
PS 最近我倾向于在RawValue的帮助下延迟可选或动态类型的 JSON 部分的解码。但是序列化它们很棘手，因为RawValue是借用的。例如，为了帮助序列化，可以实习a RawValue，将其推广到'static生命周期：
use serde_json::value::{RawValue as RawJson};
fn intern_raw_json(raw_json: Box<RawJson>) -> &'static RawJson {
use parking_lot::Mutex;
use std::mem::transmute;
static BUF: Mutex<Vec<Pin<Box<RawJson>>>> = Mutex::new(Vec::new());
let buf = BUF.lock();
let raw_json: Pin<Box<RawJson>> = raw_json.into();
let pt: &'static RawJson = {
let pt: &RawJson = &*raw_json;
transmute(pt)
};
buf.push(raw_json);
pt
}
如果性能不是问题，那么可以将动态部分反序列化为Value。
同样，如果 usingValue是一个选项，则可以通过实现来简化自定义反序列化TryFrom<Value>。