[英]How to implement `serde::Serialize` for a boxed trait object?
I ran into a problem trying to create a generic vector for a struct.我在尝试为结构创建通用向量时遇到了问题。 This was my first attempt:
这是我的第一次尝试:
#[derive(Serialize)]
struct Card {
sections: Vec<Section<dyn WidgetTrait>>
}
#[derive(Serialize)]
struct Section<T: WidgetTrait> {
header: String,
widgets: Vec<T>
}
This has brought me to an error that Sized
is not implemented and WidgetTrait
size is not known at compile time.这给我带来了一个错误,即未实现
Sized
并且在编译时不知道WidgetTrait
大小。
My next attempt was to use Box<dyn WidgetTrait>
like so:我的下一个尝试是像这样使用
Box<dyn WidgetTrait>
:
#[derive(Serialize)]
struct Section {
header: String,
widgets: Vec<Box<dyn WidgetTrait>>
}
This has led me to an error:这导致我犯了一个错误:
error[E0277]: the trait bound `WidgetTrait: serde::Serialize` is not satisfied
--> src/main.rs:11:10
|
11 | #[derive(Serialize)]
| ^^^^^^^^^ the trait `serde::Serialize` is not implemented for `WidgetTrait`
|
= note: required because of the requirements on the impl of `serde::Serialize` for `std::boxed::Box<dyn WidgetTrait>`
= note: required because of the requirements on the impl of `serde::Serialize` for `std::vec::Vec<std::boxed::Box<dyn WidgetTrait>>`
= note: required by `serde::ser::SerializeStruct::serialize_field`
My goal is for the widgets vector in Section
struct to be able to accept different types of widgets that implement WidgetTrait
trait, just like you would with an interface.我的目标是让
Section
结构中的小部件向量能够接受实现WidgetTrait
特征的不同类型的小部件,就像使用接口一样。
For serializing Serde trait objects you should use erased-serde
.要序列化 Serde 特征对象,您应该使用
erased-serde
。
#[macro_use]
extern crate serde_derive;
#[macro_use]
extern crate erased_serde;
extern crate serde;
extern crate serde_json;
#[derive(Serialize)]
struct Card {
sections: Vec<Section>,
}
#[derive(Serialize)]
struct Section {
header: String,
widgets: Vec<Box<dyn WidgetTrait>>,
}
#[derive(Serialize)]
struct Image {
image_url: String,
}
#[derive(Serialize)]
struct KeyValue {
top_label: String,
content: String,
}
trait WidgetTrait: erased_serde::Serialize {}
impl WidgetTrait for Image {}
impl WidgetTrait for KeyValue {}
serialize_trait_object!(WidgetTrait);
fn main() {
let card = Card {
sections: vec![
Section {
header: "text".to_owned(),
widgets: vec![
Box::new(Image {
image_url: "img".to_owned(),
}),
Box::new(KeyValue {
top_label: "text".to_owned(),
content: "text".to_owned(),
}),
],
},
],
};
println!("{}", serde_json::to_string_pretty(&card).unwrap());
}
I got around the compiler errors:我解决了编译器错误:
#[macro_use]
extern crate serde_derive;
extern crate serde_json;
extern crate serde;
use serde::ser::{Serialize, Serializer, SerializeStruct};
#[derive(Serialize)]
struct Card {
sections: Vec<Section>
}
#[derive(Serialize)]
struct Section {
header: String,
widgets: Vec<Box<dyn WidgetTrait>>
}
#[derive(Serialize)]
struct Image {
#[serde(rename = "imageUrl")]
image_url: String
}
#[derive(Serialize)]
struct KeyValue {
#[serde(rename = "topLabel")]
top_label: String,
content: String
}
trait WidgetTrait {}
impl Serialize for WidgetTrait {
fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
where S: Serializer {
let s = serializer.serialize_struct("???", 3)?;
s.end()
}
}
impl WidgetTrait for Image {}
impl WidgetTrait for KeyValue {}
fn main() {
// let test = ResponseMessage {
// text: None,
// cards: Some(
// vec![Card { sections: vec![
// Section { header: format!("text"), widgets: vec![
// Box::new(Image { image_url: format!("img") })
// ]},
// Section { header: format!("text"), widgets: vec![
// Box::new(KeyValue { top_label: format!("text"), content: format!("text") }),
// Box::new(KeyValue { top_label: format!("text"), content: format!("text") })
// ]}
// ]}])
// }
}
Steps for a working solution.工作解决方案的步骤。
as_any()
implementations for your structs that implement WidgetTrait
as per How to get a reference to a concrete type from a trait object?WidgetTrait
的结构编写as_any()
实现? . Serialize
of type Box<dyn WidgetTrait>
Box<dyn WidgetTrait>
类型的 trait Serialize
添加实现Box<Widget>
to the struct so we know the type using as_any()
and downcast_ref()
Box<Widget>
到结构体,所以我们知道使用as_any()
和downcast_ref()
的类型#[macro_use]
extern crate serde_derive;
extern crate serde_json;
extern crate serde;
use serde::ser::{Serialize, Serializer, SerializeStruct};
use std::any::Any;
#[derive(Serialize)]
struct Card {
sections: Vec<Section>
}
#[derive(Serialize)]
struct Section {
header: String,
widgets: Vec<Box<dyn WidgetTrait>>
}
#[derive(Serialize)]
struct Image {
#[serde(rename = "imageUrl")]
image_url: String
}
#[derive(Serialize)]
struct KeyValue {
#[serde(rename = "topLabel")]
top_label: String,
content: String
}
trait WidgetTrait {
fn as_any(&self) -> &Any;
}
impl Serialize for Box<dyn WidgetTrait> {
fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
where S: Serializer {
return match self.as_any().downcast_ref::<Image>() {
Some(img) => {
let mut widget_serializer = serializer.serialize_struct("Image", 1)?;
widget_serializer.serialize_field("imageUrl", &img.image_url)?;
widget_serializer.end()
},
None => {
let key_value: &KeyValue = match self.as_any().downcast_ref::<KeyValue>() {
Some(k) => k,
None => panic!("Unknown type!")
};
let mut widget_serializer = serializer.serialize_struct("KeyValue", 2)?;
widget_serializer.serialize_field("topLabel", &key_value.top_label)?;
widget_serializer.serialize_field("content", &key_value.content)?;
widget_serializer.end()
}
};
}
}
impl WidgetTrait for Image {
fn as_any(&self) -> &Any {
self
}
}
impl WidgetTrait for KeyValue {
fn as_any(&self) -> &Any {
self
}
}
fn main() {
// let test = ResponseMessage {
// text: None,
// cards: Some(
// vec![Card { sections: vec![
// Section { header: format!("text"), widgets: vec![
// Box::new(Image { image_url: format!("img") })
// ]},
// Section { header: format!("text"), widgets: vec![
// Box::new(KeyValue { top_label: format!("text"), content: format!("text") }),
// Box::new(KeyValue { top_label: format!("text"), content: format!("text") })
// ]}
// ]}])
// }
}
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