prost-reflect

A protobuf library extending prost with reflection support and dynamic messages.

Usage

This crate provides support for dynamic protobuf messages. These are useful when the protobuf type definition is not known ahead of time.

The main entry points into the API of this crate are:

• [DescriptorPool] wraps a FileDescriptorSet output by the protobuf compiler to provide an API for inspecting type definitions.
• [DynamicMessage] provides encoding, decoding and reflection of an arbitrary protobuf message definition described by a [MessageDescriptor].

Example - decoding

DynamicMessage does not implement [Default] since it needs a message descriptor to function. To decode a protobuf byte stream into an instance of this type, use [DynamicMessage::decode] to create a default value for the MessageDescriptor instance and merge into it:

```rust use prost::Message; use prosttypes::FileDescriptorSet; use prostreflect::{DynamicMessage, DescriptorPool, Value};

let pool = DescriptorPool::decode(includebytes!("filedescriptorset.bin").asref()).unwrap(); let messagedescriptor = pool.getmessagebyname("package.MyMessage").unwrap();

let dynamicmessage = DynamicMessage::decode(messagedescriptor, b"\x08\x96\x01".as_ref()).unwrap();

asserteq!(dynamicmessage.getfieldbyname("foo").unwrap().asref(), &Value::I32(150)); ```

Example - JSON mapping

When the serde feature is enabled, DynamicMessage can be deserialized to and from the canonical JSON mapping defined for protobuf messages.

```rust use prost::Message; use prostreflect::{DynamicMessage, DescriptorPool, Value}; use serdejson::de::Deserializer;

let pool = DescriptorPool::decode(includebytes!("filedescriptorset.bin").asref()).unwrap(); let messagedescriptor = pool.getmessagebyname("package.MyMessage").unwrap();

let json = r#"{ "foo": 150 }"#; let mut deserializer = Deserializer::fromstr(json); let dynamicmessage = DynamicMessage::deserialize(message_descriptor, &mut deserializer).unwrap(); deserializer.end().unwrap();

asserteq!(dynamicmessage.getfieldbyname("foo").unwrap().asref(), &Value::I32(150)); ```

Example - implementing ReflectMessage

The [ReflectMessage] trait provides a .descriptor() method to get type information for a message. By default it is just implemented for DynamicMessage.

When the reflect-well-known-types feature is enabled, it is implemented for the well-known-types provided by prost-types.

When the derive feature is enabled, it can be derived for Message implementations. The derive macro takes the following parameters:

| Name | Value | |-----------------|-------| | descriptorpool | An expression that resolves to a [DescriptorPool] containing the message type. The descriptor should be cached to avoid re-building it. | | messagename | The full name of the message, used to look it up within [DescriptorPool]. |

```rust use prost::Message; use prostreflect::{DescriptorPool, ReflectMessage}; use oncecell::sync::Lazy;

static DESCRIPTORPOOL: Lazy = Lazy::new(|| DescriptorPool::decode(includebytes!("filedescriptorset.bin").as_ref()).unwrap());

[derive(Message, ReflectMessage)]

[prostreflect(descriptorpool = "DESCRIPTORPOOL", messagename = "package.MyMessage")]

pub struct MyMessage {}

let message = MyMessage {}; asserteq!(message.descriptor().fullname(), "package.MyMessage"); ```

If you are using prost-build, the prost-reflect-build crate provides helpers to generate ReflectMessage implementations:

rust,no_run prost_reflect_build::Builder::new() .compile_protos(&["src/package.proto"], &["src"]) .unwrap();

Minimum Supported Rust Version

Rust 1.56 or higher.

The minimum supported Rust version may be changed in the future, but it will be done with a minor version bump.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.