Crate prost_reflect

Expand description

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:

use prost::Message;
use prost_types::FileDescriptorSet;
use prost_reflect::{DynamicMessage, DescriptorPool, Value};

let pool = DescriptorPool::decode(include_bytes!("file_descriptor_set.bin").as_ref()).unwrap();
let message_descriptor = pool.get_message_by_name("package.MyMessage").unwrap();

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

assert_eq!(dynamic_message.get_field_by_name("foo").unwrap().as_ref(), &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.

use prost::Message;
use prost_reflect::{DynamicMessage, DescriptorPool, Value};
use serde_json::de::Deserializer;

let pool = DescriptorPool::decode(include_bytes!("file_descriptor_set.bin").as_ref()).unwrap();
let message_descriptor = pool.get_message_by_name("package.MyMessage").unwrap();

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

assert_eq!(dynamic_message.get_field_by_name("foo").unwrap().as_ref(), &Value::I32(150));

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
descriptor_pool	An expression that resolves to a `DescriptorPool` containing the message type. The descriptor should be cached to avoid re-building it.
message_name	The full name of the message, used to look it up within `DescriptorPool`.

use prost::Message;
use prost_reflect::{DescriptorPool, ReflectMessage};
use once_cell::sync::Lazy;

static DESCRIPTOR_POOL: Lazy<DescriptorPool>
    = Lazy::new(|| DescriptorPool::decode(include_bytes!("file_descriptor_set.bin").as_ref()).unwrap());

#[derive(Message, ReflectMessage)]
#[prost_reflect(descriptor_pool = "DESCRIPTOR_POOL", message_name = "package.MyMessage")]
pub struct MyMessage {}

let message = MyMessage {};
assert_eq!(message.descriptor().full_name(), "package.MyMessage");

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

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

Re-exports

pub use prost;

pub use prost::bytes;

pub use prost_types;

Structs

DescriptorError

An error that may occur while creating a DescriptorPool.

DescriptorPool

A DescriptorPool is a collection of related descriptors. Typically it will be created from a FileDescriptorSet output by the protobuf compiler (see DescriptorPool::from_file_descriptor_set) but it may also be built up manually by adding individual files.

DeserializeOptionsserde

Options to control deserialization of messages.

DynamicMessage

DynamicMessage provides encoding, decoding and reflection of a protobuf message.

EnumDescriptor

A protobuf enum type.

EnumValueDescriptor

A value in a protobuf enum type.

ExtensionDescriptor

A protobuf extension field definition.

FieldDescriptor

A protobuf message definition.