在C++里面, 我们可以根据一个消息的名称, 动态的创建一个实例
google::protobuf::Descriptor* desc =
google::protobuf::DescriptorPool::generated_pool()
->FindMessageTypeByName("mypkg.MyType");
google::protobuf::Message* message =
google::protobuf::MessageFactory::generated_factory()
->GetPrototype(desc)->New();
这个在protobuf里面是集成进去了, 在其他语言也有类似的东西.
通过这个, 我们就让轻松实现编解码库, 而不需去构造一个映射表.
但是, 但是在rust里面, 是没有这种东西的. 比较难的地方是rust全局变量必须要实现Send trait, 否则是不能被共享的, 这样做确实安全, 但是对于我们实现MessageFactory就变得困难.
好在rust有thread_local和build.rs, 我们可以通过build.rs在编译proto文件的时候去遍历, 把每个消息添加到一个thread_local的hash map里面去, 从而曲线救国.
//首先proto message的信息
#[derive(Debug)]
struct ProtoMessageInfo {
file: PathBuf,
file_name: String,
messages : Vec<String>,
}
//接下来生成一个factory.rs
fn generate_factory_file(path:&str, v: &Vec<ProtoMessageInfo>) {
let mut template = File::open((path.to_string() + "/factory.rs.template").as_str()).unwrap();
let mut contents = Vec::new();
template.read_to_end(&mut contents);
let mut mod_file = File::create((path.to_string() + "/mod.rs").as_str()).unwrap();
let mut factory_file = File::create((path.to_string() + "/factory.rs").as_str()).unwrap();
mod_file.write(b"pub mod factory;\n");
factory_file.write_all(&contents[..]);
factory_file.write(b"\n\n");
for item in v.iter() {
factory_file.write_fmt(format_args!("use crate::proto::{};\n", item.file_name));
mod_file.write_fmt(format_args!("pub mod {};\n", item.file_name));
}
factory_file.write(b"\npub fn init_descriptors() {");
for file in v.iter() {
for msg in file.messages.iter() {
factory_file.write_fmt(format_args!("\n register_message::<{}::{}>();", file.file_name, msg));
}
}
factory_file.write(b"\n}\n");
}
fn get_proto_list(v: &Vec<ProtoMessageInfo>) -> Vec<&str> {
let mut r = Vec::new();
for f in v.iter() {
r.push(f.file.to_str().unwrap());
}
r
}
比如我们有一个t.proto文件
syntax="proto3";
package t111;
option optimize_for = SPEED;
message RpcRequestHandShake
{
int32 server_id = 1;
int64 start_time = 2;
}
在build.rs内添加这样的代码:
fn main() {
let proto_path = "src/proto";
let v = get_protos_info(proto_path);
let protos = get_proto_list(&v);
protoc_rust::run(protoc_rust::Args {
out_dir: proto_path,
input: &protos,
includes: &[proto_path],
customize: Customize {
..Default::default()
},
}).expect("protoc");
generate_factory_file(proto_path, &v);
}
然后就可以看到生成的factory.rs:
use protobuf::reflect::MessageDescriptor;
use protobuf::Message;
use std::cell::RefCell;
use std::collections::HashMap;
thread_local! {
pub static GLOBAL_MAP : RefCell<HashMap<String, &‘static MessageDescriptor>> = RefCell::new(HashMap::new());
}
pub fn register_message<M: Message>() {
GLOBAL_MAP.with(|x| {
let mut m = x.borrow_mut();
let name = M::descriptor_static().full_name().to_string();
if !m.contains_key(&name) {
m.insert(name, M::descriptor_static());
}
})
}
pub fn get_descriptor(full_name: String) -> Option<&‘static MessageDescriptor> {
GLOBAL_MAP.with(move |x| {
{
let m = x.borrow_mut();
if m.len() == 0 {
drop(m);
init_descriptors()
}
}
{
let m = x.borrow_mut();
match m.get(&full_name) {
Some(r) => Some(*r),
None => None,
}
}
})
}
use crate::proto::t;
pub fn init_descriptors() {
register_message::<t::RpcRequestHandShake>();
}
在main.rs里面这样使用:
mod proto;
use proto::factory::*;
use proto::rpc::*;
fn main() {
let desc = get_descriptor("t111.RpcRequestHandShake".to_string()).unwrap();
println!("{}", desc.full_name());
let msg = desc.new_instance();
println!("msg: {:?}", msg);
}
cargo run:
就可以看到
t111.RpcRequestHandShake
msg:
这时候就可以拿到MessageDesriptor, 通过这个对象可以new一个instance
还未整理代码, 到时候上传到crates.io上面去
关键字: MessageFactory, Protobuf, Rust
原文：https://www.cnblogs.com/egmkang/p/11481929.html