NF: explaining the bridge between languages

It was, honestly, quite confusing. And if I got it wrong, I guess this
code review will be the time for me to learn what was wrong.

docs/protobuf.md

@@ -1,5 +1,19 @@
 # Protocol Buffers
 
+ProtoBuf is a format used both to save data in storage and transmit
+data between services. You can think of it as similar to JSON with
+schemas, given that you can use basic types, list and records. Except
+that it's usually represented in an efficient byteform and not in a
+human readable way.
+
+In this document we'll first explain how to encode and decode protobuf
+messages in the three languages used in anki. Typescript, python and
+rust. Please note that AnkiDroid and AnkiMobile are out of scope for
+this document. In the second part, we'll explain how the code in those
+three languages can communicate, usually using protobuf rpc.
+
+# Reading and creating protobuf messages.
+
 Anki uses [different implementations of Protocol Buffers](./architecture.md#protobuf)
 and each has its own peculiarities. This document highlights some aspects relevant
 to Anki and hopefully helps to avoid some common pitfalls.
@@ -116,3 +130,56 @@ Inside the `pb` module you will find all generated Rust types and their implemen
   a valid variant, so the Rust code needs to deal with a lot of `Option`s. As we
   don't expect other parts of Anki to send invalid messages, using an `InvalidInput`
   error or `unwrap_or_default()` is usually fine.
+
+# Communication between languages
+
+As far as I understand, Rust, being the back-end, does not send
+message to Python or Ts which are both front-ends. Instead, the front
+end can request data, that Rust returns as part of a RPC initiated by
+the front end. Those RPC are discussed below.
+
+## Sending a message to typescript.
+
+This is the only case that don't use protobuf. Any python code, having
+access to a webview `wv` can send an arbitrary
+`wv.web.eval(js_command)` or `wv.web.evalWithCallback(js _command,
+cb)` to execute an arbitrary javascript command.
+
+If your command need to be executed inside a svelte script, you can add it to `+page.svelte`, with
+
+```js
+globalThis.anki || = {};
+globalThis.anki.methodName = function() {// or async
+  body
+}
+```
+
+You then simply have to execute `anki.methodName()`.
+
+Note that if the function is asynchronous, you can't directly send the
+result to a callback. Instead your function will have to call a post
+method that will be sent to python or rust. Which leads us to the next
+section.
+
+## From typescript to Python
+
+In this section, I assume you want to send a rpc to python only, and not rust.
+
+In this case:
+
+- Add your rpc declaration `rpc NameOfMyRpc(InputType) returns (OutputType)` to [frontend.proto](/proto/anki/frontend.proto)'s FrontendService,
+- In typescript you add `import {NameOfMyRpc} from "@generated/backend"` to use this RPC, you can then use `NameOfMyRpc(args)`,
+- In [mediasrv.py](/qt/aqt/mediasrv.py) you create a method `def name_of_my_rpc(args) -> bytes`, and you add `name_of_my_rpc` to `post_handler_list`.
+
+As you don't want to block the thread, unless the result can be really
+quickly computed, you'll want to return `b""`, and use
+`aqt.mw.taskman.run_on_main(function_doing_actual_work)`.
+
+## From Typescript or Python to Rust
+
+For the sake of the example, I'll consider that you want to add a RPC in the DecksService. Any service work similarly but `FrontendService`.
+
+- In [decks.proto](/proto/anki/decks.proto)'s DecksService, you add your new rpc `rpc NameOfMyRpc(InputType) returns (OutputType)`.
+- In [services.rs](/rslib/src/decks/service.rs)'s `impl crate::services::DecksService for Collection` you add the implementation `fn name_of_my_rpc(&mut self, args) -> Error::Result<OutputType>`
+- In typescript you add `import {NameOfMyRpc} from "@generated/backend"` to use this RPC, you can then use `NameOfMyRpc(args)`,
+- In Python you add can call `col._backend.name_of_my_rpc(args)`, with `col` being your collection object.