From 7c3ac9f215f05841c9a95e0e197795b985b73900 Mon Sep 17 00:00:00 2001
From: clucle <wjdenwls123@gmail.com>
Date: Sat, 20 Apr 2024 05:25:42 +0900
Subject: [PATCH] docs: add OneofValue type helper (#1034)

ref: https://github.com/stephenh/ts-proto/pull/1023

In previous helper, it is not possible to construct a message with the
oneof field passed as a function argument.

This helper allows for strict type checking and construct message using
oneof fields as an argument.

Example

```proto
syntax = "proto3";

package logs;

message SignUp {
    string name = 1;
}

message Login {
    string account_id = 1;
}

message Logout {
    string account_id = 1;
    string reason = 2;
}

message LogPayload {
  string created_at = 1;
  oneof payload {
    SignUp signup = 2;
    Login login = 3;
    Logout lougout = 4;
  }
}
```

```typesecript
// pass by argument
foo<P extends PaloadNames>(
  payload: OneOfValue<LogPayload['payload'], P>
) {
  const req: LogPayload = {
    createdAt: new Date().toUTCString(),
    payload,
  };

  send(req);
}
```

```typescript
// Usage

// Good
this.foo( {
  $case: 'login',
  login: {
    accountId: 'accountId',
  },
});

// Build Fail because case and field is not matched
// Argument of type '{ $case: string; login: { accountId: string; }; }' is not assignable to parameter of type 'never'.
this.foo( {
  $case: 'logout',
  login: {
    accountId: 'accountId',
  },
});
```

Signed-off-by: clucle <wjdenwls123@gmail.com>
---
 README.markdown | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/README.markdown b/README.markdown
index 1b7e8bfc2..062d7f927 100644
--- a/README.markdown
+++ b/README.markdown
@@ -796,6 +796,14 @@ type OneOfCase<T, K extends OneOfCases<T>> = T extends {
 }
   ? T[U]
   : never;
+
+/** Extracts the specific type of a value type from a oneOf field */
+export type OneOfValue<T, K extends OneOfCases<T>> = T extends {
+  $case: K;
+  [key: string]: unknown;
+}
+  ? T
+  : never;
 ```
 
 # Default values and unset fields