From e602894f50435b8cd62096d4823b40daa8822d1a Mon Sep 17 00:00:00 2001
From: Brian Schiller <bgschiller@gmail.com>
Date: Thu, 5 Sep 2024 09:57:41 -0700
Subject: [PATCH] Fix Event.emit type error for enums
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Rust enums are represented in TS as a type union. For some reason, `T extends null` collapses the type into an intersection, which makes it a type error to call `SomeEvent.emit(x)` for any value x.

Here's a minimal example to try in the typescript playground. The definition of __EventObj__ is as it is *before* my patch.

```ts
// These definitions come from @tauri-apps/api/event
interface Event<T> {
    /** Event name */
    event: EventName;
    /** Event identifier used to unlisten */
    id: number;
    /** Event payload */
    payload: T;
}
declare function emit(event: string, payload?: unknown): Promise<void>;
type EventCallback<T> = (event: Event<T>) => void;
type EventName = string & Record<never, never>

// This is generated by tauri-specta based on my rust types
export type SocketAction =
  | { MessageForClient: { conn_id: string; message: string } }
  | { CloseSocket: { conn_id: string } }

// This is also from tauri-specta, but doesn't depend on my types
type __EventObj__<T> = {
  emit: T extends null
    ? (payload?: T) => ReturnType<typeof emit>
    : (payload: T) => ReturnType<typeof emit>
}

// pretend we have one of these
declare const eo: __EventObj__<SocketAction>;

const x = { MessageForClient: { conn_id: 'string', message: 'str' }} satisfies SocketAction

eo.emit(x) // 💥 Type error
```
---
 examples/app/src/bindings.ts | 2 +-
 src/lang/globals.ts          | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/examples/app/src/bindings.ts b/examples/app/src/bindings.ts
index 3ec8f4c..5b4519c 100644
--- a/examples/app/src/bindings.ts
+++ b/examples/app/src/bindings.ts
@@ -94,7 +94,7 @@ type __EventObj__<T> = {
 	once: (
 		cb: TAURI_API_EVENT.EventCallback<T>,
 	) => ReturnType<typeof TAURI_API_EVENT.once<T>>;
-	emit: T extends null
+	emit: null extends T
 		? (payload?: T) => ReturnType<typeof TAURI_API_EVENT.emit>
 		: (payload: T) => ReturnType<typeof TAURI_API_EVENT.emit>;
 };
diff --git a/src/lang/globals.ts b/src/lang/globals.ts
index 645d343..a100a27 100644
--- a/src/lang/globals.ts
+++ b/src/lang/globals.ts
@@ -12,7 +12,7 @@ type __EventObj__<T> = {
 	once: (
 		cb: TAURI_API_EVENT.EventCallback<T>,
 	) => ReturnType<typeof TAURI_API_EVENT.once<T>>;
-	emit: T extends null
+	emit: null extends T
 		? (payload?: T) => ReturnType<typeof TAURI_API_EVENT.emit>
 		: (payload: T) => ReturnType<typeof TAURI_API_EVENT.emit>;
 };