Add new docs about assigning variables to global scope (#186)

* Add new docs abbout assigning variables to global scope

* fix typo

* review comments

Author: Perryvw

docs/assigning-global-variables.md

@@ -0,0 +1,195 @@
+---
+title: Setting global variables or functions
+---
+
+import { SideBySide } from "@site/src/components/SideBySide";
+
+In some Lua environments, the host application expects you to define global variables or functions as part of the API. For example, some engines might allow you to define some event handlers in your Lua, that will be called by the engine when different events happen:
+
+```lua title=example.lua
+function OnStart()
+    -- start event handling code
+end
+
+function OnStateChange(newState)
+    -- state change event handler code
+end
+```
+
+Due to the way TSTL translates module code, functions will be `local` after translation, causing the engine not to find them:
+
+<SideBySide>
+
+```typescript title=input.ts
+function OnStart(this: void) {
+  // start event handling code
+}
+function OnStateChange(this: void, newState: State) {
+  // state change event handler code
+}
+```
+
+```lua title=output.lua
+local function OnStart()
+end
+local function OnStateChange(newState)
+end
+```
+
+</SideBySide>
+
+This means we need extra helper code to correctly register these global variables so your environment can access them.
+
+## Assigning to globals with declarations
+
+The easiest way to set global variables is to first declare them as existing globals, and then assign their values. As an example:
+
+```ts
+declare var OnStart: (this: void) => void;
+declare var OnStateChanged: (this: void, newState: State) => void;
+
+OnStart = () => {
+  // start event handling code
+};
+OnStateChanged = (newState: State) => {
+  // state change event handler code
+};
+```
+
+In the example above, the declarations are in the same file as the value assignments. Alternatively, you could choose to put them in a .d.ts file included in your project. If these globals have pre-defined names specified by the engine the API, it is also possible to include these declarations in the .d.ts files (or types package) for this environment.
+
+## Setting global variables with a helper function
+
+Another way to assign global variables and functions is to use a helper function. The benefit is that you can strictly type the helper functions to ensure correct types are assigned, as well as having the ability to do additional logic like wrapping or modifying the value.
+
+The simplest helper function looks like this:
+
+```typescript
+function registerEventHandler<TArgs extends unknown[]>(
+  handlerName: string,
+  handler: (this: void, ...args: TArgs) => void,
+): void {
+  // @ts-ignore tell TS to ignore us 'illegally' writing to global scope
+  globalThis[handlerName] = handler;
+}
+```
+
+This helper function can be added in some shared TypeScript helper file and imported wherever you need it.
+
+You can now write the example code like this:
+
+```typescript
+registerEventHandler("OnStart", () => {
+  // start event handling code
+});
+registerEventHandler("OnStateChanged", (newState: State) => {
+  // state change event handler code
+});
+```
+
+Of course you can modify `registerEventHandler` to your needs if you need to assign variables of different types to the global scope. For example, you could add a second `register` function for assigning non-function values if needed:
+
+```typescript
+function registerGlobalVariable<T>(variableName: string, value: T): void {
+  // @ts-ignore tell TS to ignore us 'illegally' writing to global scope
+  globalThis[handlerName] = value;
+}
+```
+
+## Registering functions as class methods with a decorator
+
+Sometimes you don't want to register just a loose function, but instead register a class or class method. A nice way to do this is to use decorators (they unfortunately only work on classes, and not for loose functions).
+
+One example of such a decorator is:
+
+```typescript
+function registerEventHandler<TReturn, TArgs extends unknown[]>(
+  method: (...args: TArgs) => TReturn,
+  context: ClassMethodDecoratorContext,
+) {
+  /** @noSelf - the engine will not pass self parameter so wrap in lambda without self */
+  const contextless = (...args: TArgs) => method(...args);
+  // We can read the name of the method from the context
+  const globalName = context.name;
+  // @ts-ignore tell TS to ignore us 'illegally' writing to global scope
+  globalThis[globalName] = contextless;
+}
+```
+
+You can then write above example as:
+
+```typescript
+class EventHandlers {
+  @registerEventHandler
+  public OnStart() {
+    // start event handling code
+  }
+  @registerEventHandler
+  public OnStateChanged(newState: State) {
+    // state change event handler code
+  }
+}
+```
+
+:::note
+In the above example, `this` will be `nil` in the methods, do not try to use other members in the EventHandlers class!
+:::
+
+## Registering classes with a decorator
+
+Sometimes you want to register classes instead of functions, you can also do that with a decorator:
+
+```typescript
+function registerClass<TClass, TArgs extends unknown[]>(
+  c: new (...args: TArgs) => TClass,
+  context: ClassDecoratorContext,
+) {
+  if (context.name) {
+    // @ts-ignore tell TS to ignore us 'illegally' writing to global scope
+    globalThis[context.name] = c;
+  }
+}
+```
+
+You can now register any class by simply adding the decorator:
+
+```typescript
+@registerClass
+class EventHandlers {
+  public OnStart() {
+    // start event handling code
+  }
+  public OnStateChanged(newState: State) {
+    // state change event handler code
+  }
+}
+```
+
+### Custom global name decorator
+
+In the examples above, the decorators directly used the name of the decorated class or method, but with decorator parameters you can also specify custom override names:
+
+```typescript
+const registerClass =
+  (globalName: string) =>
+  <TClass, TArgs extends unknown[]>(c: new (...args: TArgs) => TClass, context: ClassDecoratorContext) => {
+    if (context.name) {
+      // @ts-ignore tell TS to ignore us 'illegally' writing to global scope
+      globalThis[globalName] = c;
+    }
+  };
+```
+
+Now instead of taking the global name from the class, you can specify a custom name yourself:
+
+```typescript
+@registerClass("CustomGlobalName")
+class EventHandlers {
+  public OnStart() {
+    // start event handling code
+  }
+  public OnStateChanged(newState: State) {
+    // state change event handler code
+  }
+}
+```

sidebars.json

@@ -9,6 +9,7 @@
                 "caveats",
                 "the-self-parameter",
                 "advanced/writing-declarations",
+                "assigning-global-variables",
                 "external-code",
                 "publishing-modules",
                 "editor-support"