[base-ui] Create hooks contribution guide

packages/mui-base/CONTRIBUTING.md

@@ -0,0 +1,180 @@
+# Contributing
+
+## Creating a new hook
+
+### File structure
+
+When creating a new hook, follow the file structure found in other hooks' directories:
+
+Taking the imaginary `useAwesomeControl` as an example:
+
+- 📂 `useAwesomeControl`
+  - 📄 `index.ts` - aggregates the public exports from all the other files in the directory
+  - 📄 `useAwesomeControl.test.tsx` - unit tests
+  - 📄 `useAwesomeControl.spec.tsx` - (optional) type tests
+  - 📄 `useAwesomeControl.ts` - the implementation of the hook
+  - 📄 `useAwesomeControl.types.ts` - type definitions
+
+### Implementation
+
+Most Base UI hooks have a similar structure:
+
+1. [Parameters destructuring](#parameters-destructuring)
+2. Hook-specific logic
+3. [Event handler factory functions](#event-handler-factory-functions)
+4. [Slot props resolvers](#slot-props-resolvers)
+
+#### Parameters destructuring
+
+The parameters type must be called `[HookName]Parameters`.
+There are docs generation scripts that require this pattern.
+
+#### Event handler factory functions
+
+We don't define event handlers directly as functions because they must be able to access and call other handlers provided by developers in slot props resolvers.
+
+In other words, instead of defining the `handleClick(event: React.MouseEvent) => void` function, we define the `createHandleClick(otherHandlers: EventHandlers) => (event: React.MouseEvent) => void`. The `otherHandlers` parameter contains external handlers provided by developers.
+
+By convention, we call them `createHandle[EventName]`.
+
+If we allow a developer to skip running our logic for a given handler, we check the `event.defaultMuiPrevented` field. It's an equivalent of the native `defaultPrevented` that works just for Base UI code:
+
+```tsx
+const createHandleKeyDown = (otherHandlers: EventHandlers) => (event: React.KeyboardEvent & MuiCancellableEvent) => {
+  // Run the external handler first.
+  // It possibly can set the defaultMuiPrevented field.
+  otherHandlers.onKeyDown?.(event);
+
+  // If the field is set, do not execute the usual handler logic.
+  if (event.defaultMuiPrevented) {
+    return;
+  }
+
+  // handler-specific logic...
+```
+
+#### Slot props resolvers
+
+These are functions called `get[SlotName]Props` that accept additional (optional) props and return props to be placed on slots of a component.
+Many components have just one slot (that we call "root"), but more complex components can have more.
+
+The order of merging props for the resulting object is important so that users can override the build-in props when necessary:
+
+1. built-in props
+2. external props
+3. ref
+4. event handlers
+
+Refs and event handlers can be placed in any order.
+They just have to be after external props.
+
+Example:
+
+```tsx
+const getRootProps = <OtherHandlers extends EventHandlers>(
+  otherHandlers: OtherHandlers = {} as OtherHandlers,
+): UseAwesomeControlRootSlotProps<OtherHandlers> => {
+  return {
+    id,
+    disabled,
+    role: 'button' as const,
+    ...otherHandlers, // if `id`, `disabled`, or `role` is provided here, they will override the default values set by us.
+    ref: handleListboxRef, // refs mustn't be overridden, so they come after `...otherHandlers`
+    onMouseDown: createHandleMouseDown(otherHandlers), // here we execute the event handler factory supplying it with external props
+  };
+};
+```
+
+It's common that a hook uses other hooks and combines their `get*Props` with its own.
+To handle these cases, we have the `combineHooksSlotProps` utility.
+It creates a function that merges two other slot resolver functions:
+
+```tsx
+const createHandleClick = (otherHandlers: EventHandlers) => (event: React.KeyboardEvent) => {
+  /* ... */
+}
+
+const { getRootProps as getListRootProps } = useList(/* ... */);
+const getOwnRootEventHandlers = (otherHandlers: EventHandlers = {}) => ({
+  onClick: createHandleClick(otherHandlers),
+});
+
+const getRootProps = <TOther extends EventHandlers>(
+    otherHandlers: TOther = {} as TOther,
+  ): UseAwesomeControlRootSlotProps => {
+    const getCombinedRootProps = combineHooksSlotProps(getOwnRootEventHandlers, getListRootProps);
+    return {
+      ...getCombinedRootProps(otherHandlers),
+      role: 'combobox'
+    }
+  }
+
+```
+
+#### Ref handling
+
+When a hook needs to access the DOM node it operates on, it should create a ref and return it in the `get*Props` function.
+However, since the user of the hook may already have a ref to the element, we accept the refs in parameters and merge them with our refs using the `useForkRef` function, so the callers of the hook don't have to do it.
+
+Since hooks can operate on many elements (when dealing with multiple slots), we call refs in input parameters `[slotName]Ref`.
+
+Example:
+
+```ts
+
+interface AwesomeControlHookParameters {
+  rootRef?: React.Ref<Element>;
+  // ...
+}
+
+const useAwesomeControlHook = (parameters: AwesomeControlHookParameters) {
+  const { rootRef: externalRef } = parameters;
+  const innerRef = React.useRef<HTMLDivElement | null>(null);
+
+  const handleRef = useForkRef(externalRef, innerRef);
+
+  return {
+    // parameters omitted for the sake of brevity
+    getRootProps: () => {
+      ref: handleRef
+    },
+    rootRef: handleRef
+  }
+}
+```
+
+Note that the merged ref (`handleRef`) is not only returned as a part of root props but also as a field of the hook's return object.
+This is helpful in situations where the ref needs to be merged with yet another ref.
+
+### Types
+
+Defining proper types can tremendously help developers use the hooks.
+The following types are required for each hook:
+
+- [HookName]Parameters - input parameters
+- [HookName]ReturnValue - the shape of the object returned by the hook
+- [HookName][SlotName]SlotProps - return values of slot props resolvers
+
+The parameters and return value types are usually straightforward.
+The definition of slot props, however, is more complex as it must take into consideration the object passed as an argument to the props resolver function:
+
+```ts
+export interface UseMenuReturnValue {
+  getListboxProps: <TOther extends EventHandlers>(
+    otherHandlers?: TOther,
+  ) => UseMenuListboxSlotProps;
+  // ...
+}
+
+interface UseMenuListboxSlotEventHandlers {
+  onBlur: React.FocusEventHandler;
+  onKeyDown: React.KeyboardEventHandler;
+}
+
+export type UseMenuListboxSlotProps<TOther = {}> = UseListRootSlotProps<
+  Omit<TOther, keyof UseMenuListboxSlotEventHandlers> & UseMenuListboxSlotEventHandlers
+> & {
+  ref: React.RefCallback<Element> | null;
+  role: React.AriaRole;
+};
+```

packages/mui-base/README.md

@@ -19,3 +19,7 @@ yarn add @mui/base
 <!-- #default-branch-switch -->
 
 [The documentation](https://mui.com/base-ui/getting-started/)
+
+## Contributing
+
+See [the contributing guide](./CONTRIBUTING.md) if you wish to implement Base UI components or hooks.