Add docs for models

Author: JoviDeCroock

content/en/guide/v10/signals.md

@@ -693,3 +693,230 @@ function Component() {
 	);
 }
 ```
+
+## Models
+
+Models provide a structured way to build reactive state containers that encapsulate signals, computed values, effects, and actions. They offer a clean pattern for organizing complex state logic while ensuring automatic cleanup and batched updates.
+
+### createModel(factory)
+
+The `createModel(factory)` function creates a model constructor from a factory function. The factory function can accept arguments for initialization and should return an object containing signals, computed values, and action methods.
+
+```js
+import { signal, computed, effect, createModel } from '@preact/signals-core';
+
+const CounterModel = createModel((initialCount = 0) => {
+	const count = signal(initialCount);
+	const doubled = computed(() => count.value * 2);
+
+	effect(() => {
+		console.log('Count changed:', count.value);
+	});
+
+	return {
+		count,
+		doubled,
+		increment() {
+			count.value++;
+		},
+		decrement() {
+			count.value--;
+		}
+	};
+});
+
+// Create a new model instance using `new`
+const counter = new CounterModel(5);
+counter.increment(); // Updates are automatically batched
+console.log(counter.count.value); // 6
+console.log(counter.doubled.value); // 12
+
+// Clean up all effects when done
+counter[Symbol.dispose]();
+```
+
+#### Key Features
+
+- **Factory arguments**: Factory functions can accept arguments for initialization, making models reusable with different configurations.
+- **Automatic batching**: All methods returned from the factory are automatically wrapped as actions, meaning state updates within them are batched and untracked.
+- **Automatic effect cleanup**: Effects created during model construction are captured and automatically disposed when the model is disposed via `Symbol.dispose`.
+- **Composable models**: Models compose naturally - effects from nested models are captured by the parent and disposed together when the parent is disposed.
+
+#### Model Composition
+
+Models can be nested within other models. When a parent model is disposed, all effects from nested models are automatically cleaned up:
+
+```js
+const TodoItemModel = createModel((text) => {
+	const completed = signal(false);
+
+	return {
+		text,
+		completed,
+		toggle() {
+			completed.value = !completed.value;
+		}
+	};
+});
+
+const TodoListModel = createModel(() => {
+	const items = signal([]);
+
+	return {
+		items,
+		addTodo(text) {
+			const todo = new TodoItemModel(text);
+			items.value = [...items.value, todo];
+		},
+		removeTodo(todo) {
+			items.value = items.value.filter(t => t !== todo);
+			todo[Symbol.dispose]();
+		}
+	};
+});
+
+const todoList = new TodoListModel();
+todoList.addTodo('Buy groceries');
+todoList.addTodo('Walk the dog');
+
+// Disposing the parent also cleans up all nested model effects
+todoList[Symbol.dispose]();
+```
+
+### action(fn)
+
+The `action(fn)` function wraps a function to run in a batched and untracked context. This is useful when you need to create standalone actions outside of a model:
+
+```js
+import { signal, action } from '@preact/signals-core';
+
+const count = signal(0);
+
+const incrementBy = action((amount) => {
+	count.value += amount;
+});
+
+incrementBy(5); // Batched update
+```
+
+### useModel(modelOrFactory)
+
+The `useModel` hook is available in both `@preact/signals` and `@preact/signals-react` packages. It handles creating a model instance on first render, maintaining the same instance across re-renders, and automatically disposing the model when the component unmounts.
+
+```jsx
+import { signal, createModel } from '@preact/signals-core';
+import { useModel } from '@preact/signals'; // or '@preact/signals-react'
+
+const CounterModel = createModel(() => ({
+	count: signal(0),
+	increment() {
+		this.count.value++;
+	}
+}));
+
+function Counter() {
+	const model = useModel(CounterModel);
+
+	return (
+		<button onClick={() => model.increment()}>
+			Count: {model.count}
+		</button>
+	);
+}
+```
+
+For models that require constructor arguments, wrap the instantiation in a factory function:
+
+```jsx
+const CounterModel = createModel((initialCount) => ({
+	count: signal(initialCount),
+	increment() {
+		this.count.value++;
+	}
+}));
+
+function Counter({ initialValue }) {
+	// Use a factory function to pass arguments
+	const model = useModel(() => new CounterModel(initialValue));
+
+	return (
+		<button onClick={() => model.increment()}>
+			Count: {model.count}
+		</button>
+	);
+}
+```
+
+### Recommended Patterns
+
+#### Explicit ReadonlySignal Pattern
+
+For better encapsulation, declare your model interface explicitly and use `ReadonlySignal` for signals that should only be modified through actions:
+
+```ts
+import { signal, computed, createModel, ReadonlySignal } from '@preact/signals-core';
+
+interface Counter {
+	count: ReadonlySignal<number>;
+	doubled: ReadonlySignal<number>;
+	increment(): void;
+	decrement(): void;
+}
+
+const CounterModel = createModel<Counter>(() => {
+	const count = signal(0);
+	const doubled = computed(() => count.value * 2);
+
+	return {
+		count,
+		doubled,
+		increment() {
+			count.value++;
+		},
+		decrement() {
+			count.value--;
+		}
+	};
+});
+
+const counter = new CounterModel();
+counter.increment(); // OK
+counter.count.value = 10; // TypeScript error: Cannot assign to 'value'
+```
+
+#### Custom Dispose Logic
+
+If your model needs custom cleanup logic that isn't related to signals (such as closing WebSocket connections), use an effect with no dependencies that returns a cleanup function:
+
+```js
+const WebSocketModel = createModel((url) => {
+	const messages = signal([]);
+	const ws = new WebSocket(url);
+
+	ws.onmessage = (e) => {
+		messages.value = [...messages.value, e.data];
+	};
+
+	// This effect runs once; its cleanup runs on dispose
+	effect(() => {
+		return () => {
+			ws.close();
+		};
+	});
+
+	return {
+		messages,
+		send(message) {
+			ws.send(message);
+		}
+	};
+});
+
+const chat = new WebSocketModel('wss://example.com/chat');
+chat.send('Hello!');
+
+// Closes the WebSocket connection on dispose
+chat[Symbol.dispose]();
+```
+
+This pattern mirrors `useEffect(() => { return cleanup }, [])` in React and ensures that cleanup happens automatically when models are composed together - parent models don't need to know about the dispose functions of nested models.