Understanding MobX Observables and Reactions in ReactJS

MobX has revolutionized state management in React applications with its simple yet powerful approach. Let’s dive deep into how Observables and Reactions work together to create responsive and efficient React applications.

The Magic of Observables

Think of Observables as special containers that can hold any piece of data - from simple numbers to complex objects. What makes them special? They keep track of who’s watching them and notify those observers whenever changes occur.

import { makeObservable, observable } from 'mobx';

class TodoStore {
    todos = [];

    constructor() {
        makeObservable(this, {
            todos: observable
        });
    }
}

Reactions: The Silent Watchers

Reactions are like vigilant guardians that spring into action whenever their observed values change. They come in three flavors:

1. autorun - Runs immediately and tracks dependencies automatically
2. reaction - More selective, lets you specify what to track
3. when - Runs once when a condition is met

import { autorun } from 'mobx';

autorun(() => {
    console.log(`Current todos: ${todoStore.todos.length}`);
});

Bringing It All Together

The real power emerges when we combine Observables and Reactions in React components:

import { observer } from 'mobx-react-lite';

const TodoList = observer(({ store }) => {
    return (
        <ul>
            {store.todos.map(todo => (
                <li key={todo.id}>{todo.text}</li>
            ))}
        </ul>
    );
});

Best Practices

1. Keep observable state minimal
2. Use computed values for derived data
3. Structure reactions to watch specific changes
4. Implement proper cleanup in component unmounting

Remember, the goal is to create maintainable and performant applications. MobX gives us the tools - it’s up to us to use them wisely.