Impressed by fine-grained reactivity concept from solid-js, I've tried to build a library that brings it to react. Some react issues I was going to solve where:

• Provide possibility to re-render just those elements, which related data has changed.
• Enable easy 2-way binding, however maintaining unidirectional data flow.
• Remove necessity to overflow the code by explicitly mentioning all dependencies, as we currently do with useEffect, useCallback and useMemo.
• Issues with encapsulation and modularisation when using redux or context as state management (I ❤️ redux btw).

Now I'm going to present you main concepts of the library within a TodoMVC app example. You can find full source code here. Note that example fits in less than 60 lines of code.

First of all let define our component:

export const App = seal(() => {
  // ...
});

seal is an import from react-mlyn, it's a wrapper of React.memo, which compare function always returns true. Which means, component should never re-render by incoming properties change (those are not supposed to ever change). All children re-renders will be triggered by mlyn reactivity system.
Now let define the state:

const state$ = useSubject({
  todos: [],
  newTitle: ""
});

useSubject is a react-hook, that will convert initial state to a subject. A subject in mlyn is a proxy object, which can we used in 4 different ways:

• you can read from it:

// will return actual state
state$();

• you can write to it:

// will set `newTitle` to `hello`
state$({
  ...state$(),
  newTitle: "hello",
}); 

• you can subscribe to it:

useMlynEffect(() => {
  // will log the `state$` value every time it's updated
  console.log(state$());
});

By reading state$ inside of useMlynEffect hook we automatically set it as a dependency, which will re-run the hook every time state$ has been updated.

• you can lens it:

state$.newTitle("hello");
state$.newTitle(); // hello
state$(); // { newTitle: "hello", todos: [] }

Every lens behave like a subject, but when updated bubbles an immutable update to the root subject. Also within lens you can subscribe to updates of just a portions of the state.

Now let go back to our TodoMVC app, let create a synchroniser of todos to the local storage:

// this hook accepts a subject and a string key for localstorage
const useSyncronize = (subject$, key) => {
  // if localStorage already contains info for that key,
  // let write it to `subject$` as initial state
  if (localStorage[key]) {
    const preloadedState = JSON.parse(localStorage[key]);
    subject$(preloadedState);
  }
  // create a subscription to `subject$` and write
  // write it to localStorage when updated
  useMlynEffect(() => {
    localStorage[key] = JSON.stringify(subject$()); 
  });
};

Invocation of this hook in the component code:

// create a lens to `state$.todos` and
// bind it to localStorage `todos` key.
useSyncronize(state$.todos, "todos");

Let create methods for adding / deleting todos:

const addItem = () => {
  state$({
    todos: [
      // remember to use `()` when reading from a subject.
      ...state$.todos(),
      {
        title: state$.newTitle(),
        createdAt: new Date().toISOString(),
        done: false
      }
    ],
    newTitle: ""
  });
};

This looks very similar to normal react update, but you don't need to wrap it with useCallback since with mlyn component is not going to be re-rendered.

const removeItem = (i) => {
  state$.todos([
    ...state$.todos().slice(0, i),
    ...state$.todos().slice(i + 1)
  ]);
};

Note that since here you need to update just todos you can directly write to state$.todos without taking care of rest of the state. This is very handy, when passing a lens as a property to a child.
And finally jsx:

return (
  <>
    <h3>Simple Todos Example</h3>
    <Mlyn.input
      type="text"
      placeholder="enter todo and click +"
      bindValue={state$.newTitle}
    />
    <button onClick={addItem}>+</button>
    <For
      each={state$.todos}
      getKey={({ createdAt }) => createdAt}
    >
      {(todo$, index$) => (
        <div>
          <Mlyn.input type="checkbox" bindChecked={todo$.done} />
          <Mlyn.input bindValue={todo$.title} />
          <button onClick={() => removeItem(index$())}>x</button>
        </div>
      )}
    </For>
  </>
);

Notice that for inputs we use special tag Mlyn.input it has some properties which enables subscriptions to mlyn reactivity. One of those is bindValue. When you pass state$.newTitle to it, it will both update the input when the newTitle is updated, and write to newTitle when input is changed. In short, this is 2-way binding.

<Mlyn.input
  type="text"
  placeholder="enter todo and click +"
  bindValue={state$.newTitle}
/>

Now let analyse how the For component, that is used to display collections works:

<For
  // pass subject which holds array to display
  each={state$.todos}
  // key extractor, it's used not only by react reconciliation,
  // but also by `For` component logic.
  getKey={({ createdAt }) => createdAt}
>
  {(todo$, index$) => (
    <div>
      <Mlyn.input type="checkbox" bindChecked={todo$.done} />
      <Mlyn.input bindValue={todo$.title} />
      <button onClick={() => removeItem(index$())}>x</button>
    </div>
  )}
</For>

The first parameter $todo of function child prop is still a 2-way lens. Which means, by updating it, you'll update todos array and in general entire state. So writing:

todo$.title("new value");

Is like writing something similar to bellow in plain react:

setState({
  ...state,
  todos: state.todos.map(item => {
    if (getKey(item) === getKey(todo)) {
      return { ...item, title: "new value" };
    }
    return item;
  }),
});

You probably noticed that one input is a checkbox toggle for boolean value:

<Mlyn.input type="checkbox" bindChecked={todo$.done} />

bindChecked is similar to bindValue but it creates 2-way binding for a boolean subject value to input checked field.