import { decorate, observable, action } from 'mobx';
import React from 'react';

class TheTimer {
    currentTick = 0; // In seconds since start

    tick = () => {
        this.currentTick = Math.floor(performance.now() / 1000);
    }
}

decorate(TheTimer, {
    currentTick: observable,
    tick: action
});

// Bare store
export const timerStore = new TheTimer();
// Store as React context
export const TimerStoreContext = React.createContext(timerStore);

// This makes the timer go
setInterval(timerStore.tick, 100);

import React from 'react';
import { configure } from 'mobx';
import { observer } from 'mobx-react';

import { timerStore } from './store';

configure({ enforceActions: 'observed' });

export default const App = observer(() => {
    return (
        <p>{timerStore.currentTick}</p>
    );
});

import React from 'react';
import { configure } from 'mobx';
import { observer } from 'mobx-react';

import { TimerStoreContext } from './store';

configure({ enforceActions: 'observed' });

export default const App = observer(() => {
    const timerStore = React.useContext(TimerStoreContext);

    return (
        <p>{timerStore.currentTick}</p>
    );
});