Reactivity and Events

How to observe and react to changes in Polotno store using MobX and event listeners

Polotno's state is an mobx-state-tree store, so every change is observable through MobX.

In React, you get automatic UI updates by wrapping your components in observer():

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

// the component will be automatically updated when number of children is changed
export const App = observer(({ store }) => (
  <div>
    <p>Elements on the current page: {store.activePage?.children.length}</p>
  </div>
));

You can also use any data from the store as dependency to react hooks:

const App = observer(({ store }) => {
  React.useEffect(() => {
    console.log('width of the store is changed');
  }, [store.width]);
  return <></>;
})

React hooks + MobX reactions

If you want logic that runs outside the render cycle, you can combine useEffect with mobx.reaction (or autorun):

import { reaction } from 'mobx';
import { useEffect } from 'react';

export function useElementCounter(store) {
  useEffect(() => {
    // fires every time the number of elements on the active page changes
    const dispose = reaction(
      () => store.activePage?.children.length,
      (length) => {
        console.log('Element count changed:', length);
      }
    );
    return dispose;          // clean-up on unmount
  }, [store]);               // deps
}

Under the hood, MobX tracks exactly what you read inside the first function and re-runs the second function only when that value changes—no manual listeners, no polling.

Events

store.on('change', handler) is the only event we fire. It triggers on any mutation: add, remove, move, resize, undo, redo, you name it.

const unsubscribe = store.on('change', () => {
  console.log('Something in the store changed');
});

Detecting specific actions

Because every action funnels through the same event, you filter manually.

Example: Simple tracking of adding/removing elements

let prevCount = store.activePage?.children.length ?? 0;

const unsub = store.on('change', () => {
  const newCount = store.activePage?.children.length ?? 0;

  if (newCount > prevCount) {
    console.log('Element added');
  } else if (newCount < prevCount) {
    console.log('Element removed');
  }

  prevCount = newCount;
});

Example: deeper tracking of adding/removing elements

If you need more gradual control, you can save more references to previous data to have a deeper diff.

let lastIds = {};

store.on('change', () => {
   const newIds = {};
   store.find(item => {
     newIds[item.id] = item;
   })
   for (const id in lastIds) {
      const deleted = !newIds[id];
      if (deleted) {
         console.log(id, 'deleted');
      }
   }
   for (const id in newIds) {
      const added = !lastIds[id];
      if (added) {
         console.log(id, 'added');
      }
   }
   lastIds = newIds;
});

Example: Changes on one element

import { reaction } from 'mobx';

function watchElement(element) {
  return reaction(
    () => ({
      x: element.x,
      y: element.y,
      width: element.width,
      height: element.height,
      rotation: element.rotation,
    }),
    (coords) => {
      console.log('Element changed', coords);
    }
  );
}

// later
const dispose = watchElement(store.activePage.children[0]);

Example: Page-level changes (title, background, etc.)

reaction(
  () => ({
    title: store.activePage?.name,
    bg: store.activePage?.background,
  }),
  (data) => console.log('Page updated', data)
);

Performance

change event can fire dozens of times per second during drag/resize. Avoid heavy work inside the handler - wrap it in throttle, or debounce.

// define function to save design to backend
const saveDesign = async () => {
  // export the design
  const json = store.toJSON();
  // save it to the backend
  await fetch('https://example.com/designs', {
    method: 'POST',
    body: JSON.stringify(json),
  });
}

// write a function for throttle saving
// it will call save no more then 1 time per second
let timeout = null;
const requestSave = () => {
  // if save is already requested - do nothing
  if (timeout) {
    return;
  }
  // schedule saving to the backend
  timeout = setTimeout(() => {
    // reset timeout
    timeout = null;
    saveDesign();
  }, 1000);
};

// request saving operation on any changes
store.on('change', requestSave);

Reactivity and Events

React hooks + MobX reactions

Events

Detecting specific actions

Performance

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