Feature request: add previous global state to addCallback parameters

First of all let me tell you how I love this library. It is simple, powerful and made my work much, much better and enjoyable. Thanks mate!

To the point now.

I am having a case where I would like to call a certain function whenever I update the global state, but only when a particular property of the state has been changed.

I thought that the addCallback helper function could help me. The thing is that I cannot compare the new state with the previous one. Is it difficult to add the prevGlobal as the second parameter of the addCallback helper function?

Or maybe there is some other approach I did not think of?

[CharlesStover]

@monteiz Would you be able to unblock yourself with something like this?

let previousState = {};
addCallback((newState, _, stateChange) => {
  // Check state difference here.
  for (const [changedKey, newValue] of Object.entries(stateChange)) {
    console.log(`${changedKey}: ${previousState[changedKey]} => ${newValue}`);
  }

  // Update previous state for future comparisons.
  previousState = newState;
});

[monteiz]

I solved it.

Actually the addCallback helper function has an undocumented 2nd and a 3rd parameters, which are - respectively - the dispatcherMap and the stateChange.

With the stateChange parameter I could very easily achieve what I was after.

I am more and more amazed by the elegance of this library.

@CharlesStover if you want, I can submit a pull request with the documentation updated.