/**
* @module changeListener
*/
/**
* @typedef {Function} ext.popups.ChangeListener
* @param {Object} oldState The previous state
* @param {Object} newState The current state
*/
/**
* Registers a change listener, which is bound to the
* [store](http://redux.js.org/docs/api/Store.html).
*
* A change listener is a function that is only invoked when the state in the
* [store](http://redux.js.org/docs/api/Store.html) changes. N.B. that there
* may not be a 1:1 correspondence with actions being dispatched to the store
* and the state in the store changing.
*
* See [Store#subscribe](http://redux.js.org/docs/api/Store.html#subscribe)
* for more information about what change listeners may and may not do.
*
* @param {Redux.Store} store
* @param {ext.popups.ChangeListener} callback
*/
export default function registerChangeListener( store, callback ) {
// This function is based on the example in [the documentation for
// Store#subscribe](http://redux.js.org/docs/api/Store.html#subscribe),
// which was written by Dan Abramov.
let previousState;
store.subscribe( () => {
const state = store.getState();
if ( previousState !== state ) {
callback( previousState, state );
previousState = state;
}
} );
}