changeListener.js
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changeListener.js
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	/**
	* @module changeListener
	* @private
	*/

	/**
	* @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;
	}
	} );
	}