mediawiki-extensions-Popups/resources/ext.popups/changeListener.js

( function ( mw ) {

	/**
	 * @typedef {Function} ext.popups.ChangeListener
	 * @param {Object} prevState The previous state
	 * @param {Object} state 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 {Store} store
	 * @param {ext.popups.ChangeListener} callback
	 */
	mw.popups.registerChangeListener = function ( 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.

		var state;

		store.subscribe( function () {
			var prevState = state;

			state = store.getState();

			if ( prevState !== state ) {
				callback( prevState, state );
			}
		} );
	};

}( mediaWiki ) );