change_listener.md
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	# Change Listeners

	Redux's [`Store#subscribe`](http://redux.js.org/docs/api/Store.html#subscribe)
	allows you to subscribe to updates to the state tree. These updates are
	delivered every time an action is dispatched to the store, which may or may not
	result in a change of state.

	In the Extension:Popups codebase, a change listener is a function that is
	only called when the state tree has changed. As such, change listeners are
	predominantly responsible for updating the UI so that it matches the state in
	the store.

	## Registering Change Listeners

	Change listeners are registered automatically during
	[boot](./resources/ext.popups/boot.js) in the `registerChangeListeners`
	function. It expects the values of the `mw.popups.changeListeners` map to be
	factory functions that accept, currently, the [bound action
	creators](http://redux.js.org/docs/api/bindActionCreators.html), i.e.

	```javascript
	mw.popups.changeListeners.foo = function ( boundActions ) {
	var $link = $( '<a>' )
	.attr( 'href', '#' )
	.click( boundActions.showSettings );

	return function ( oldState, newState ) {
	// ...
	}
	};
	```

	You'll note that the above change listener is effectful and maintains some
	local state (`$link`), both of which are acceptable. The former is unavoidable
	and the latter is to avoid populating the state tree with unimportant data.