observeSelector pattern

observeSelector.coffee

#
# Observable pattern for Redux selectors.
# Compares using _ deep equality.
# See: https://github.com/rackt/redux/issues/303#issuecomment-125184409
#
_ = require 'underscore'

deepEqualityTest = _.isEqual
objectEqualityTest = (a,b) -> a is b

observeSelector = (store, selector, onChange, observeInitialValue = true, equalityTest = deepEqualityTest) ->
	lastState = undefined
	watcher = ->
		nextState = selector(store.getState())
		if not equalityTest(nextState, lastState)
			lastState = nextState
			onChange(nextState)

	unsubscriber = store.subscribe(watcher)
	if observeInitialValue then watcher()
	unsubscriber

stopObserving = (unsubscriber) -> unsubscriber()

module.exports = { observeSelector, stopObserving }