state: wp.data adapter adjustments

This addresses a couple of PR comments about some optimization and
additional commenting.

client/state/wp-data/calypso-registry-plugin.js

@@ -17,9 +17,9 @@ const calypsoRegistryPlugin = calypsoStore => registry => {
 	calypsoStore.subscribe( () => {
 		const calypsoState = calypsoStore.getState();
 		const hasChanged = calypsoState !== lastCalypsoState;
-		lastCalypsoState = calypsoState;
 
 		if ( hasChanged ) {
+			lastCalypsoState = calypsoState;
 			calypsoListener();
 		}
 	} );
@@ -46,6 +46,12 @@ const calypsoRegistryPlugin = calypsoStore => registry => {
 		namespaces[ reducerKey ].actions = mapValues( newActions, createBoundAction );
 	}
 
+	/**
+	 * Registers a store within the registry.
+	 * @param {String} reducerKey The desired key to denote this store.
+	 * @param {Object} options The options for the store.
+	 * @return {Object} The newly registered store.
+	 */
 	function registerStore( reducerKey, options ) {
 		if ( options.useCalypsoStore ) {
 			return registerToCalypsoStore( reducerKey, options );
@@ -55,6 +61,13 @@ const calypsoRegistryPlugin = calypsoStore => registry => {
 		return registry.registerStore( reducerKey, options );
 	}
 
+	/**
+	 * Subscribe to registry change events.
+	 * This subscribes to all store updates in the registry,
+	 * calypso and otherwise.
+	 * @param {Function} listener The listener function to be added.
+	 * @returns {Function} The unsubscribe function for this listener.
+	 */
 	function subscribe( listener ) {
 		listeners.push( listener );
 		const registryUnsub = registry.subscribe( listener );
@@ -65,6 +78,11 @@ const calypsoRegistryPlugin = calypsoStore => registry => {
 		};
 	}
 
+	/**
+	 * Gets selectors for a desired store.
+	 * @param {String} reducerKey The reducerKey for the desired store.
+	 * @return {Object} A mapping of selectors for the desired store.
+	 */
 	function select( reducerKey ) {
 		const namespace = namespaces[ reducerKey ];
 		if ( namespace ) {
@@ -73,6 +91,11 @@ const calypsoRegistryPlugin = calypsoStore => registry => {
 		return registry.select( reducerKey );
 	}
 
+	/**
+	 * Gets action creators for a desired store.
+	 * @param {String} reducerKey The reducerKey for the desired store.
+	 * @return {Object} Action creators mapped to dispatch for the desired store.
+	 */
 	function dispatch( reducerKey ) {
 		const namespace = namespaces[ reducerKey ];
 		if ( namespace ) {

client/state/wp-data/calypso-wp-data-provider.js

@@ -12,20 +12,16 @@ import calypsoRegistryPlugin from './calypso-registry-plugin';
 import calypsoWPDataStore from './calypso-wp-data-store';
 
 class CalypsoWPDataProvider extends Component {
-	constructor() {
+	constructor( props ) {
 		super( ...arguments );
-		this.registry = undefined;
+		this.updateRegistry( props.calypsoStore );
 	}
 
 	updateRegistry( calypsoStore ) {
 		this.registry = use( calypsoRegistryPlugin( calypsoStore ) );
 		this.registry.registerStore( 'calypso', calypsoWPDataStore );
 	}
 
-	componentDidMount() {
-		this.updateRegistry( this.props.calypsoStore );
-	}
-
 	componentDidUpdate( prevProps ) {
 		if ( prevProps.calypsoStore !== this.props.calypsoStore ) {
 			this.updateRegistry( this.props.calypsoStore );