Implement SessionStore

This wraps session-related state into a basic flux store. The localStorage item 'mx_pass' is the only thing managed by this store for now but it could easily be extended to track other items (like the teamToken which is passed around through props a lot)

src/stores/SessionStore.js

@@ -0,0 +1,63 @@
+import dis from '../dispatcher';
+import EventEmitter from 'events';
+
+/**
+ * A class for storing application state to do with the session. This is a simple flux
+ * store that listens for actions and updates its state accordingly, informing any
+ * listeners (views) of state changes via the 'update' event.
+ */
+function SessionStore() {
+    // Initialise state
+    this._state = {
+        cachedPassword: localStorage.getItem('mx_pass'),
+    };
+
+    dis.register(this._onAction.bind(this));
+}
+
+// Inherit from EventEmitter
+SessionStore.prototype = EventEmitter.prototype;
+
+SessionStore.prototype._update = function() {
+    // Persist state to localStorage
+    if (this._state.cachedPassword) {
+        localStorage.setItem('mx_pass', this._state.cachedPassword);
+    } else {
+        localStorage.removeItem('mx_pass', this._state.cachedPassword);
+    }
+
+    this.emit('update');
+};
+
+SessionStore.prototype._setState = function(newState) {
+    this._state = Object.assign(this._state, newState);
+    this._update();
+};
+
+SessionStore.prototype._onAction = function(payload) {
+    switch (payload.action) {
+        case 'cached_password':
+            this._setState({
+                cachedPassword: payload.cachedPassword,
+            });
+            break;
+        case 'password_changed':
+            this._setState({
+                cachedPassword: null,
+            });
+            break;
+    }
+};
+
+SessionStore.prototype.getCachedPassword = function() {
+    return this._state.cachedPassword;
+};
+
+// Export singleton getter
+let singletonSessionStore = null;
+export default function getSessionStore() {
+    if (!singletonSessionStore) {
+        singletonSessionStore = new SessionStore();
+    }
+    return singletonSessionStore;
+}