Save recent network balances in local storage (#5843)

* Use selector for state.metamask.accounts in all cases.

* Default to cached balance when selecting metamask accounts

* Adds the cached-balances controller

* Documentation and small codes fixes for #5843

Co-Authored-By: danjm <danjm.com@gmail.com>

2 files changed, 92 insertions, 0 deletions

diff --git a/app/scripts/controllers/cached-balances.js b/app/scripts/controllers/cached-balances.js
new file mode 100644
index 000000000..925c45334
--- /dev/null
+++ b/app/scripts/controllers/cached-balances.js
@@ -0,0 +1,83 @@
+const ObservableStore = require('obs-store')
+const extend = require('xtend')
+
+/**
+ * @typedef {Object} CachedBalancesOptions
+ * @property {Object} accountTracker An {@code AccountTracker} reference
+ * @property {Function} getNetwork A function to get the current network
+ * @property {Object} initState The initial controller state
+ */
+
+/**
+ * Background controller responsible for maintaining
+ * a cache of account balances in local storage
+ */
+class CachedBalancesController {
+  /**
+   * Creates a new controller instance
+   *
+   * @param {CachedBalancesOptions} [opts] Controller configuration parameters
+   */
+  constructor (opts = {}) {
+    const { accountTracker, getNetwork } = opts
+
+    this.accountTracker = accountTracker
+    this.getNetwork = getNetwork
+
+    const initState = extend({
+      cachedBalances: {},
+    }, opts.initState)
+    this.store = new ObservableStore(initState)
+
+    this._registerUpdates()
+  }
+
+  /**
+   * Updates the cachedBalances property for the current network. Cached balances will be updated to those in the passed accounts
+   * if balances in the passed accounts are truthy.
+   *
+   * @param {Object} obj The the recently updated accounts object for the current network
+   * @returns {Promise<void>}
+   */
+  async updateCachedBalances ({ accounts }) {
+    const network = await this.getNetwork()
+    const balancesToCache = await this._generateBalancesToCache(accounts, network)
+    this.store.updateState({
+      cachedBalances: balancesToCache,
+    })
+  }
+
+  _generateBalancesToCache (newAccounts, currentNetwork) {
+    const { cachedBalances } = this.store.getState()
+    const currentNetworkBalancesToCache = { ...cachedBalances[currentNetwork] }
+
+    Object.keys(newAccounts).forEach(accountID => {
+      const account = newAccounts[accountID]
+
+      if (account.balance) {
+        currentNetworkBalancesToCache[accountID] = account.balance
+      }
+    })
+    const balancesToCache = {
+      ...cachedBalances,
+      [currentNetwork]: currentNetworkBalancesToCache,
+    }
+
+    return balancesToCache
+  }
+
+  /**
+   * Sets up listeners and subscriptions which should trigger an update of cached balances. These updates will
+   * happen when the current account changes. Which happens on block updates, as well as on network and account
+   * selections.
+   *
+   * @private
+   *
+   */
+  _registerUpdates () {
+    const update = this.updateCachedBalances.bind(this)
+    this.accountTracker.store.subscribe(update)
+  }
+}
+
+module.exports = CachedBalancesController
diff --git a/app/scripts/metamask-controller.js b/app/scripts/metamask-controller.js
index 7d6f06f92..fe806e47e 100644
--- a/app/scripts/metamask-controller.js
+++ b/app/scripts/metamask-controller.js
@@ -29,6 +29,7 @@ const ShapeShiftController = require('./controllers/shapeshift')
 const AddressBookController = require('./controllers/address-book')
 const InfuraController = require('./controllers/infura')
 const BlacklistController = require('./controllers/blacklist')
+const CachedBalancesController = require('./controllers/cached-balances')
 const RecentBlocksController = require('./controllers/recent-blocks')
 const MessageManager = require('./lib/message-manager')
 const PersonalMessageManager = require('./lib/personal-message-manager')
@@ -142,6 +143,12 @@ module.exports = class MetamaskController extends EventEmitter {
       }
     })
 
+    this.cachedBalancesController = new CachedBalancesController({
+      accountTracker: this.accountTracker,
+      getNetwork: this.networkController.getNetworkState.bind(this.networkController),
+      initState: initState.CachedBalancesController,
+    })
+
     // ensure accountTracker updates balances after network change
     this.networkController.on('networkDidChange', () => {
       this.accountTracker._updateAccounts()
@@ -241,6 +248,7 @@ module.exports = class MetamaskController extends EventEmitter {
       ShapeShiftController: this.shapeshiftController.store,
       NetworkController: this.networkController.store,
       InfuraController: this.infuraController.store,
+      CachedBalancesController: this.cachedBalancesController.store,
     })
 
     this.memStore = new ComposableObservableStore(null, {
@@ -248,6 +256,7 @@ module.exports = class MetamaskController extends EventEmitter {
       AccountTracker: this.accountTracker.store,
       TxController: this.txController.memStore,
       BalancesController: this.balancesController.store,
+      CachedBalancesController: this.cachedBalancesController.store,
       TokenRatesController: this.tokenRatesController.store,
       MessageManager: this.messageManager.memStore,
       PersonalMessageManager: this.personalMessageManager.memStore,