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>

1 files changed, 83 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