controllers/token-rates.js

const ObservableStore = require('obs-store')
+
+// By default, poll every 3 minutes
+const DEFAULT_INTERVAL = 180 * 1000
+
+/**
+ * A controller that polls for token exchange
+ * rates based on a user's current token list
+ */
+class TokenRatesController {
+  /**
+   * Creates a TokenRatesController
+   *
+   * @param {Object} [config] - Options to configure controller
+   */
+  constructor ({ interval = DEFAULT_INTERVAL, preferences } = {}) {
+    this.store = new ObservableStore()
+    this.preferences = preferences
+    this.interval = interval
+  }
+
+  /**
+   * Updates exchange rates for all tokens
+   */
+  async updateExchangeRates () {
+    if (!this.isActive) { return }
+    const contractExchangeRates = {}
+    for (const i in this._tokens) {
+      const address = this._tokens[i].address
+      contractExchangeRates[address] = await this.fetchExchangeRate(address)
+    }
+    this.store.putState({ contractExchangeRates })
+  }
+
+  /**
+   * Fetches a token exchange rate by address
+   *
+   * @param {String} address - Token contract address
+   */
+  async fetchExchangeRate (address) {
+    try {
+      const response = await fetch(`https://exchanges.balanc3.net/prices?from=${address}&to=ETH&autoConversion=false&summaryOnly=true`)
+      const json = await response.json()
+      return json && json.length ? json[0].averagePrice : 0
+    } catch (error) { }
+  }
+
+  /**
+   * @type {Number}
+   */
+  set interval (interval) {
+    this._handle && clearInterval(this._handle)
+    if (!interval) { return }
+    this._handle = setInterval(() => { this.updateExchangeRates() }, interval)
+  }
+
+  /**
+   * @type {Object}
+   */
+  set preferences (preferences) {
+    this._preferences && this._preferences.unsubscribe()
+    if (!preferences) { return }
+    this._preferences = preferences
+    this.tokens = preferences.getState().tokens
+    preferences.subscribe(({ tokens = [] }) => { this.tokens = tokens })
+  }
+
+  /**
+   * @type {Array}
+   */
+  set tokens (tokens) {
+    this._tokens = tokens
+    this.updateExchangeRates()
+  }
+}
+
+module.exports = TokenRatesController
+