Add exchange rate feature with multi-currency transactions and transfers support (#1099)

Co-authored-by: Pedro J. Aramburu <pedro@joakin.dev>

app/javascript/controllers/transaction_form_controller.js

@@ -0,0 +1,60 @@
+import ExchangeRateFormController from "controllers/exchange_rate_form_controller";
+
+// Connects to data-controller="transaction-form"
+export default class extends ExchangeRateFormController {
+  static targets = [
+    ...ExchangeRateFormController.targets,
+    "account",
+    "currency"
+  ];
+
+  hasRequiredExchangeRateTargets() {
+    if (!this.hasAccountTarget || !this.hasCurrencyTarget || !this.hasDateTarget) {
+      return false;
+    }
+
+    return true;
+  }
+
+  getExchangeRateContext() {
+    if (!this.hasRequiredExchangeRateTargets()) {
+      return null;
+    }
+
+    const accountId = this.accountTarget.value;
+    const currency = this.currencyTarget.value;
+    const date = this.dateTarget.value;
+
+    if (!accountId || !currency) {
+      return null;
+    }
+
+    const accountCurrency = this.accountCurrenciesValue[accountId];
+    if (!accountCurrency) {
+      return null;
+    }
+
+    return {
+      fromCurrency: currency,
+      toCurrency: accountCurrency,
+      date
+    };
+  }
+
+  isCurrentExchangeRateState(fromCurrency, toCurrency, date) {
+    if (!this.hasRequiredExchangeRateTargets()) {
+      return false;
+    }
+
+    const currentAccountId = this.accountTarget.value;
+    const currentCurrency = this.currencyTarget.value;
+    const currentDate = this.dateTarget.value;
+    const currentAccountCurrency = this.accountCurrenciesValue[currentAccountId];
+
+    return fromCurrency === currentCurrency && toCurrency === currentAccountCurrency && date === currentDate;
+  }
+
+  onCurrencyChange() {
+    this.checkCurrencyDifference();
+  }
+}