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

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

app/javascript/controllers/transfer_form_controller.js

@@ -0,0 +1,59 @@
+import ExchangeRateFormController from "controllers/exchange_rate_form_controller";
+
+// Connects to data-controller="transfer-form"
+export default class extends ExchangeRateFormController {
+  static targets = [
+    ...ExchangeRateFormController.targets,
+    "fromAccount",
+    "toAccount"
+  ];
+
+  hasRequiredExchangeRateTargets() {
+    if (!this.hasFromAccountTarget || !this.hasToAccountTarget || !this.hasDateTarget) {
+      return false;
+    }
+
+    return true;
+  }
+
+  getExchangeRateContext() {
+    if (!this.hasRequiredExchangeRateTargets()) {
+      return null;
+    }
+
+    const fromAccountId = this.fromAccountTarget.value;
+    const toAccountId = this.toAccountTarget.value;
+    const date = this.dateTarget.value;
+
+    if (!fromAccountId || !toAccountId) {
+      return null;
+    }
+
+    const fromCurrency = this.accountCurrenciesValue[fromAccountId];
+    const toCurrency = this.accountCurrenciesValue[toAccountId];
+
+    if (!fromCurrency || !toCurrency) {
+      return null;
+    }
+
+    return {
+      fromCurrency,
+      toCurrency,
+      date
+    };
+  }
+
+  isCurrentExchangeRateState(fromCurrency, toCurrency, date) {
+    if (!this.hasRequiredExchangeRateTargets()) {
+      return false;
+    }
+
+    const currentFromAccountId = this.fromAccountTarget.value;
+    const currentToAccountId = this.toAccountTarget.value;
+    const currentFromCurrency = this.accountCurrenciesValue[currentFromAccountId];
+    const currentToCurrency = this.accountCurrenciesValue[currentToAccountId];
+    const currentDate = this.dateTarget.value;
+
+    return fromCurrency === currentFromCurrency && toCurrency === currentToCurrency && date === currentDate;
+  }
+}