Watch for account changes and recalculate destination amount for transfers on the Add Transfer Transaction dialog

src/views/base/transactions/TransactionEditPageBase.ts

@@ -16,7 +16,7 @@ import type { LocalizedTimezoneInfo } from '@/core/timezone.ts';
 import { TransactionType } from '@/core/transaction.ts';
 import { TemplateType } from '@/core/template.ts';
 import { DISPLAY_HIDDEN_AMOUNT } from '@/consts/numeral.ts';
-import { TRANSACTION_MAX_PICTURE_COUNT } from '@/consts/transaction.ts';
+import { TRANSACTION_MAX_PICTURE_COUNT, TRANSACTION_MIN_AMOUNT, TRANSACTION_MAX_AMOUNT } from '@/consts/transaction.ts';
 
 import { Account, type CategorizedAccountWithDisplayBalance } from '@/models/account.ts';
 import type { TransactionCategory } from '@/models/transaction_category.ts';
@@ -26,13 +26,19 @@ import { Transaction } from '@/models/transaction.ts';
 import { TransactionTemplate } from '@/models/transaction_template.ts';
 
 import {
-    isArray
+    isArray,
+    isNumber
 } from '@/lib/common.ts';
 
 import {
-    getExchangedAmountByRate
+    getExchangedAmountByRate,
+    getAmountWithDecimalNumberCount
 } from '@/lib/numeral.ts';
 
+import {
+    getCurrencyFraction
+} from '@/lib/currency.ts';
+
 import {
     getUtcOffsetByUtcOffsetMinutes,
     getTimezoneOffsetMinutes,
@@ -421,6 +427,57 @@ export function useTransactionEditPageBase(type: TransactionEditPageType, initMo
         }
     });
 
+    // Watch for account changes and recalculate destination amount for transfers
+    watch(() => [transaction.value.sourceAccountId, transaction.value.destinationAccountId], ([newSourceAccountId, newDestinationAccountId], [oldSourceAccountId, oldDestinationAccountId]) => {
+        if (mode.value === TransactionEditPageMode.View || loading.value) {
+            return;
+        }
+
+        if (transaction.value.type !== TransactionType.Transfer) {
+            return;
+        }
+
+        // Only recalculate if accounts actually changed
+        if (newSourceAccountId === oldSourceAccountId && newDestinationAccountId === oldDestinationAccountId) {
+            return;
+        }
+
+        // Only recalculate if accounts actually changed (skip initial watch call)
+        if (oldSourceAccountId !== undefined && oldDestinationAccountId !== undefined) {
+            if (newSourceAccountId === oldSourceAccountId && newDestinationAccountId === oldDestinationAccountId) {
+                return;
+            }
+        }
+
+        const sourceAccount = allAccountsMap.value[transaction.value.sourceAccountId];
+        const destinationAccount = allAccountsMap.value[transaction.value.destinationAccountId];
+
+        if (!sourceAccount || !destinationAccount) {
+            return;
+        }
+
+        // Recalculate destination amount based on source amount and exchange rate
+        if (sourceAccount.currency !== destinationAccount.currency) {
+            const exchangedAmount = exchangeRatesStore.getExchangedAmount(transaction.value.sourceAmount, sourceAccount.currency, destinationAccount.currency);
+
+            if (isNumber(exchangedAmount)) {
+                const decimalNumberCount = getCurrencyFraction(destinationAccount.currency);
+                let newDestinationAmount = Math.trunc(exchangedAmount);
+
+                if (isNumber(decimalNumberCount)) {
+                    newDestinationAmount = getAmountWithDecimalNumberCount(newDestinationAmount, decimalNumberCount);
+                }
+
+                if (TRANSACTION_MIN_AMOUNT <= newDestinationAmount && newDestinationAmount <= TRANSACTION_MAX_AMOUNT) {
+                    transaction.value.destinationAmount = newDestinationAmount;
+                }
+            }
+        } else {
+            // Same currency, just copy the source amount
+            transaction.value.destinationAmount = transaction.value.sourceAmount;
+        }
+    });
+
     return {
         // constants
         isSupportGeoLocation,