show warning if exchanging with slow fee (#579)

* show warning if exchanging with slow fee

* Update setDefaultTransactionPriority for return type void; update for use MoneroTransactionPriority.automatic instead of MoneroTransactionPriority.regular.

Co-authored-by: M <m@cakewallet.com>

lib/src/screens/exchange/exchange_page.dart

@@ -111,6 +111,10 @@ class ExchangePage extends BasePage {
     WidgetsBinding.instance
         .addPostFrameCallback((_) => _setReactions(context, exchangeViewModel));
 
+    if (exchangeViewModel.isLowFee) {
+      _showFeeAlert(context);
+    }
+
     return KeyboardActions(
         disableScroll: true,
         config: KeyboardActionsConfig(
@@ -794,5 +798,23 @@ class ExchangePage extends BasePage {
     return address;
   }
 
+  void _showFeeAlert(BuildContext context) async {
+    await Future<void>.delayed(Duration(seconds: 1));
+    final confirmed = await showPopUp<bool>(
+        context: context,
+        builder: (dialogContext) {
+          return AlertWithTwoActions(
+              alertTitle: S.of(context).low_fee,
+              alertContent: S.of(context).low_fee_alert,
+              leftButtonText: S.of(context).ignor,
+              rightButtonText: S.of(context).use_suggested,
+              actionLeftButton: () => Navigator.of(context).pop(false),
+              actionRightButton: () => Navigator.of(context).pop(true));
+        }) ?? false;
+    if (confirmed) {
+      exchangeViewModel.setDefaultTransactionPriority();
+    }
+  }
+
   void disposeBestRateSync() => exchangeViewModel.bestRateSync?.cancel();
 }