GUI: use cached balance in overviewpage and sendcoinsdialog.

Plus, calculate the cached balance right when the wallet model, so the wallet widgets don't need to redo the same balance calculation multiple times when they are waiting for the model balance polling timer.

Author: furszy

src/qt/overviewpage.cpp

@@ -277,14 +277,13 @@ void OverviewPage::setWalletModel(WalletModel *model)
         ui->listTransactions->setModelColumn(TransactionTableModel::ToAddress);
 
         // Keep up to date with wallet
-        interfaces::Wallet& wallet = model->wallet();
-        interfaces::WalletBalances balances = wallet.getBalances();
-        setBalance(balances);
+        setBalance(model->getCachedBalance());
         connect(model, &WalletModel::balanceChanged, this, &OverviewPage::setBalance);
 
         connect(model->getOptionsModel(), &OptionsModel::displayUnitChanged, this, &OverviewPage::updateDisplayUnit);
 
-        updateWatchOnlyLabels(wallet.haveWatchOnly() && !model->wallet().privateKeysDisabled());
+        interfaces::Wallet& wallet = model->wallet();
+        updateWatchOnlyLabels(wallet.haveWatchOnly() && !wallet.privateKeysDisabled());
         connect(model, &WalletModel::notifyWatchonlyChanged, [this](bool showWatchOnly) {
             updateWatchOnlyLabels(showWatchOnly && !walletModel->wallet().privateKeysDisabled());
         });

src/qt/sendcoinsdialog.cpp

@@ -718,7 +718,7 @@ void SendCoinsDialog::setBalance(const interfaces::WalletBalances& balances)
 
 void SendCoinsDialog::refreshBalance()
 {
-    setBalance(model->wallet().getBalances());
+    setBalance(model->getCachedBalance());
     ui->customFee->setDisplayUnit(model->getOptionsModel()->getDisplayUnit());
     updateSmartFeeLabel();
 }

src/qt/walletmodel.cpp

@@ -66,6 +66,10 @@ WalletModel::~WalletModel()
 
 void WalletModel::startPollBalance()
 {
+    // Update the cached balance right away, so every view can make use of it,
+    // so them don't need to waste resources recalculating it.
+    pollBalanceChanged();
+
     // This timer will be fired repeatedly to update the balance
     // Since the QTimer::timeout is a private signal, it cannot be used
     // in the GUIUtil::ExceptionSafeConnect directly.