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.

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test wise: `WalletTests` now need to trigger the walletModel balance changed manually. So the model updates its internal state and can be used by the widgets.

This is because the test does not start the balance polling timer, in the same way as does not initialize several parts of the GUI workflow. All the objects (wallet, models, views, etc) that are used on this test are manually created instead of using the `WalletController` class flow.

Rationale is that this unit test is focused on verifying the GUI widgets/views behavior only: update the presented information, etc. when they receive different signals and/or function calls from outside (in other words, focus is on the signal slots/receiver side). It's not about whether the wallet balance polling timer is functioning as expected or not (which we definitely create a new test case for it in a follow-up work).

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/test/wallettests.cpp

@@ -129,6 +129,13 @@ void BumpFee(TransactionView& view, const uint256& txid, bool expectDisabled, st
     QVERIFY(text.indexOf(QString::fromStdString(expectError)) != -1);
 }
 
+void CompareBalance(WalletModel& walletModel, CAmount expected_balance, QLabel* balance_label_to_check)
+{
+    BitcoinUnit unit = walletModel.getOptionsModel()->getDisplayUnit();
+    QString balanceComparison = BitcoinUnits::formatWithUnit(unit, expected_balance, false, BitcoinUnits::SeparatorStyle::ALWAYS);
+    QCOMPARE(balance_label_to_check->text().trimmed(), balanceComparison);
+}
+
 //! Simple qt wallet tests.
 //
 // Test widgets can be debugged interactively calling show() on them and
@@ -193,15 +200,10 @@ void TestGUI(interfaces::Node& node)
     sendCoinsDialog.setModel(&walletModel);
     transactionView.setModel(&walletModel);
 
-    {
-        // Check balance in send dialog
-        QLabel* balanceLabel = sendCoinsDialog.findChild<QLabel*>("labelBalance");
-        QString balanceText = balanceLabel->text();
-        BitcoinUnit unit = walletModel.getOptionsModel()->getDisplayUnit();
-        CAmount balance = walletModel.wallet().getBalance();
-        QString balanceComparison = BitcoinUnits::formatWithUnit(unit, balance, false, BitcoinUnits::SeparatorStyle::ALWAYS);
-        QCOMPARE(balanceText, balanceComparison);
-    }
+    // Update walletModel cached balance which will trigger an update for the 'labelBalance' QLabel.
+    walletModel.pollBalanceChanged();
+    // Check balance in send dialog
+    CompareBalance(walletModel, walletModel.wallet().getBalance(), sendCoinsDialog.findChild<QLabel*>("labelBalance"));
 
     // Send two transactions, and verify they are added to transaction list.
     TransactionTableModel* transactionTableModel = walletModel.getTransactionTableModel();
@@ -221,12 +223,8 @@ void TestGUI(interfaces::Node& node)
     // Check current balance on OverviewPage
     OverviewPage overviewPage(platformStyle.get());
     overviewPage.setWalletModel(&walletModel);
-    QLabel* balanceLabel = overviewPage.findChild<QLabel*>("labelBalance");
-    QString balanceText = balanceLabel->text().trimmed();
-    BitcoinUnit unit = walletModel.getOptionsModel()->getDisplayUnit();
-    CAmount balance = walletModel.wallet().getBalance();
-    QString balanceComparison = BitcoinUnits::formatWithUnit(unit, balance, false, BitcoinUnits::SeparatorStyle::ALWAYS);
-    QCOMPARE(balanceText, balanceComparison);
+    walletModel.pollBalanceChanged(); // Manual balance polling update
+    CompareBalance(walletModel, walletModel.wallet().getBalance(), overviewPage.findChild<QLabel*>("labelBalance"));
 
     // Check Request Payment button
     ReceiveCoinsDialog receiveCoinsDialog(platformStyle.get());

src/qt/walletmodel.cpp

@@ -67,6 +67,10 @@ 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.