GUI: 'getAvailableBalance', use cached balance if the user did not select UTXO manually

No need to walk through the entire wallet's tx map. Used for 'walletModel::prepareTransaction' and 'useAvailable' flow in sendcoinsdialog.

Author: furszy

src/qt/sendcoinsdialog.cpp

@@ -795,7 +795,7 @@ void SendCoinsDialog::useAvailableBalance(SendCoinsEntry* entry)
     m_coin_control->fAllowWatchOnly = model->wallet().privateKeysDisabled() && !model->wallet().hasExternalSigner();
 
     // Calculate available amount to send.
-    CAmount amount = model->wallet().getAvailableBalance(*m_coin_control);
+    CAmount amount = model->getAvailableBalance(m_coin_control.get());
     for (int i = 0; i < ui->entries->count(); ++i) {
         SendCoinsEntry* e = qobject_cast<SendCoinsEntry*>(ui->entries->itemAt(i)->widget());
         if (e && !e->isHidden() && e != entry) {

src/qt/walletmodel.cpp

@@ -203,7 +203,9 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
         return DuplicateAddress;
     }
 
-    CAmount nBalance = m_wallet->getAvailableBalance(coinControl);
+    // If no coin was manually selected, use the cached balance
+    // Future: can merge this call with 'createTransaction'.
+    CAmount nBalance = getAvailableBalance(&coinControl);
 
     if(total > nBalance)
     {
@@ -608,3 +610,8 @@ uint256 WalletModel::getLastBlockProcessed() const
 {
     return m_client_model ? m_client_model->getBestBlockHash() : uint256{};
 }
+
+CAmount WalletModel::getAvailableBalance(const CCoinControl* control)
+{
+    return control && control->HasSelected() ? wallet().getAvailableBalance(*control) : getCachedBalance().balance;
+}

src/qt/walletmodel.h

@@ -160,6 +160,10 @@ class WalletModel : public QObject
     // Retrieve the cached wallet balance
     interfaces::WalletBalances getCachedBalance() const;
 
+    // If coin control has selected outputs, searches the total amount inside the wallet.
+    // Otherwise, uses the wallet's cached available balance.
+    CAmount getAvailableBalance(const wallet::CCoinControl* control);
+
 private:
     std::unique_ptr<interfaces::Wallet> m_wallet;
     std::unique_ptr<interfaces::Handler> m_handler_unload;