Handle "Delete confirmation" dialog result asynchronously

This is to avoid creating nested event loops as discussed in
https://github.com/qbittorrent/qBittorrent/pull/10786#issuecomment-502795822

src/gui/deletionconfirmationdialog.cpp

@@ -39,10 +39,12 @@ DeletionConfirmationDialog::DeletionConfirmationDialog(QWidget *parent, const in
     , m_ui(new Ui::DeletionConfirmationDialog)
 {
     m_ui->setupUi(this);
+
     if (size == 1)
         m_ui->label->setText(tr("Are you sure you want to delete '%1' from the transfer list?", "Are you sure you want to delete 'ubuntu-linux-iso' from the transfer list?").arg(name.toHtmlEscaped()));
     else
         m_ui->label->setText(tr("Are you sure you want to delete these %1 torrents from the transfer list?", "Are you sure you want to delete these 5 torrents from the transfer list?").arg(QString::number(size)));
+
     // Icons
     const QSize iconSize = Utils::Gui::largeIconSize();
     m_ui->labelWarning->setPixmap(UIThemeManager::instance()->getIcon("dialog-warning").pixmap(iconSize));
@@ -62,21 +64,11 @@ DeletionConfirmationDialog::~DeletionConfirmationDialog()
     delete m_ui;
 }
 
-bool DeletionConfirmationDialog::shouldDeleteLocalFiles() const
+bool DeletionConfirmationDialog::isDeleteFileSelected() const
 {
     return m_ui->checkPermDelete->isChecked();
 }
 
-bool DeletionConfirmationDialog::askForDeletionConfirmation(QWidget *parent, bool &deleteLocalFiles, const int size, const QString &name)
-{
-    DeletionConfirmationDialog dlg(parent, size, name, deleteLocalFiles);
-    if (dlg.exec() == QDialog::Accepted) {
-        deleteLocalFiles = dlg.shouldDeleteLocalFiles();
-        return true;
-    }
-    return false;
-}
-
 void DeletionConfirmationDialog::updateRememberButtonState()
 {
     m_ui->rememberBtn->setEnabled(m_ui->checkPermDelete->isChecked() != Preferences::instance()->deleteTorrentFilesAsDefault());

src/gui/deletionconfirmationdialog.h

@@ -46,8 +46,7 @@ public:
     DeletionConfirmationDialog(QWidget *parent, int size, const QString &name, bool defaultDeleteFiles);
     ~DeletionConfirmationDialog();
 
-    bool shouldDeleteLocalFiles() const;
+    bool isDeleteFileSelected() const;
-    static bool askForDeletionConfirmation(QWidget *parent, bool &deleteLocalFiles, int size, const QString &name);
 
 private slots:
     void updateRememberButtonState();

src/gui/transferlistwidget.cpp

@@ -96,6 +96,14 @@ namespace
 
         return false;
     }
+
+    void removeTorrents(const QVector<BitTorrent::TorrentHandle *> &torrents, const bool isDeleteFileSelected)
+    {
+        auto *session = BitTorrent::Session::instance();
+        const DeleteOption deleteOption = isDeleteFileSelected ? TorrentAndFiles : Torrent;
+        for (const BitTorrent::TorrentHandle *torrent : torrents)
+            session->deleteTorrent(torrent->hash(), deleteOption);
+    }
 }
 
 TransferListWidget::TransferListWidget(QWidget *parent, MainWindow *mainWindow)
@@ -363,19 +371,25 @@ void TransferListWidget::permDeleteSelectedTorrents()
     deleteSelectedTorrents(true);
 }
 
-void TransferListWidget::deleteSelectedTorrents(bool deleteLocalFiles)
+void TransferListWidget::deleteSelectedTorrents(const bool deleteLocalFiles)
 {
     if (m_mainWindow->currentTabWidget() != this) return;
 
     const QVector<BitTorrent::TorrentHandle *> torrents = getSelectedTorrents();
     if (torrents.empty()) return;
 
-    if (Preferences::instance()->confirmTorrentDeletion()
+    if (Preferences::instance()->confirmTorrentDeletion()) {
-        && !DeletionConfirmationDialog::askForDeletionConfirmation(this, deleteLocalFiles, torrents.size(), torrents[0]->name()))
+        auto *dialog = new DeletionConfirmationDialog(this, torrents.size(), torrents[0]->name(), deleteLocalFiles);
-        return;
+        dialog->setAttribute(Qt::WA_DeleteOnClose);
-    const DeleteOption deleteOption = deleteLocalFiles ? TorrentAndFiles : Torrent;
+        connect(dialog, &DeletionConfirmationDialog::accepted, this, [dialog, torrents]()
-    for (const BitTorrent::TorrentHandle *torrent : torrents)
+        {
-        BitTorrent::Session::instance()->deleteTorrent(torrent->hash(), deleteOption);
+            removeTorrents(torrents, dialog->isDeleteFileSelected());
+        });
+        dialog->open();
+    }
+    else {
+        removeTorrents(torrents, deleteLocalFiles);
+    }
 }
 
 void TransferListWidget::deleteVisibleTorrents()
@@ -386,14 +400,18 @@ void TransferListWidget::deleteVisibleTorrents()
     for (int i = 0; i < m_sortFilterModel->rowCount(); ++i)
         torrents << m_listModel->torrentHandle(mapToSource(m_sortFilterModel->index(i, 0)));
 
-    bool deleteLocalFiles = false;
+    if (Preferences::instance()->confirmTorrentDeletion()) {
-    if (Preferences::instance()->confirmTorrentDeletion()
+        auto *dialog = new DeletionConfirmationDialog(this, torrents.size(), torrents[0]->name(), false);
-        && !DeletionConfirmationDialog::askForDeletionConfirmation(this, deleteLocalFiles, torrents.size(), torrents[0]->name()))
+        dialog->setAttribute(Qt::WA_DeleteOnClose);
-        return;
+        connect(dialog, &DeletionConfirmationDialog::accepted, this, [dialog, torrents]()
-
+        {
-    const DeleteOption deleteOption = deleteLocalFiles ? TorrentAndFiles : Torrent;
+            removeTorrents(torrents, dialog->isDeleteFileSelected());
-    for (const BitTorrent::TorrentHandle *torrent : asConst(torrents))
+        });
-        BitTorrent::Session::instance()->deleteTorrent(torrent->hash(), deleteOption);
+        dialog->open();
+    }
+    else {
+        removeTorrents(torrents, false);
+    }
 }
 
 void TransferListWidget::increaseQueuePosSelectedTorrents()