Qt OPC UA Viewer Example

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 #include "certificatedialog.h"
 #include "ui_certificatedialog.h"
 #include <QFile>
 #include <QPushButton>

 CertificateDialog::CertificateDialog(QWidget *parent) :
     QDialog(parent),
     ui(new Ui::CertificateDialog)
 {
     ui->setupUi(this);
     connect(ui->btnTrust, &QPushButton::clicked, this, &CertificateDialog::saveCertificate);
 }

 CertificateDialog::~CertificateDialog()
 {
     delete ui;
 }

 /** Returns 0 if the connect should be aborted, 1 if it should be resumed. */
 int CertificateDialog::showCertificate(const QString &message, const QByteArray &der, const QString &trustListDirectory)
 {
     QList<QSslCertificate> certs = QSslCertificate::fromData(der, QSsl::Der);

     m_trustListDirectory = trustListDirectory;

     // if it is a unstrusted self-signed certificate we can allow to trust it
     if (certs.count() == 1 && certs[0].isSelfSigned()) {
         m_cert = certs[0];
         ui->btnTrust->setEnabled(true);
     } else {
         ui->btnTrust->setEnabled(false);
     }

     for (const QSslCertificate &cert : qAsConst(certs))
         ui->certificate->appendPlainText(cert.toText());

     ui->message->setText(message);
     ui->certificate->moveCursor(QTextCursor::Start);
     ui->certificate->ensureCursorVisible();

     return exec();
 }

 void CertificateDialog::saveCertificate()
 {
    const QByteArray digest = m_cert.digest();
    const QString path = m_trustListDirectory + QLatin1Char('/')
        + QLatin1String(digest.toHex()) + QLatin1String(".der");

    QFile file(path);
    if (file.open(QIODevice::WriteOnly)) {
        file.write(m_cert.toDer());
        file.close();
    }
 }