CAN-Example: Fix two issues with the status timer

1. Fix a crash when device open fails

If opening of m_canDevice failed, m_canDevice is reset to
nullptr in line 154. Dereferencing that leads to a crash.

2. Avoid connecting multiple slots to status timer

Opening and closing multiple times lead to multiple
connections of the lambda slot, which triggered the
call to m_canDevice->busStatus() unnecessarily often.

The solution is to do the connection only once,
and stop the timer when m_canDevice becomes nullptr.
It is started again after a successful open.

Amends f83587a9978bd212d

[ChangeLog][Examples] Fixed a crash in CAN-Example when
opening the CAN bus device failed.

Change-Id: I4d19f382593fa26fa3b8166f7121e7ff14fd2e74
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>

2 files changed, 33 insertions, 23 deletions

diff --git a/examples/serialbus/can/mainwindow.cpp b/examples/serialbus/can/mainwindow.cpp
index a1c10fe..e8d688b 100644
--- a/examples/serialbus/can/mainwindow.cpp
+++ b/examples/serialbus/can/mainwindow.cpp
@@ -75,6 +75,8 @@ MainWindow::MainWindow(QWidget *parent) :
 
     initActionsConnections();
     QTimer::singleShot(50, m_connectDialog, &ConnectDialog::show);
+
+    connect(m_busStatusTimer, &QTimer::timeout, this, &MainWindow::busStatus);
 }
 
 MainWindow::~MainWindow()
@@ -177,32 +179,39 @@ void MainWindow::connectDevice()
             m_status->setText(tr("Plugin: %1, connected to %2")
                     .arg(p.pluginName).arg(p.deviceInterfaceName));
         }
-    }
 
-    connect(m_busStatusTimer, &QTimer::timeout, this, [this]() {
-        switch (m_canDevice->busStatus()) {
-        case QCanBusDevice::CanBusStatus::Good:
-            m_ui->busStatus->setText("CAN bus status: Good.");
-            break;
-        case QCanBusDevice::CanBusStatus::Warning:
-            m_ui->busStatus->setText("CAN bus status: Warning.");
-            break;
-        case QCanBusDevice::CanBusStatus::Error:
-            m_ui->busStatus->setText("CAN bus status: Error.");
-            break;
-        case QCanBusDevice::CanBusStatus::BusOff:
-            m_ui->busStatus->setText("CAN bus status: Bus Off.");
-            break;
-        default:
-            m_ui->busStatus->setText("CAN bus status: Unknown.");
-            break;
-        }
-    });
+        if (m_canDevice->hasBusStatus())
+            m_busStatusTimer->start(2000);
+        else
+            m_ui->busStatus->setText(tr("No CAN bus status available."));
+    }
+}
 
-    if (m_canDevice->hasBusStatus())
-        m_busStatusTimer->start(2000);
-    else
+void MainWindow::busStatus()
+{
+    if (!m_canDevice || !m_canDevice->hasBusStatus()) {
         m_ui->busStatus->setText(tr("No CAN bus status available."));
+        m_busStatusTimer->stop();
+        return;
+    }
+
+    switch (m_canDevice->busStatus()) {
+    case QCanBusDevice::CanBusStatus::Good:
+        m_ui->busStatus->setText("CAN bus status: Good.");
+        break;
+    case QCanBusDevice::CanBusStatus::Warning:
+        m_ui->busStatus->setText("CAN bus status: Warning.");
+        break;
+    case QCanBusDevice::CanBusStatus::Error:
+        m_ui->busStatus->setText("CAN bus status: Error.");
+        break;
+    case QCanBusDevice::CanBusStatus::BusOff:
+        m_ui->busStatus->setText("CAN bus status: Bus Off.");
+        break;
+    default:
+        m_ui->busStatus->setText("CAN bus status: Unknown.");
+        break;
+    }
 }
 
 void MainWindow::disconnectDevice()
diff --git a/examples/serialbus/can/mainwindow.h b/examples/serialbus/can/mainwindow.h
index 2e910d1..febe72d 100644
--- a/examples/serialbus/can/mainwindow.h
+++ b/examples/serialbus/can/mainwindow.h
@@ -82,6 +82,7 @@ private slots:
     void sendFrame(const QCanBusFrame &frame) const;
     void processErrors(QCanBusDevice::CanBusError) const;
     void connectDevice();
+    void busStatus();
     void disconnectDevice();
     void processFramesWritten(qint64);