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Qt

Safe-Guarding the (L)GPL Future of Qt

The little known KDE Free Qt Foundation makes sure that Qt stays free and open-source. It guarantees that all Qt modules currently licensed under LGPLv3 must continue to be available under LGPLv3 in the future. This covers all modules from Qt Essentials and many add-on modules. If The Qt Company discontinued the FOSS version of Qt, the Foundation would have the right to make Qt available under the BSD license. This is a very powerful protection of free and open-source Qt.

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Detecting Overdraw in QML HMIs with GammaRay

Overdraw happens when one QML item fully eclipses another QML item. The QML renderer always draws both items, although there is no need to draw the eclipsed item. You must help out the renderer by explicitly setting visible: false on the eclipsed item.

On embedded systems, heavy overdraw makes animations or flicking jerky. In the worst case, it freezes your HMI. Fortunately, the Qt experts at KDAB developed a tool, GammaRay, which makes detecting overdraw a piece of cake. I’ll show you how to build GammaRay, how to detect overdraw in the home screen of a harvester HMI, and how to fix the overdraw.

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Deploying Qt Projects to Embedded Devices with CMake

When you run a Qt project, QtCreator first deploys the project to a remote embedded Linux device via ssh and then runs the executable on the remote device. This feature enables nearly instant feedback how your Qt application works on the embedded device.

Deployment works fine with the INSTALLS variable of qmake. It does not work out of the box with the install functions of CMake. Fortunately, the Qt trolls provide a workaround. I’ll explain the workaround with an example CMakeLists.txt file.

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QCOMPARE with Custom Qt Types

When you write unit tests, you will have to compare the actual value and the expected value. A simplified example with QStrings would look like this.

void MyTest::testQCompare() {
    auto actualStr = QString{"abba"};
    auto expectedStr = QString{"juhu"};
    QCOMPARE(actualStr, expectedStr);
}

When you run this unit test, the QTest framework will print the actual value "abba" and the expected value "juhu". This is often enough to know what went wrong.

FAIL!  : MyTest::testQCompare() Compared values are not the same
   Actual   (actualStr)  : "abba"
   Expected (expectedStr): "juhu"

If two values of a custom type differ, the QTest framework will only print the first line: Compared values are not the same. This is not very helpful. How can you make QCOMPARE print the actual and expected for custom types as well?

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Printing Custom Qt Types with qDebug()

When you define your own C++ types for a Qt application, you want to print their values with qDebug(), qWarning() or qCritical() eventually. 

auto frame1 = QCanBusFrame{0x18ef0201U, QByteArray::fromHex("018A010000000000")};
qDebug() << "frame1 =" << frame1;

The C++ compiler will complain with this error message:

error: no match for ‘operator<<’ (operand types are ‘QDebug’ and ‘QCanBusFrame’)

The error message tells you to write your own output operator for the QDebug stream and for QCanBusFrame.

A custom C++ type that wants to play nicely with Qt should always define a QDebug output operator. Even the developers of the Qt library seem to forget it sometimes, as the example of QCanBusFrame shows.

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