lvgl_sim_qt

Provides a small example of how to embed an lvgl application in both a Qt Widgets and Qt Quick C++ application. Mouse events are supported, and in widgets, basic keyboard is supported (the keyboard driver is hooked into an automatically created group). Treating the mousewheel as an encoder is left as an exercise to the reader. Demos that require lv_drivers are not currently supported. 32, 16, 8, and 1-bit color are supported, though most examples do not work well with 1-bit color.

Building

This software requires:

• C++ >= 14
• CMake >= 3.10
• Qt >= 5.9 or Qt >= 6.2

git clone https://github.com/JonTheBurger/lvgl_sim_qt.git --recursive
cd lvgl_sim_qt
cmake -Bbuild -S.
cmake --build build

Executables will be placed in build/lvgl_qt_quick and build/lvgl_qt_widgets.

Modifying

Edits lvgl_sim_qt has made to lv_conf.h and lv_demo_conf.h can be found by searching for /** lvgl_sim_qt */, which precedes all edits. To change which demo is being run, replace the lv_demo function call found in each application's main function. To change the tick period, modify LvglRenderer::Tick_Period_Ms. The default value is 16. Double buffering can be disabled by providing lv_disp_buf_init with nullptr instead of display_frame2_.

This project is setup to use lvgl v8.1.0. When upgrading, be sure to bump both the lvgl and lv_demos submodules to matching versions. At the time of writing, the lvgl repo unstable main branch appears to be absorbing lv_demos.

Implementation Notes

• If attempting to port code, note that LvglGraphicsView is added in Qt Designer using promote widget.
• Qt5 does not support 8-bit 332 color, so a quick and dirty 8-bit color palette is generated in this case.
• Both applications render to a QImage, so this common functionality is extracted into lvgl_qt/LvglRenderer.
• Forward declarations of classes are used when possible to speed up preprocess time.
• All render callbacks are piped through the Qt event loop, so no additional multithreading considerations should be necessary for this example.
• mouseMoveEvent only registers movement on a mouse click by default, so a simpler position query is used instead.
• The Qt official docs suggest calling C++ from QML over calling QML from C++ for maintainability reasons. We set QML properties from C++ when the values are shared between the Quick and Widgets applications, and C++ from QML otherwise.
• Decrease Tick_Period_Ms to improve responsiveness.
• Resize is supported, and mouse input coordinates are properly scaled.
• The LVGL CMake targets do not properly include all necessary include directories by default, so they are augmented in external/CMakeLists.txt.