Add qt6

The goal is to support both Qt5 and Qt6 when building the examples and also when building the Cxx tarballs.

src/Admin/VTKQtCMakeLists

@@ -15,26 +15,42 @@ find_package(VTK COMPONENTS ZZZ
   QUIET
 )
 
-if (NOT VTK_FOUND)
-  message("Skipping XXX: ${VTK_NOT_FOUND_MESSAGE}")
-  return ()
+if(NOT VTK_FOUND)
+  message(FATAL_ERROR "XXX: ${VTK_NOT_FOUND_MESSAGE}")
 endif()
-message (STATUS "VTK_VERSION: ${VTK_VERSION}")
 
-if(vtkGUISupportQt_LOADED OR TARGET VTK::GUISupportQt)
-  if (VTK_VERSION VERSION_LESS "8.90.0")
-    # Instruct CMake to run moc automatically when needed.
-    set(CMAKE_AUTOMOC ON)
-  else()
-     # Instruct CMake to run moc and uic automatically when needed.
-    set(CMAKE_AUTOMOC ON)
-    set(CMAKE_AUTOUIC ON)
-  endif()
-  # We have ui files, this will bring in the macro: qt5_wrap_ui
-  find_package(Qt5Widgets REQUIRED QUIET)
+if(NOT(vtkGUISupportQt_LOADED OR TARGET VTK::GUISupportQt))
+  message(FATAL_ERROR "XXX: VTK not built with Qt support.")
+endif()
+
+if(NOT DEFINED VTK_QT_VERSION)
+  set(VTK_QT_VERSION 5)
+endif()
+
+set(qt_components Core Gui Widgets)
+if(${VTK_QT_VERSION} VERSION_GREATER_EQUAL 6)
+  list(APPEND qt_components OpenGLWidgets)
+endif()
+list(SORT qt_components)
+# We have ui files, so this will also bring in the macro:
+#   qt5_wrap_ui or qt_wrap_ui from Widgets.
+find_package(Qt${VTK_QT_VERSION} QUIET
+  REQUIRED COMPONENTS ${qt_components}
+)
+
+foreach(_qt_comp IN LISTS qt_components)
+  list(APPEND qt_modules "Qt${VTK_QT_VERSION}::${_qt_comp}")
+endforeach()
+
+message (STATUS "VTK_VERSION: ${VTK_VERSION}, Qt Version: ${Qt${VTK_QT_VERSION}Widgets_VERSION}")
+
+if (VTK_VERSION VERSION_LESS "8.90.0")
+# Instruct CMake to run moc automatically when needed.
+  set(CMAKE_AUTOMOC ON)
 else()
-  message(STATUS "GuiSupportQt not found.")
-  return()
+# Instruct CMake to run moc and uic automatically when needed.
+  set(CMAKE_AUTOMOC ON)
+  set(CMAKE_AUTOUIC ON)
 endif()
 
 include_directories(${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_CURRENT_BINARY_DIR})
@@ -43,7 +59,14 @@ file(GLOB UI_FILES *.ui)
 file(GLOB QT_WRAP *.h)
 file(GLOB CXX_FILES *.cxx)
 
-qt5_wrap_ui(UISrcs ${UI_FILES})
+# For VTK versions greater than or equal to 8.90.0:
+#  CMAKE_AUTOUIC is ON so we handle uic automatically for Qt targets.
+#  CMAKE_AUTOMOC is ON so we handle moc automatically for Qt targets.
+# However we have to do the following for VTK versions less than 8.90.0,
+#  and we also assume, in this case, that Qt5 is being used.
+if (VTK_VERSION VERSION_LESS "8.90.0")
+  qt5_wrap_ui(UISrcs ${EXAMPLE}.ui)
+endif()
 
 if (VTK_VERSION VERSION_LESS "8.90.0")
   # old system
@@ -51,20 +74,20 @@ if (VTK_VERSION VERSION_LESS "8.90.0")
   # CMAKE_AUTOMOC in ON so the MOC headers will be automatically wrapped.
   add_executable(XXX MACOSX_BUNDLE
     ${CXX_FILES} ${UISrcs} ${QT_WRAP})
-  if (Qt5Widgets_VERSION VERSION_LESS "5.11.0")
-    qt5_use_modules(XXX Core Gui Widgets)
+  if (Qt${VTK_QT_VERSION}Widgets_VERSION VERSION_LESS "5.11.0")
+    qt5_use_modules(XXX ${qt_components})
   else()
-    target_link_libraries(XXX Qt5::Core Qt5::Gui Qt5::Widgets)
+    target_link_libraries(XXX ${qt_modules})
   endif()
   target_link_libraries(XXX ${VTK_LIBRARIES})
 else()
   # CMAKE_AUTOMOC in ON so the MOC headers will be automatically wrapped.
   add_executable(XXX MACOSX_BUNDLE
     ${CXX_FILES} ${UISrcs} ${QT_WRAP})
-  if (Qt5Widgets_VERSION VERSION_LESS "5.11.0")
-    qt5_use_modules(XXX Core Gui Widgets)
+  if (Qt${VTK_QT_VERSION}Widgets_VERSION VERSION_LESS "5.11.0")
+    qt5_use_modules(XXX ${qt_components})
   else()
-    target_link_libraries(XXX Qt5::Core Qt5::Gui Qt5::Widgets)
+    target_link_libraries(XXX ${qt_modules})
   endif()
   target_link_libraries(XXX ${VTK_LIBRARIES})
   # vtk_module_autoinit is needed