Create a target-based CMake command as alternative to AUTOUIC and qt_wrap_uic

With CMake, there are several ways to handle Qt UI files.

CMake's AUTOUIC has some bugs that don't seem fixable without user-breaking changes. Compare CMake upstream issues #16776 and #23523 to name just two.

What AUTOUIC offers is currently:

• users don't have to specify UI files in target sources
• the #include "ui_form.h" statement in C++ sources triggers the generation of ui_form.h from form.ui

We currently don't see a way to fix #16776 without requiring users to add UI files to target sources. That means one advantage of AUTOUIC is going to vanish anyway. Also, IDE users tend to want UI files in their target sources to have them visible in the project tree (#19513).

Generating header files from Qt's UI files doesn't require magic from CMake (unlike moc). It would be possible to implement support for UI files purely in CMake language. There's already qt_wrap_uic, but that command is not target-based, and it doesn't allow multiple UI files of the same name.

This suggestion is about creating a target-based CMake command for UI files.

Requirements

The functionality must allow for multiple UI files in different subfolders.

It must be possible to specify additional arguments for uic per source file, per target (maybe initialized with a variable).

AUTOUIC supports setting such options as INTERFACE for libraries. Compare INTERFACE_AUTOUIC_OPTIONS in the AUTOUIC documentation.

There should be some convenient way to retrieve the include directory that's added to the target. Compare https://stackoverflow.com/questions/44746308/how-to-make-include-directories-added-with-autouic-available-to-downstream-targe/60203765#60203765 and #17456
The use case for this are libraries that have an ui_form.h include in their headers (which is bad style, but whatever).

possibility: qt_add_ui

# CMakeLists.txt
qt_add_executable(myapp main.cpp 
    aboutdialog.cpp aboutdialog.h
    mainwindow.cpp mainwindow.h
)
qt_add_ui(myapp 
    FILES dialogs/aboutdialog.ui mainwindow.ui
)

The following files are generated:

${CMAKE_CURRENT_BINARY_DIR}/.ui/ui_mainwindow.h
${CMAKE_CURRENT_BINARY_DIR}/.ui/dialogs/ui_aboutdialog.h

This is added to the target's include path:

${CMAKE_CURRENT_BINARY_DIR}/.ui

We mandate that include statements match the subdirectories:

// aboutdialog.cpp
#include <dialogs/ui_aboutdialog.h>

// mainwindow.cpp
#include <ui_mainwindow.h>

possibility: use the target's sources

Similar to the latter possibility with the difference that we don't add a new CMake command but handle UI files in the finalizer.

qt_add_executable(myapp main.cpp 
    aboutdialog.cpp aboutdialog.h
    mainwindow.cpp mainwindow.h
    aboutdialog.ui mainwindow.ui
)

Here, the target finalizer checks the target's SOURCES and calls an internal command that does what's described for qt_add_ui.

Disadvantages of adding this new command

Already released Qt versions don't benefit from this. Unless we provide a compatibility file like suggested in QTBUG-114587.