CMake: Deduce install prefix of example in qt_internal_add_example

We want to get rid of the INSTALL_EXAMPLEDIR and INSTALL_EXAMPLESDIR
code in each example project.

This was an internal workaround to ensure examples are installed into
a relative path somewhere under $qt_prefix/examples and not in
$qt_prefix/bin or similar.

To achieve that we do two things.

First, deduce the install prefix for each example in the
implementation of qt_internal_add_example (our add_subdirectory
wrapper) and assign it to CMAKE_INSTALL_PREFIX before calling
add_subdirectory. We need to make sure to remove the default
value of CMAKE_INSTALL_PREFIX in the generated
cmake_install.cmake file.

Second, we set an internal variable called
QT_INTERNAL_SET_EXAMPLE_INSTALL_DIR_TO_DOT before the add_subdirectory
call, which will be checked whenever find_package(Qt6Core) is called
in an example project. If the variable is set, the
INSTALL_EXAMPLEDIR var is set to "." in the scope of where Qt6Core
is found.

This ensures that the hardcoded INSTALL_EXAMPLEDIR values in our
example projects are changed to ".".

With both changes, our example project install(TARGET DESTINATION)
calls will now install to
"${CMAKE_INSTALL_PREFIX}/examples/${example_relative_dir}/."

Once all repositories are updated to use qt_internal_add_example
instead of add_subdirectory, we can get rid of the
QT_INTERNAL_SET_EXAMPLE_INSTALL_DIR_TO_DOT workaround.

For repositories that still don't use qt_internal_add_example,
the install prefix rewriting will not happen, but the examples will
still be installed to the proper place because they use their own
hardcoded INSTALL_EXAMPLEDIR value.

Amends d97fd7af2bc5c89a0ad9e5fac080041b78d01179

Pick-to: 6.2 6.3
Task-number: QTBUG-96232
Task-number: QTBUG-98545
Change-Id: I78c118e10c41b519c570c7d87529afd15aeed900
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>

2 files changed, 12 insertions, 0 deletions

diff --git a/src/corelib/Qt6CoreConfigExtras.cmake.in b/src/corelib/Qt6CoreConfigExtras.cmake.in
index d7ce721bbf..3b9cbb883d 100644
--- a/src/corelib/Qt6CoreConfigExtras.cmake.in
+++ b/src/corelib/Qt6CoreConfigExtras.cmake.in
@@ -55,3 +55,5 @@ endif()
 if(EMSCRIPTEN)
     include("${CMAKE_CURRENT_LIST_DIR}/@QT_CMAKE_EXPORT_NAMESPACE@WasmMacros.cmake")
 endif()
+
+_qt_internal_override_example_install_dir_to_dot()
diff --git a/src/corelib/Qt6CoreMacros.cmake b/src/corelib/Qt6CoreMacros.cmake
index 588b6cc0ec..7ed5e2b1a6 100644
--- a/src/corelib/Qt6CoreMacros.cmake
+++ b/src/corelib/Qt6CoreMacros.cmake
@@ -2205,6 +2205,16 @@ if(NOT QT_NO_CREATE_VERSIONLESS_FUNCTIONS)
     endfunction()
 endif()
 
+# TODO: Remove once all repositories use qt_internal_add_example instead of add_subdirectory.
+macro(_qt_internal_override_example_install_dir_to_dot)
+    # Set INSTALL_EXAMPLEDIR to ".".
+    # This overrides the install destination of unclean Qt example projects to install directly
+    # to CMAKE_INSTALL_PREFIX.
+    if(QT_INTERNAL_SET_EXAMPLE_INSTALL_DIR_TO_DOT)
+        set(INSTALL_EXAMPLEDIR ".")
+    endif()
+endmacro()
+
 function(qt6_allow_non_utf8_sources target)
     set_target_properties("${target}" PROPERTIES QT_NO_UTF8_SOURCE TRUE)
 endfunction()