Detect the QT_INSTALL_DOCS path via qtpaths or qmake

doc/CMakeLists.txt

@@ -1,13 +1,45 @@
 # SPDX-FileCopyrightText: 2022-2024 Paul Colby <[email protected]>
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
-# Include Qt doc tagfiles, if provided.
-if (QT_TAGFILES_PATH)
+# Attempt to detect the QT_INSTALL_DOCS path via qtpaths or qmake.
+if(NOT QT_INSTALL_DOCS)
+  if(QT_VERSION VERSION_GREATER_EQUAL 6.2.0 AND TARGET Qt${QT_VERSION_MAJOR}::qtpaths)
+    message(DEBUG "Fetching location of imported Qt${QT_VERSION_MAJOR}::qtpaths command")
+    get_target_property(COMMAND Qt${QT_VERSION_MAJOR}::qtpaths LOCATION)
+  elseif(TARGET Qt${QT_VERSION_MAJOR}::qmake)
+    message(DEBUG "Fetching location of imported Qt${QT_VERSION_MAJOR}::qmake command")
+    get_target_property(COMMAND Qt${QT_VERSION_MAJOR}::qmake LOCATION)
+  else()
+    message(STATUS "Neither Qt${QT_VERSION_MAJOR}::qtpaths nor Qt${QT_VERSION_MAJOR}::qmake available")
+  endif()
+  if (COMMAND)
+    message(VERBOSE "Fetching QT_INSTALL_DOCS from ${COMMAND}")
+    execute_process(
+      COMMAND "${COMMAND}" -query QT_INSTALL_DOCS
+      RESULT_VARIABLE COMMAND_RESULT
+      OUTPUT_VARIABLE COMMAND_OUTPUT
+      ERROR_VARIABLE COMMNAD_ERROR
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_STRIP_TRAILING_WHITESPACE)
+    message(DEBUG "${COMMAND} result: ${COMMAND_RESULT}")
+    message(DEBUG "${COMMAND} output: ${COMMAND_OUTPUT}")
+    message(DEBUG "${COMMAND} error: ${COMMAND_ERROR}")
+    if(COMMAND_RESULT EQUAL 0)
+      set(QT_INSTALL_DOCS "${COMMAND_OUTPUT}")
+      message(STATUS "Found QT_INSTALL_DOCS: ${QT_INSTALL_DOCS}")
+    else()
+      message(STATUS "Failed to find QT_INSTALL_DOCS: ${COMMAND_OUTPUT} ${COMMAND_ERROR} [${COMMAND_RESULT}]")
+    endif()
+  endif()
+endif()
+
+# Include Qt doc tagfiles, if available.
+if (QT_INSTALL_DOCS)
   set(QT_DOCS_BASEURL "https://doc.qt.io/qt-${QT_VERSION_MAJOR}/")
-  message(STATUS "Using Qt doc tagfiles ${QT_TAGFILES_PATH} (${QT_DOCS_BASEURL})")
+  message(STATUS "Using Qt doc tagfiles ${QT_INSTALL_DOCS} (${QT_DOCS_BASEURL})")
   set(DOX_TAGFILES "\\
-  \"${QT_TAGFILES_PATH}/qtcore/qtcore.tags=${QT_DOCS_BASEURL}\" \\
-  \"${QT_TAGFILES_PATH}/qtbluetooth/qtbluetooth.tags=${QT_DOCS_BASEURL}\" \\")
+  \"${QT_INSTALL_DOCS}/qtcore/qtcore.tags=${QT_DOCS_BASEURL}\" \\
+  \"${QT_INSTALL_DOCS}/qtbluetooth/qtbluetooth.tags=${QT_DOCS_BASEURL}\" \\")
 endif()
 
 find_package(Doxygen)