Make build system usable as a git submodule

Replace CMAKE_SOURCE_DIR/CMAKE_BINARY_DIR with PROJECT_SOURCE_DIR/
PROJECT_BINARY_DIR so paths resolve correctly from add_subdirectory().
Guard global settings (CMAKE_CXX_STANDARD, CMAKE_BUILD_RPATH,
qt_standard_project_setup, demo) behind PROJECT_IS_TOP_LEVEL.
Export OPEN62541_INSTALL_DIR and QTOPCUA_INSTALL_DIR as CACHE INTERNAL
so parent projects can set up RPATH and plugin paths.

4 files changed, 39 insertions, 28 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index f35b498..51b90e8 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,9 +1,11 @@
 cmake_minimum_required(VERSION 3.16)
 project(BobinkQtOpcUa LANGUAGES CXX)
 
-set(CMAKE_CXX_STANDARD 17)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+if(PROJECT_IS_TOP_LEVEL)
+  set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+  set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+endif()
 
 # Build external dependencies (open62541, qtopcua) if not already built
 include(cmake/BuildDeps.cmake)
@@ -19,21 +21,27 @@ set(Qt6DeclarativeOpcua_DIR "${QTOPCUA_INSTALL_DIR}/lib/cmake/Qt6DeclarativeOpcu
 list(PREPEND CMAKE_PREFIX_PATH "${OPEN62541_INSTALL_DIR}")
 
 find_package(Qt6 6.10.2 REQUIRED COMPONENTS Core Qml Quick OpcUa)
-qt_standard_project_setup(REQUIRES 6.10.2)
-
-# Set path for qmllint
-set(QML_IMPORT_PATH
-    "${CMAKE_CURRENT_BINARY_DIR}/qml"
-    CACHE STRING "Path to locally built qml")
-# Generate .qmlls.ini for QML Language Server. Useful once QtOpcUa is installed
-# globally (qt-cmake --install build/deps/qtopcua-build) so qmlls can resolve
-# all Qt QML imports without extra importPaths. set(QT_QML_GENERATE_QMLLS_INI ON
-# CACHE BOOL "")
-
-# Ensure the local QtOpcUa and open62541 libs are findable at runtime (needed
-# because the Qt plugin loader dlopen's the open62541 backend).
-set(CMAKE_BUILD_RPATH "${QTOPCUA_INSTALL_DIR}/lib"
-                      "${OPEN62541_INSTALL_DIR}/lib")
+
+if(PROJECT_IS_TOP_LEVEL)
+  qt_standard_project_setup(REQUIRES 6.10.2)
+
+  # Set path for qmllint
+  set(QML_IMPORT_PATH
+      "${CMAKE_CURRENT_BINARY_DIR}/qml"
+      CACHE STRING "Path to locally built qml")
+  # Generate .qmlls.ini for QML Language Server. Useful once QtOpcUa is installed
+  # globally (qt-cmake --install build/deps/qtopcua-build) so qmlls can resolve
+  # all Qt QML imports without extra importPaths. set(QT_QML_GENERATE_QMLLS_INI ON
+  # CACHE BOOL "")
+
+  # Ensure the local QtOpcUa and open62541 libs are findable at runtime (needed
+  # because the Qt plugin loader dlopen's the open62541 backend).
+  set(CMAKE_BUILD_RPATH "${QTOPCUA_INSTALL_DIR}/lib"
+                        "${OPEN62541_INSTALL_DIR}/lib")
+endif()
 
 add_subdirectory(src)
-add_subdirectory(demo)
+
+if(PROJECT_IS_TOP_LEVEL)
+  add_subdirectory(demo)
+endif()
diff --git a/cmake/BuildDeps.cmake b/cmake/BuildDeps.cmake
index 8f819f3..83791a2 100644
--- a/cmake/BuildDeps.cmake
+++ b/cmake/BuildDeps.cmake
@@ -8,12 +8,14 @@
 # skipped. Delete the file to rebuild.
 # ======================================
 
-set(OPEN62541_SOURCE_DIR "${CMAKE_SOURCE_DIR}/deps/open62541")
-set(OPEN62541_BUILD_DIR "${CMAKE_BINARY_DIR}/deps/open62541-build")
-set(OPEN62541_INSTALL_DIR "${CMAKE_BINARY_DIR}/deps/open62541-install")
-set(QTOPCUA_SOURCE_DIR "${CMAKE_SOURCE_DIR}/deps/qtopcua")
-set(QTOPCUA_BUILD_DIR "${CMAKE_BINARY_DIR}/deps/qtopcua-build")
-set(QTOPCUA_INSTALL_DIR "${CMAKE_BINARY_DIR}/deps/qtopcua-install")
+set(OPEN62541_SOURCE_DIR "${PROJECT_SOURCE_DIR}/deps/open62541")
+set(OPEN62541_BUILD_DIR "${PROJECT_BINARY_DIR}/deps/open62541-build")
+set(OPEN62541_INSTALL_DIR "${PROJECT_BINARY_DIR}/deps/open62541-install"
+    CACHE INTERNAL "open62541 install prefix")
+set(QTOPCUA_SOURCE_DIR "${PROJECT_SOURCE_DIR}/deps/qtopcua")
+set(QTOPCUA_BUILD_DIR "${PROJECT_BINARY_DIR}/deps/qtopcua-build")
+set(QTOPCUA_INSTALL_DIR "${PROJECT_BINARY_DIR}/deps/qtopcua-install"
+    CACHE INTERNAL "QtOpcUa install prefix")
 
 # Verify submodules are initialized
 if(NOT EXISTS "${OPEN62541_SOURCE_DIR}/CMakeLists.txt")
@@ -150,7 +152,7 @@ endif()
 
 if(WIN32)
   # On Windows DLLs must be next to the .exe (no RPATH). Copy them to bin/.
-  file(MAKE_DIRECTORY "${CMAKE_BINARY_DIR}/bin")
+  file(MAKE_DIRECTORY "${PROJECT_BINARY_DIR}/bin")
   file(GLOB _dep_dlls "${OPEN62541_INSTALL_DIR}/bin/*.dll"
        "${QTOPCUA_INSTALL_DIR}/bin/*.dll")
   foreach(_dll ${_dep_dlls})
diff --git a/demo/CMakeLists.txt b/demo/CMakeLists.txt
index 03c19d3..d2158cb 100644
--- a/demo/CMakeLists.txt
+++ b/demo/CMakeLists.txt
@@ -12,7 +12,7 @@ qt_add_qml_module(
 
 # Executable goes to bin/ to avoid clashing with the QML module directory
 set_target_properties(BobinkDemo PROPERTIES RUNTIME_OUTPUT_DIRECTORY
-                                            "${CMAKE_BINARY_DIR}/bin")
+                                            "${PROJECT_BINARY_DIR}/bin")
 
 target_link_libraries(BobinkDemo PRIVATE Qt6::Quick BobinkQtOpcUaplugin)
 
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index b0781cd..0aff9d4 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -14,6 +14,7 @@ qt_add_qml_module(
   OpcUaMonitoredNode.h
   OpcUaMonitoredNode.cpp
   OUTPUT_DIRECTORY
-  "${CMAKE_BINARY_DIR}/qml/Bobink")
+  "${PROJECT_BINARY_DIR}/qml/Bobink")
 
+target_compile_features(BobinkQtOpcUa PUBLIC cxx_std_17)
 target_link_libraries(BobinkQtOpcUa PUBLIC Qt6::Core Qt6::Quick Qt6::OpcUa)