From 9fb3bbe179698fb10339d4911b98531b0847cfa1 Mon Sep 17 00:00:00 2001
From: Martchus <martchus@gmx.net>
Date: Fri, 1 Jan 2021 18:42:22 +0100
Subject: [PATCH] Export a target for using only headers of a library

Some backend libraries of Syncthing Tray and Reflective RapidJSON only use
certain headers of qtutilities/c++utilities. The current solution did not
really work because it did not distinguish between the build and install
interface and also did not take compile definitions and options into
account.
---
 cmake/modules/3rdParty.cmake      |  5 ++++-
 cmake/modules/LibraryTarget.cmake | 15 +++++++++++++++
 cmake/templates/Config.cmake.in   | 11 ++++++++++-
 3 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/cmake/modules/3rdParty.cmake b/cmake/modules/3rdParty.cmake
index d854534..f2fa51b 100644
--- a/cmake/modules/3rdParty.cmake
+++ b/cmake/modules/3rdParty.cmake
@@ -45,7 +45,7 @@ endfunction ()
 
 function (parse_arguments_for_use_functions)
     # parse arguments
-    set(OPTIONAL_ARGS OPTIONAL)
+    set(OPTIONAL_ARGS OPTIONAL ONLY_HEADERS)
     set(ONE_VALUE_ARGS VISIBILITY LIBRARIES_VARIABLE PACKAGES_VARIABLE PKG_CONFIG_MODULES_VARIABLE TARGET_NAME PACKAGE_NAME)
     set(MULTI_VALUE_ARGS PKG_CONFIG_MODULES PACKAGE_ARGS)
     cmake_parse_arguments(ARGS "${OPTIONAL_ARGS}" "${ONE_VALUE_ARGS}" "${MULTI_VALUE_ARGS}" ${ARGN})
@@ -103,6 +103,9 @@ function (parse_arguments_for_use_functions)
     set(ARGS_OPTIONAL
         "${ARGS_OPTIONAL}"
         PARENT_SCOPE)
+    set(ARGS_ONLY_HEADERS
+        "${ARGS_ONLY_HEADERS}"
+        PARENT_SCOPE)
     if (NOT ARGS_OPTIONAL)
         set(ARGS_FIND_PACKAGE
             "REQUIRED"
diff --git a/cmake/modules/LibraryTarget.cmake b/cmake/modules/LibraryTarget.cmake
index f9653b1..7f205bb 100644
--- a/cmake/modules/LibraryTarget.cmake
+++ b/cmake/modules/LibraryTarget.cmake
@@ -172,6 +172,18 @@ else ()
     if (META_TARGET_NAME MATCHES "lib.*")
         set_target_properties(${META_TARGET_NAME} PROPERTIES PREFIX "")
     endif ()
+
+    # add target for pulling only headers because some libraries contain header-only parts which are useful on their own
+    if (NOT META_PLUGIN_CATEGORY)
+        add_library(${META_TARGET_NAME}-headers INTERFACE)
+        target_include_directories(
+            ${META_TARGET_NAME}-headers INTERFACE $<BUILD_INTERFACE:${TARGET_INCLUDE_DIRECTORY_BUILD_INTERFACE}>
+                                                  $<INSTALL_INTERFACE:${HEADER_INSTALL_DESTINATION}> ${PUBLIC_INCLUDE_DIRS})
+        target_compile_definitions(${META_TARGET_NAME}-headers INTERFACE "${META_PUBLIC_COMPILE_DEFINITIONS}"
+                                                                         "${META_PRIVATE_COMPILE_DEFINITIONS}")
+        target_compile_options(${META_TARGET_NAME}-headers INTERFACE "${META_PUBLIC_COMPILE_OPTIONS}"
+                                                                     "${META_PRIVATE_COMPILE_OPTIONS}")
+    endif ()
 endif ()
 
 # populate META_PUBLIC_LIB_DEPENDS
@@ -454,6 +466,9 @@ if (NOT META_NO_INSTALL_TARGETS AND ENABLE_INSTALL_TARGETS)
             list(APPEND TARGETS_TO_EXPORT ${BUNDLED_TARGET})
         endif ()
     endforeach ()
+    if (NOT META_HEADER_ONLY_LIB AND NOT META_PLUGIN_CATEGORY)
+        list(APPEND TARGETS_TO_EXPORT "${META_TARGET_NAME}-headers")
+    endif ()
     install(
         TARGETS ${TARGETS_TO_EXPORT}
         EXPORT "${META_PROJECT_NAME}${META_CONFIG_SUFFIX}Targets"
diff --git a/cmake/templates/Config.cmake.in b/cmake/templates/Config.cmake.in
index f16db5a..8762c8a 100644
--- a/cmake/templates/Config.cmake.in
+++ b/cmake/templates/Config.cmake.in
@@ -118,9 +118,18 @@ function(use_@META_PROJECT_VARNAME@)
 
     # link against library (also "link" against header-only libs to get compile defs and interface link libs)
     link_directories(${@META_PROJECT_VARNAME_UPPER@_LIB_DIR})
-    set("${ARGS_LIBRARIES_VARIABLE}" "${${ARGS_LIBRARIES_VARIABLE}};${@META_PROJECT_VARNAME_UPPER@_LIB}" PARENT_SCOPE)
+    if (ARGS_ONLY_HEADERS)
+        set(TARGET_NAME "${@META_PROJECT_VARNAME_UPPER@_LIB}-headers")
+    else ()
+        set(TARGET_NAME "${@META_PROJECT_VARNAME_UPPER@_LIB}")
+    endif ()
+    set("${ARGS_LIBRARIES_VARIABLE}" "${${ARGS_LIBRARIES_VARIABLE}};${TARGET_NAME}" PARENT_SCOPE)
     set("${ARGS_PACKAGES_VARIABLE}" "${${ARGS_PACKAGES_VARIABLE}};${@META_PROJECT_VARNAME_UPPER@_PACKAGE}" PARENT_SCOPE)
 
+    if (ARGS_ONLY_HEADERS)
+        return ()
+    endif ()
+
     # add required Qt and KF modules
     set(IMPORTED_QT_MODULES
         ${IMPORTED_QT_MODULES}