From e05f7429d5dc54ea1228e127857c2d6d0d2c0d41 Mon Sep 17 00:00:00 2001
From: Vito Gamberini <vito@gamberini.email>
Date: Wed, 21 Jun 2023 15:33:18 -0400
Subject: [PATCH] Build: Remove SDL requirement for packaging

This allows the "headers-only" target to be installed even if SDL wasn't
found during configuration, enabling a "bring your own SDL" style of
development for devs who vendor specific SDL versions or binaries in
their codebase.
---
 CMakeLists.txt               | 47 +++++++++++++++++++++---------------
 cmake/centurion-config.cmake |  8 +++---
 cmake/centurion.cmake        | 36 ++++++++++++++++++++++++++-
 3 files changed, 67 insertions(+), 24 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e060642d1..a2e453d3e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -38,29 +38,36 @@ set(CENTURION_TEST_TARGET centurion-tests)
 set(CENTURION_MOCK_TARGET centurion-mocks)
 
 # System dependencies
-find_package(SDL2 CONFIG REQUIRED)
-find_package(SDL2_image CONFIG REQUIRED)
-find_package(SDL2_mixer CONFIG REQUIRED)
-find_package(SDL2_ttf CONFIG REQUIRED)
+find_package(SDL2 CONFIG)
+find_package(SDL2_image CONFIG)
+find_package(SDL2_mixer CONFIG)
+find_package(SDL2_ttf CONFIG)
 
 # CMake Targets
 add_library(centurion-headers-only INTERFACE)
 add_library(centurion::centurion-headers-only ALIAS centurion-headers-only)
+set(CEN_TARGETS centurion-headers-only)
 add_subdirectory("${CEN_SOURCE_DIR}")
 
-add_library(centurion INTERFACE)
-add_library(centurion::centurion ALIAS centurion)
-target_link_libraries(centurion INTERFACE
-  centurion-headers-only
-  $<IF:$<TARGET_EXISTS:SDL2::SDL2>,SDL2::SDL2,SDL2::SDL2-static>
-  $<IF:$<TARGET_EXISTS:SDL2_image::SDL2_image>,SDL2_image::SDL2_image,SDL2_image::SDL2_image-static>
-  $<IF:$<TARGET_EXISTS:SDL2_mixer::SDL2_mixer>,SDL2_mixer::SDL2_mixer,SDL2_mixer::SDL2_mixer-static>
-  $<IF:$<TARGET_EXISTS:SDL2_ttf::SDL2_ttf>,SDL2_ttf::SDL2_ttf,SDL2_ttf::SDL2_ttf-static>
-)
+cen_has_SDL(SDL_SHARED SDL_STATIC)
+
+if (SDL_SHARED OR SDL_STATIC)
+  add_library(centurion INTERFACE)
+  add_library(centurion::centurion ALIAS centurion)
+  list(APPEND CEN_TARGETS centurion)
+  target_link_libraries(centurion INTERFACE
+    centurion-headers-only
+    $<IF:$<TARGET_EXISTS:SDL2::SDL2>,SDL2::SDL2,SDL2::SDL2-static>
+    $<IF:$<TARGET_EXISTS:SDL2_image::SDL2_image>,SDL2_image::SDL2_image,SDL2_image::SDL2_image-static>
+    $<IF:$<TARGET_EXISTS:SDL2_mixer::SDL2_mixer>,SDL2_mixer::SDL2_mixer,SDL2_mixer::SDL2_mixer-static>
+    $<IF:$<TARGET_EXISTS:SDL2_ttf::SDL2_ttf>,SDL2_ttf::SDL2_ttf,SDL2_ttf::SDL2_ttf-static>
+  )
+endif ()
 
-if (TARGET SDL2::SDL2-static)
+if (SDL_STATIC)
   add_library(centurion-static INTERFACE)
   add_library(centurion::centurion-static ALIAS centurion-static)
+  list(APPEND CEN_TARGETS centurion-static)
   target_link_libraries(centurion-static INTERFACE
     centurion-headers-only
     SDL2::SDL2-static
@@ -70,6 +77,12 @@ if (TARGET SDL2::SDL2-static)
   )
 endif ()
 
+if (BUILD_TESTS OR BUILD_EXAMPLES)
+  if (NOT TARGET centurion::centurion)
+    message(FATAL_ERROR "Unable to find required SDL libraries")
+  endif()
+endif ()
+
 if (BUILD_TESTS)
   # Vcpkg test dependencies
   find_package(GTest CONFIG REQUIRED)
@@ -111,15 +124,11 @@ install(FILES
 )
 
 install(
-  TARGETS centurion-headers-only centurion
+  TARGETS ${CEN_TARGETS}
   EXPORT centurionTargets
   FILE_SET HEADERS
 )
 
-if(TARGET centurion-static)
-  install(TARGETS centurion-static EXPORT centurionTargets)
-endif()
-
 install(
   EXPORT centurionTargets
   NAMESPACE centurion::
diff --git a/cmake/centurion-config.cmake b/cmake/centurion-config.cmake
index 9564c2133..4ed69d198 100644
--- a/cmake/centurion-config.cmake
+++ b/cmake/centurion-config.cmake
@@ -1,6 +1,6 @@
-find_package(SDL2 CONFIG REQUIRED)
-find_package(SDL2_image CONFIG REQUIRED)
-find_package(SDL2_mixer CONFIG REQUIRED)
-find_package(SDL2_ttf CONFIG REQUIRED)
+find_package(SDL2 CONFIG)
+find_package(SDL2_image CONFIG)
+find_package(SDL2_mixer CONFIG)
+find_package(SDL2_ttf CONFIG)
 
 include("${CMAKE_CURRENT_LIST_DIR}/centurionTargets.cmake")
diff --git a/cmake/centurion.cmake b/cmake/centurion.cmake
index 56831ac30..21aa3cad3 100644
--- a/cmake/centurion.cmake
+++ b/cmake/centurion.cmake
@@ -52,4 +52,38 @@ function(cen_set_basic_compiler_options target)
       target_compile_options(${target} PRIVATE -Werror)
     endif ()
   endif ()
-endfunction()
\ No newline at end of file
+endfunction()
+
+# Checks if targets exists and stores the result in a variable.
+#   var: the name of the result variable
+#   ARGN: list of targets to check the existence of
+function(cen_has_targets var)
+  set(${var} FALSE PARENT_SCOPE)
+  foreach(target IN LISTS ARGN)
+    if (NOT TARGET ${target})
+      return()
+    endif ()
+  endforeach ()
+  set(${var} TRUE PARENT_SCOPE)
+endfunction()
+
+# Checks if the relevant SDL targets exist and stores the result in the
+# corresponding variable
+#   shared: the name of the shared libraries variable
+#   static: the name of the static libraries variable
+function(cen_has_SDL shared static)
+  cen_has_targets(_shared
+    SDL2::SDL2
+    SDL2_image::SDL2_image
+    SDL2_mixer::SDL2_mixer
+    SDL2_ttf::SDL2_ttf
+  )
+  set(${shared} ${_shared} PARENT_SCOPE)
+  cen_has_targets(_static 
+    SDL2::SDL2-static
+    SDL2_image::SDL2_image-static
+    SDL2_mixer::SDL2_mixer-static
+    SDL2_ttf::SDL2_ttf-static
+  )
+  set(${static} ${_static} PARENT_SCOPE)
+endfunction()