Add GTK4 support

.gitignore

@@ -5,3 +5,6 @@
 
 # Mac OS X rubbish
 .DS_Store
+
+# CMake build directory
+/build/

CMakeLists.txt

@@ -26,6 +26,6 @@ set (CMAKE_CXX_STANDARD 17)
 add_subdirectory(src)
 
 option(NFD_BUILD_TESTS "Build tests for nfd" OFF)
-if(${NFD_BUILD_TESTS})
+if(NFD_BUILD_TESTS)
     add_subdirectory(test)
 endif()

src/CMakeLists.txt

@@ -12,8 +12,27 @@ endif()
 
 if(nfd_PLATFORM STREQUAL PLATFORM_LINUX)
   find_package(PkgConfig REQUIRED)
-  pkg_check_modules(GTK3 REQUIRED gtk+-3.0)
-  message("Using GTK version: ${GTK3_VERSION}")
+  set(NFD_GTK_VERSION "" CACHE STRING "GTK version for Linux builds ('3' or '4')")
+  set_property(CACHE NFD_GTK_VERSION PROPERTY STRINGS "" 3 4)
+  # For Linux, we support both GTK3 and GTK4.
+  # If NFD_GTK_VERSION is not explicitly set, then we take one that is available.
+  # Otherwise, we find the version that the user wants.
+  if(NFD_GTK_VERSION STREQUAL "")
+    pkg_search_module(GTK REQUIRED gtk+-3.0 gtk4)
+    if(DEFINED GTK_gtk+-3.0_VERSION)
+      set(GTK_VERSION ${GTK_gtk+-3.0_VERSION})
+    elseif(DEFINED GTK_gtk4_VERSION)
+      set(GTK_VERSION ${GTK_gtk4_VERSION})
+    endif()
+  elseif(NFD_GTK_VERSION STREQUAL 3)
+    pkg_check_modules(GTK REQUIRED gtk+-3.0)
+  elseif(NFD_GTK_VERSION STREQUAL 4)
+    pkg_check_modules(GTK REQUIRED gtk4)
+  else()
+    message(FATAL_ERROR "Unsupported GTK version: ${NFD_GTK_VERSION}")
+  endif()
+
+  message("Using GTK version: ${GTK_VERSION}")
   list(APPEND SOURCE_FILES nfd_gtk.cpp)
 endif()
 
@@ -32,9 +51,12 @@ target_include_directories(${TARGET_NAME}
 
 if(nfd_PLATFORM STREQUAL PLATFORM_LINUX)
   target_include_directories(${TARGET_NAME}
-    PRIVATE ${GTK3_INCLUDE_DIRS})
+    PRIVATE ${GTK_INCLUDE_DIRS})
   target_link_libraries(${TARGET_NAME}
-    PRIVATE ${GTK3_LIBRARIES})
+    PRIVATE ${GTK_LIBRARIES})
+  string(REPLACE "." ";" GTK_VERSION_LIST ${GTK_VERSION})
+  list(GET GTK_VERSION_LIST 0 GTK_VERSION_MAJOR)
+  target_compile_definitions(${TARGET_NAME} PUBLIC NFD_GTK_VERSION=${GTK_VERSION_MAJOR})
 endif()
 
 if(nfd_PLATFORM STREQUAL PLATFORM_MACOS)

src/include/nfd.h

@@ -28,6 +28,9 @@ typedef char nfdnchar_t;
 typedef void nfdpathset_t;
 typedef struct {
     void* ptr;
+#if defined(NFD_GTK_VERSION) && NFD_GTK_VERSION == 4
+    unsigned int next_index;
+#endif
 } nfdpathsetenum_t;
 
 typedef unsigned int nfdfiltersize_t;
@@ -43,6 +46,10 @@ typedef struct {
     const nfdnchar_t* spec;
 } nfdnfilteritem_t;
 
+/* Conventions:
+ * Output parameter (typically outPath/outPaths), if any, will not be modified if the function
+ * returns something other than NFD_OKAY. This applies everywhere, including helper functions. */
+
 /* nfd_<targetplatform>.c */
 
 /* free a file path that was returned by the dialogs */