modules: make it possible to use SDL2 as a CMake subproject.

doc/changelog.dox

@@ -315,6 +315,9 @@ See also:
     [mosra/homebrew-magnum#6](https://github.com/mosra/homebrew-magnum/pull/6))
 -   Various changes to Vcpkg packages to account for newly added libraries and
     plugin interfaces ([mosra/magnum#485](https://github.com/mosra/magnum/issues/485))
+-   The `FindSDL2.cmake` module was updated to allow using SDL2 as a
+    subproject. See the @ref Platform-Sdl2Application-usage "Platform::Sdl2Application docs"
+    for more information. See also [mosra/magnum#496](https://github.com/mosra/magnum/issues/496).
 
 @subsection changelog-latest-bugfixes Bug fixes
 

modules/FindSDL2.cmake

@@ -42,6 +42,63 @@
 #   DEALINGS IN THE SOFTWARE.
 #
 
+# If we have a CMake subproject, use the targets directly. I'd prefer the
+# static variant, however SDL2 defines its own SDL2::SDL2 alias for only the
+# dynamic variant since https://github.com/libsdl-org/SDL/pull/4074 and so I'm
+# forced to use that, if available.
+if(TARGET SDL2)
+    # In case we don't have https://github.com/libsdl-org/SDL/pull/4074 yet,
+    # do the alias ourselves.
+    if(NOT TARGET SDL2::SDL2)
+        # Aliases of (global) targets are only supported in CMake 3.11, so we
+        # work around it by this. This is easier than fetching all possible
+        # properties (which are impossible to track of) and then attempting to
+        # rebuild them into a new target.
+        add_library(SDL2::SDL2 INTERFACE IMPORTED)
+        set_target_properties(SDL2::SDL2 PROPERTIES INTERFACE_LINK_LIBRARIES SDL2)
+    endif()
+
+    # Just to make FPHSA print some meaningful location, nothing else. Not
+    # using the INTERFACE_INCLUDE_DIRECTORIES as that contains
+    # $<BUILD_INTERFACE and looks ugly in the output. Funnily enough, the
+    # BUILD_INTERFACE thing works here without having to override it with
+    # custom-found paths like I do in FindAssimp and elsewhere. Needs further
+    # investigation.
+    get_target_property(_SDL2_SOURCE_DIR SDL2 SOURCE_DIR)
+    include(FindPackageHandleStandardArgs)
+    find_package_handle_standard_args("SDL2" DEFAULT_MSG _SDL2_SOURCE_DIR)
+
+    if(CORRADE_TARGET_WINDOWS)
+        # .dll is in LOCATION, .lib is in IMPLIB. Yay, useful!
+        get_target_property(SDL2_DLL_DEBUG SDL2 IMPORTED_LOCATION_DEBUG)
+        get_target_property(SDL2_DLL_RELEASE SDL2 IMPORTED_LOCATION_RELEASE)
+    endif()
+
+    return()
+
+# The static build is a separate target for some reason. I wonder HOW that
+# makes more sense than just having a build-time option for static/shared and
+# use the same name for both. Are all depending projects supposed to branch on
+# it like this?!
+elseif(TARGET SDL2-static)
+    # The target should not be defined by SDL itself. If it is, that's from us.
+    if(NOT TARGET SDL2::SDL2)
+        # Aliases of (global) targets are only supported in CMake 3.11, so we
+        # work around it by this. This is easier than fetching all possible
+        # properties (which are impossible to track of) and then attempting to
+        # rebuild them into a new target.
+        add_library(SDL2::SDL2 INTERFACE IMPORTED)
+        set_target_properties(SDL2::SDL2 PROPERTIES INTERFACE_LINK_LIBRARIES SDL2-static)
+    endif()
+
+    # Just to make FPHSA print some meaningful location, nothing else
+    get_target_property(_SDL2_SOURCE_DIR SDL2-static SOURCE_DIR)
+    include(FindPackageHandleStandardArgs)
+    find_package_handle_standard_args("SDL2" DEFAULT_MSG _SDL2_SOURCE_DIR)
+
+    return()
+endif()
+
 # In Emscripten SDL is linked automatically, thus no need to find the library.
 # Also the includes are in SDL subdirectory, not SDL2.
 if(CORRADE_TARGET_EMSCRIPTEN)

src/Magnum/Platform/Sdl2Application.h

@@ -217,17 +217,51 @@ find_package(Magnum REQUIRED Sdl2Application)
 target_link_libraries(your-app PRIVATE Magnum::Sdl2Application)
 @endcode
 
-Additionally, if you're using Magnum as a CMake subproject, do the following
+Additionally, if you're using Magnum as a CMake subproject, bundle the
+[SDL repository](https://github.com/libsdl-org/SDL) and do the following
 * *before* calling @cmake find_package() @ce to ensure it's enabled, as the
-library is not built by default.  Using SDL2 itself as a CMake subproject isn't
-tested at the moment, so you need to provide it as a system dependency and
-point `CMAKE_PREFIX_PATH` to its installation dir if necessary.
+library is not built by default. If you want to use system-installed SDL2,
+omit the first part and point `CMAKE_PREFIX_PATH` to its installation dir if
+necessary.
 
 @code{.cmake}
+# This is the most minimal set of features which still make Sdl2Application
+# work. If you need something from these, remove the setting. The SDL_AUDIO
+# option should not be needed either as Magnum doesn't use it, but if it's
+# disabled it causes linker errors. Either SDL_DLOPEN or SDL_LOADSO needs to be
+# enabled depending on the system to allow linking dependencies at runtime, so
+# it's better to just leave them both on.
+set(SDL_ATOMIC OFF CACHE BOOL "" FORCE)
+set(SDL_CPUINFO OFF CACHE BOOL "" FORCE)
+set(SDL_EVENTS OFF CACHE BOOL "" FORCE)
+set(SDL_FILE OFF CACHE BOOL "" FORCE)
+set(SDL_FILESYSTEM OFF CACHE BOOL "" FORCE)
+set(SDL_HAPTIC OFF CACHE BOOL "" FORCE)
+set(SDL_LOCALE OFF CACHE BOOL "" FORCE)
+set(SDL_POWER OFF CACHE BOOL "" FORCE)
+set(SDL_RENDER OFF CACHE BOOL "" FORCE)
+set(SDL_SENSOR OFF CACHE BOOL "" FORCE)
+set(SDL_THREADS OFF CACHE BOOL "" FORCE)
+set(SDL_TIMERS OFF CACHE BOOL "" FORCE)
+# This assumes you want to have SDL as a static library. If not, set SDL_STATIC
+# to OFF instead.
+set(SDL_SHARED OFF CACHE BOOL "" FORCE)
+add_subdirectory(SDL EXCLUDE_FROM_ALL)
+
 set(WITH_SDL2APPLICATION ON CACHE BOOL "" FORCE)
 add_subdirectory(magnum EXCLUDE_FROM_ALL)
 @endcode
 
+<b></b>
+
+@m_class{m-note m-warning}
+
+@par
+    While SDL itself, being a C project, builds quite fast, when using it as a
+    CMake subproject be prepared that it will *significantly* increase the
+    CMake configure time due to excessive platform checks, and pollute the
+    CMake option list with a lot of unprefixed SDL-specific options.
+
 If no other application is requested, you can also use the generic
 `Magnum::Application` alias to simplify porting. Again, see @ref building and
 @ref cmake for more information.