How can I use CMake to both build wxwidgets on-demand and link with it

Question

I have the following situation:

• I'm working on an application that depends on a number of third party libs, among them wxwidgets
• I build the application for multiple target configurations (x86, arm, Linux, Windows) using Linux as my build host system
• Due to the above mentioned multiple target configurations, I have chosen to build those third-party libs from source, using CMake's ExternalProject_Add function.
• The third-party libs are built 'on-demand' at a location separate from my application's CMAKE_BINARY_DIR so that I can wipe the build tree for my application without having to rebuild the third-party libs (takes a looooong time).
• The location of the third-party libs is different depending on what target configuration I build them for (obviously).

I'm quite new to CMake and the problem I currently face is this: The source files in my application can't find the wx include files and I need to set the correct linker flags to be able to link my application against wxwidgets.

This seems to be handled by a utility 'wx-config' that provides exactly that info as output when run with either the --cppflags or --libs flag. I can not however, figure out how to catch that output and append it to the include dirs and linked libraries I setup from my CMakeLists.txt files.

So basically what I want is.

1. Build wxwidgets (if it doesn't exist) for the current target configuration
2. Run wx-config --cppflags and --libs to find out the correct include dirs and linker flags for the current target configuration
3. Use the info from step 2 when building targets that are my own application

So far I've tried something like this:

# Set a target-configuration-specific location 
set(wxwidgetsTop ${MYPROJECT_EXTERNAL_DIR}/wxwidgets/wxwidgets_${MYPROJECT_CURRENT_TARGET_CFG})

# Build the project
ExternalProject_Add( wxWidgetsExternal
  PREFIX ${wxwidgetsTop}
  URL ${MYPROJECT_EXTERNAL_DIR}/tarballs/wxWidgets-3.0.2.tar.bz2
  SOURCE_DIR ${wxwidgetsTop}/src/wxwidgets
  CONFIGURE_COMMAND ${configure_cmdline}
  BUILD_COMMAND make -j${MYPROJECT_NCPU}
  INSTALL_COMMAND make install
  )

# Create a wxwidgets target to be used as a dependency from other code
add_library(wxWidgets IMPORTED STATIC GLOBAL)
add_dependencies(wxWidgets wxWidgetsExternal)

# (non-working) attempt to get the correct include dirs and linker
# flags for wxwidgets 
add_custom_command(TARGET wxWidgetsExternal
  POST_BUILD
  COMMAND ${INSTALL_DIR}/bin/wx-config ARGS --cppflags
  COMMENT "Running wx-config"
)

but the above does not provide a way to actually use the result from the custom command to append the cppflags and linker options when building the targets that make up my application.

What is a good way to achieve what I want?

Answer 1

I see three different ways of doing this:

Method 1: use find_package

Use wxWidgets as a standalone requirement for your project, and expect the devs to install it before building your project. In your CMakeLists.txt you will need to call find_package(wxWidgets), like this:

    find_package(wxWidgets COMPONENTS net gl core base)
    if(wxWidgets_FOUND)
        include(${wxWidgets_USE_FILE})
        # and for each of your dependent executable/library targets:
        target_link_libraries(<YourTarget> ${wxWidgets_LIBRARIES})
    endif()

This has the advantage of not rebuilding the lib if you rebuild your project, however it requires some work for your user (they need to handle the installation of wxWidgets by hand) and for you (you need to setup include paths / compile definitions / ... by hand).

Method 2: embed wxWidgets

The second option is to bundle wxWidgets in your repo (svn external or git submodule) and usually (re)write the CMakeLists.txt of this lib to be target-oriented. Then, in your top-most CMakeLists.txt, you can do the following:

    # for example, if you just need core and net:
    target_link_librairies(my_app PUBLIC wxWidgetsCore wxWidgetsNet)
    # No need to manually setup include dirs, etc...

To make a CMakeLists.txt target-oriented, you define include directories and other compilation properties for a target, not a directory. Example:

    # When defining wxWidgetsCore, for example
    add_library(wxWidgetsCore ...)
    target_include_directories(wxWidgetsCore PUBLIC someDir)
    target_compile_definitions(wxWidgetsCore PUBLIC -pedantic)
    target_link_libraries(wxWidgetsCore PUBLIC someLib)

The drawback of this approach is that rebuilding your project will trigger a rebuild of wxWidgets. However, it is possible to trick this by not using "rebuild" but "clean just my app, then build". Here is some insight on how to achieve this.

Method 3: some sort of hybrid

The big drawback of method 2 leads to the third approach: don't put wxWidgets in your project, but create a CMakeLists.txt that will "import" the lib. The idea: you ask your user for the directory where wxWidgets is installed, then this script will setup everything for your project. First, put the CMakeLists.txt here:

    /your-project-root
        /thirdparty
            /wxWidgets
                CMakeLists.txt
    /dir-where-wxwidgets-is-installed
        ...

Now, you define an imported target:

    # When defining wxWidgetsCore, for example
    set(WX_INCLUDE_DIR ${USER_SPECIFIED_WX_ROOT}/include)
    add_library(wxWidgetsCore IMPORTED GLOBAL)
    set_property(TARGET wxWidgetsCore APPEND PROPERTY
        INTERFACE_INCLUDE_DIRECTORIES ${WX_INCLUDE_DIR})

See INTERFACE_INCLUDE_DIRECTORIES and INTERFACE_LINK_LIBRARIES. You need your user to have build wxWidgets somewhere in his system, but from your point of view you just do target_link_libraries(your_app PUBLIC wxWidgets...), as in method 2. The advantage is that this approach is interchangeable with method 2 transparently, and you don't put the whole dependency in your project.

Answer 2

Setting cppflags and linker flags has to be done at CMake time, but you are trying to run wx-config at build time and you are not capturing its output anyway, so your add_custom_command() isn't doing anything useful other than printing things to the build tool's output.

Ideally, you would use the FindwxWidgets module CMake already provides. It requires wxWidgets to already be built (but see further below). Have a look at the CMake documentation for it and see if that at least sounds like what you are trying to achieve manually by using wx-config. If you can get FindwxWidgets to do the job for you, that would be a much cleaner approach.

Getting something to build at configure time so you can use it later on in your CMakeLists.txt file is a bit more tricky. ExternalProject_Add() downloads and builds things at build time, but you need wxWidgets to be built earlier at configure time. I wrote an article recently for how to do at least the downloading part at configure time and you should be able to adapt it to do the whole build at configure time instead. The article uses Google Test as its example and can be found here:

https://crascit.com/2015/07/25/cmake-gtest/

It would be trivial to make it put the wxWidgets build wherever you like, not just in the CMAKE_BINARY_DIR area. That would allow you to have different wxWidgets builds for each build configuration and to be able to wipe out your application's build tree independently of the wxWidgets builds.

Hope that points you in the right direction.

Answer 3

The solution I use checks for wxWidgets installation in the system using find_package, if it's not found, then the script downloads wxWidgets from github and links the program against downloaded library. The lib is installed in the build directory, so only the first build is slow - subsequent builds do not even check wxWidgets sources timestamps, so the process is as fast as building using preinstalled wxWidgets library.

Here's how my script does it:

1. It quietly checks for wxWidgets installation using find_package(wxWidgets QUIET),
2. If it's found, the script adds a dummy library wxWidgets_external,
3. If it's not, then it creates an ExternalProject named wxWidgets_external which downloads, builds and installs the library in the build dir, setting wxWidgets_ROOT_DIR to point to the wxWidgets installation dir,
4. Then we add another ExternalProject pointing to a folder with the main program's source files and CMakeLists.txt build script. This external projects depends on wxWidgets_external which is either a dummy library in case wxWidgets is preinstalled in the system, or an external project set up to download the library from github,
5. In the aforementioned CMakeLists.txt we again call find_package, this time with REQUIRED parameter and use the library the standard way (https://docs.wxwidgets.org/trunk/overview_cmake.html). Because we set up the dependencies and variables correctly, this call will use either preinstalled wxWidgets (if it's available) or the one downloaded from github.

There are more quirks to it, but that's the gist of it. The full sample code (tested on Linux, Windows and Mac) is available on github (https://github.com/lszl84/wx_cmake_template).

Also see full blog post which explains this in more detail: https://justdevtutorials.medium.com/wxwidgets-cmake-multiplatform-superbuild-4ea86c4e6eda