Thursday, May 2, 2019

How to include a CMake project in a Xcode project

For an iOS app I'm working on I needed to include a C++ project, have it build according to the current scheme (debug / release) and be able to debug it. I don't know much about C++, but I know that CMake is a popular build tool and for the project I was trying to import it was the preferred way to build it anyway. Turns out there are not a lot of people doing this as most of the information you'll find is either outdated (no, I'm not using Xcode 5 anymore) or requires a lot of background knowledge about both Xcode and CMake - which I'm both lacking.

To save myself and others some time in the future, I'd like to walk through an example of how you can build a CMake project for iOS in Xcode:

1. Create iOS Xcode project

Doh! Assuming you don't have one already. There is nothing specific to do here.

2. Clone the C++ project you'd like to include in your project

I'll be using glog for the purpose of this tutorial. Please note that some projects do not build out of the box on iOS for different reasons. I have created a fork of some projects that I needed on iOS, maybe you are lucky and the one you are looking for is among them:

3. CMake, meet Xcode.

First you need to clone a toolchain for iOS to teach CMake how to compile for iOS. Please note: I was not able to use the latest version at the time of writing this article. Instead, I used a specific commit.
Next, run the following one-liner inside your project folder in the terminal:
mkdir build/ && cd build/ && cmake -G "Xcode" -DCMAKE_TOOLCHAIN_FILE=../ios-cmake/ios.toolchain.cmake ../glog.ios/ && cd ../
I usually place that in a file called build.sh as you will need to rerun this whenever the C++ project changes (i.e. new files added).

The above commands will take a while to complete, but at the end you should see something along the lines of:
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/tom/workspace/cmake-sample.ios/build
Please note that you might have to install CMake first and the Xcode command line tools.

4. Xcode, meet CMake!

If the previous step succeeded and did not report any errors in the last three lines you are already halfway there! Hang tight. You should see a new .xcodeproj-file now in a folder called "build" if you look at your project using the file explorer. In my case it's called glog.xcodeproj. Drag this file into the file panel (on the left) of your Xcode project to link them. It doesn't really matter where exactly you drag it to as the file structure of a Xcode project is purely virtual.
Files created by CMake after running the above commands
This is what your Xcode project might look like after dragging the project created by CMake into it


A few more things left to click-click configure in Xcode: go to your project file and under "General" click the Add-button for "Linked Frameworks and Libraries". In the dialog that pops up you should see your C++ project show up. Add it!
Linking the framework generated by CMake

In the same file under "Build Phases" add your C++ project to "Target dependencies" (note: there are lots of targets showing up here. It's usually the one with a grey house-like icon and the exact same name as your C++ project). If you don't do this, Xcode won't automatically rebuild your C++ project if necessary.
Adding the framework as a build dependency

Last but definitely not least you have to tell Xcode where to find the headers of the C++ project. Again in the project file, under "Build Settings" search for a setting called "Header Search Paths", add the path to the header files and make it "recursive" (should work with "non-recursive" too if you put the correct folder there). One would expect this setting to change automatically, but unfortunately you'll have to do this for each C++ dependency you add.
Specifying path to lo

5. Wrap C++ with Objective-C to use it in Swift

As of now Swift does not support calling C++ directly. Instead you have to call Objective-C, which can call C++. There's a lot of stuff about this online already, so give it a go. Feel free to leave a comment if you can't get it to work.

(A few tips and tricks to fix common quirks)

  • If your project doesn't build and you are completely out of ideas what went wrong, try removing the build-folder, rerunning the commands to create it and restart Xcode (yes, that does help sometimes).
  • Sometimes, cleaning the project twice makes a difference. I swear I'm not kidding.
  • All of this is very unstable in my experience, so it could be that it stops working with the next Mac / Xcode update. CMake versions do matter too (I'm using 3.14.0).

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