Selective sketch building with PlatformIO

I switched to use PlatformIO again for tinkering with electronics and wanted to note down a quick little tip. Recently I tested some screen modules and I was looking for a way to keep different sketches around in the same project. The problem here is that by default the build process includes all .cpp files from the src directory, which is not what I want in this case. My goal was to find a way to specify a specific source file that should be used, and I found one: build_src_filter - https://docs.platformio.org/en/latest/projectconf/sections/env/options/build/build_src_filter.html#projectconf-build-src-filter

As an example, this is the directory tree of one of my projects:

.
├── include
│   └── README
├── lib
│   └── README
├── platformio.ini
├── src
│   ├── test_a
│   │   └── main.cpp
│   └── test_b
│       └── main.cpp
└── test
    └── README

In this case I only want to use the main.cpp in test_a, so my platformio.ini looks like this:

[env:lolin_s2_mini]
platform = espressif32
board = lolin_s2_mini
framework = arduino
lib_deps = bodmer/TFT_eSPI@^2.5.43
build_src_filter =
  +<test_a/*.cpp>

The last two lines are the important ones here.

update: I found an even better option: src_dir. This was the first one I tried but for some reason it didn’t work back then.

[platformio]

src_dir = examples/factory
; src_dir = examples/TFT_eSPI_Sprite
; src_dir = examples/lvgl_demo
; src_dir = examples/CameraShield
; src_dir = examples/octagon_on_amoled