DefaultApp is an open source starting point – a template. I maintained it in Objective-C for over a decade before finally porting it to Swift in 2018. Anytime I start a new app – big or small, whether or not it’s something I plan on releasing publicly or if it’s just a small prototype or utility app I’m building for myself – I start with this project.
With DefaultApp I can go from initial idea to writing actual code in thirty seconds.
That said, I would't use this as the basis for a billion dollar corporation’s enterprise app. Or with a team of “100 engineers” “solving hard problems”. But if you’re a one-person development shop or a team of just two or three engineers building a typical macOS shoebox or document based app? Please take a look.