A self-contained project that gives the candidate room to demonstrate a basic grasp on core skills. It should take no more than an hour or 2 hours at most if the candidate chooses to stay within the constraints of the project (in my experience, candidates will often throw in a few extra features since we're not monopolizing their time with a demanding list of requirements, which provides really awesome insight into the candidate). In most cases, if the candidate is capable of a simple project like that, they'd be equally capable of a larger project, and there's no use wasting anyone's time on a larger thing (unless you're focusing on the wrong stuff, like specific knowledge of one particular library).
For my purposes, I like variations on a project that asks them to implement a very basic listing call from a public API. I let them use any language they want. This shows that they can put together a project in some language, look up API and/or library documentation, provision an API key, reference external API docs and code a function that reads against it. I understand that Compose covers a different space and would need to tailor a different minimalist project.
For me, if they're capable of this minimalist code task, it demonstrates basic competency and filters out almost all of the guys that aren't worth wasting time on. It doesn't demand too much of their time and allows them maximum freedom, which allows us to see a lot of information not only about their organizational skills but also about their code ideals. It doesn't penalize a great asset for not having run across a specific language, platform, library, algorithm, or data structure in his/her past; those things can be learned quickly by good candidates. It doesn't depend on knowledge of trivia or number of times they've seen the problem in past interviews/tests. It's not overly academic and doesn't depend on how long it's been since they reviewed their compsci textbook.
The rest of the information needed to make a hiring decision is derived from an extensive discussion on their background in the field, their attitude and goals, and their immediately relevant experience.
