The complexity that lives in the GUI
The fundamental challenge of GUIs is that they have state as a core concern. Unlike most systems, state is not an implementation detail that can be refactored away. It is central to the problem domain. The end user sees and cares about having a stateful system.The hardest part of dealing with state in a complex system is maintaining consistency in different places. Some instances of this this can be avoided by creating single-sources-of-truth, but in other cases you can't unify your state, or you're dealing with an external stateful system (like the DOM), and you have no choice but to find a way to keep separate pieces of state in sync.I should probably write a blog post on this