Combine

"Customize and receive events from asynchronous sources."

"Declares that a type can transmit a sequence of values over time."

"A publisher that emits an output to each subscriber just once, and then finishes."

"A publisher that eventually produces a single value and then finishes or fails."

"A protocol that declares a type that can receive input from a publisher."

"A publisher that exposes a method for outside callers to publish elements."

"A subject that wraps a single value and publishes a new element whenever the value changes."

"A subject that broadcasts elements to downstream subscribers."

Combine with UIKit (MVVM-C) 구현하면서 알게된 점