What does the SwiftUI `@State` keyword do?

The @State keyword is a @propertyWrapper, a feature just recently introduced in Swift 5.1. As explained in the corresponding proposal, it’s sort of a value wrapper avoiding boilerplate code.

Sidenote: @propertyWrapper has previously been called @propertyDelegate, but that has changed since. See this post for more information.

The official @State documentation has the following to say:

SwiftUI manages the storage of any property you declare as a state.
When the state value changes, the view invalidates its appearance and
recomputes the body. Use the state as the single source of truth for a
given view.

A State instance isn’t the value itself; it’s a means of
reading and mutating the value. To access a state’s underlying value,
use its value property.

So when you initialize a property that’s marked @State, you’re not actually creating your own variable, but rather prompting SwiftUI to create “something” in the background that stores what you set and monitors it from now on! Your @State var just acts as a delegate to access this wrapper.

Every time your @State variable is written, SwiftUI will know as it is monitoring it. It will also know whether the @State variable was read from the View‘s body. Using this information, it will be able to recompute any View having referenced a @State variable in its body after a change to this variable.