Value down, change up

Controlled from JS, always

A Button only emits. A Slider, Switch, or Picker also holds a value — which raises the question every senior gets asked: who owns the state? The answer for a well-behaved native control is the same as for a well-behaved React input: the JS layer owns it. The native view is controlled.

• Value flows down — a value prop tells the native control what to display.
• Change flows up — an onValueChange event proposes a new value; JS decides whether to accept it and set value back down.

In an interview, say: "I build native inputs as controlled components: value down, onValueChange up, JS owns the source of truth. It keeps the native view stateless and prevents the classic bug where native state and JS state silently diverge."

Red flag: letting the native control hold its own state and only emitting on change (an uncontrolled input). It feels simpler, but the native value and your JS state drift the moment anything else sets the value — and you can't reset or validate from JS. Keep it controlled: value down, onValueChange up.