stateIn และ WhileSubscribed

UI state should be hot and cached, but your transformed flows are cold. The stateIn operator converts a cold flow into a StateFlow.

stateIn launches the upstream flow in a scope, shares one collection across all collectors, and caches the latest value for new ones.

The first argument is a scope, usually viewModelScope, which ties the running flow to the ViewModel lifecycle.

Because a StateFlow always holds a value, stateIn requires an initial one to show before the first real emission arrives.

The middle argument is the SharingStarted policy. It decides exactly when the upstream flow starts and stops running.

SharingStarted.Eagerly starts the flow immediately and never stops it, while Lazily starts on the first collector and keeps going.

WhileSubscribed only runs the upstream flow while there is at least one active collector, then stops it when the last one leaves.

Passing a small timeout to WhileSubscribed keeps the flow alive briefly after the last collector, surviving quick rotations.

During a screen rotation the old collector leaves and a new one arrives moments later. The timeout avoids a needless restart in between.

With WhileSubscribed a database or network flow pauses when the screen is off, then resumes when the user returns. Battery wins. 🔋

Put it together: transform your data, then call stateIn with the scope, WhileSubscribed, and an initial loading state.

Reason about subscriber-driven lifecycles.

Well done! stateIn turns a cold flow into a cached StateFlow, and WhileSubscribed runs it only while observed so you stop wasting work. 🎯