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kotlinlang

`viewModelScope` and `lifecycleScope` both run on the Main Dispatcher. All the official and unofficial examples of these scopes together with a `StateFlow` or `SharedFlow` show this:
`.stateIn(viewModelScope, ...) or .sharedIn(viewModelScope, ...)` . But this means that all the calculations (for example when using `combine`) then run on the main thread. Wouldn’t the better default be to always run `SharedFlow` and `StateFlow` in `viewModelScope + Dispatchers.Default` ? When collecting the flow, it will run in another coroutine anyway, and UI operations can be done on the Main dispatcher.

We want to be on the Main thread to propagate UI state or UI effects. Calculations can (should) be wrapped with `withContext(Dispatcher)` .

"calculations" is pretty vague and can include a lot of different kinds of work. For it to be worth the overhead of swapping threads your work needs to be quite heavy. Generally don't do it unless you've measured a specific problem case.