Hot reload maintains the app state while updating the UI almost instantaneously. Hot restart, by comparison, takes a little longer because it resets the app state to its initial conditions before updating the UI. Both of these are faster than doing a full restart, which requires recompiling the app.
When making significant changes, you need to stop and restart the app. On rare occasions, you might have to delete the app from your simulator/emulator or device and reinstall it.
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