Same problem here... "Preparing iPhone Simulator for Previews" forever – with a spinner. Never stops, never displays an error or a "diagnostics" button. Been spinning for two days now. Any idea how we can reset the simulator? I've tried deleting the entire Derived Data and Archive folders, emptying the trash, etc. No good. Any other ideas? Maybe re-install Xcode?

Ok so that's interesting. I tried switching to another simulator as well (hadn't tried that yet)... While waiting for "Preparing iPhone Simulator for Previews [spinner]"... Chose a different simulator (iPhone 12 Pro Max in this case, hadn't used that one before) Results: The "Preparing iPhone Simulator for Previews [spinner]"... keeps right on going... BUT... After about 20-30 seconds, I have a working SwiftUI preview again! UNFORTUNATELY... the "Preparing iPhone Simulator for Previews [spinner]"... is STILL spinning. It seems as if there's a hung process that just won't refuse to give up the ghost on the borked simulator. After the above, I tried quitting both Xcode and iOS Simulator and restarting. That seems to have stopped the wayward message/spinner problem, but now I'm afraid to go back to the borked simulator (iPhone 12 Pro) – worried it might "spread" or get even more sticky. :(

Feb 2022 and still seeing this error (SwiftUI): invalid mode 'kCFRunLoopCommonModes' provided to CFRunLoopRunSpecific - break on _CFRunLoopError_RunCalledWithInvalidMode to debug. This message will only appear once per execution