RxSwift Driver got fatal error in Swift Testing because it is not called form main thread.

Question

Bawenang OP

Created Mar ’25

Replies 1

Boosts 0

Participants 2

Hi,

I'm trying the new Swift Testing instead of XCTest for my new project. I am using RxSwift+UIKit. And when I am trying to test my ViewModel that has a Driver in it, it crashes due to the driver is not being called form main thread.

Thread 5: Fatal error: `drive*` family of methods can be only called from `MainThread`.

Here is the test code:

struct PlayerViewModelTest {
    @Test func testInit_shouldPopulateTable_withEmpty() async throws {
        // Arrange
        let disposeBag = DisposeBag()
        var expectedSongTableCellViewData: [SongTableCellViewData]?
        
        // Act
        let sut = PlayerViewModel(provideAllSongs: { return .just(mockSongList)  },
                                  provideSongByArtist: { _ in return .just(mockSongList) },
                                  disposeBag: disposeBag)
        
        sut.populateTable
            .drive(onNext: { expectedSongTableCellViewData = $0 })
            .disposed(by: disposeBag)
        
        // Assert
        #expect(expectedSongTableCellViewData != nil, "Should emit something so it should not be nil")
        #expect(expectedSongTableCellViewData!.isEmpty, "Should emit empty array")
    }
    
}

This never happen in XCTest. So I assume Swift Testing is not being run in the main thread? How do I fix this?

Thanks

Boost

Answer 1

Engineer OP

Apple

Mar ’25

If an API you use in your @Test function can only be used safely from the main actor, add @MainActor to the @Test function or its containing suite type (struct PlayerViewModelTest, in your case).

Generally, APIs which have this requirement should themselves be annotated with @MainActor and should produce compiler diagnostics giving you a hint about this. If that's not the case, I suggest filing an issue with the maintainers of the library in question (e.g. RxSwift).

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