Is there a way to change an injected (test)value after initalization?

[agrabz]

I know that it sounds a bad and non-clean idea, but what I want to test is the error handling of a list refreshing logic with the below steps:

1. Load values into a list successfully
2. Try refresh (the action called by pull to refresh) which will fail

As both steps are using the same logic (through the same dependency, let's call it DataFetcher) I couldn't figure out a way to test this with Swift Dependencies. See example code with comments below:

func test_Event_Refresh_Failure() async {
        await withMainSerialExecutor {
            //GIVEN
            let sut = withDependencies {
                $0.dataFetcher = DataFetcher()
            } operation: {
                ListViewModel()
            }
            
            await sut.handleAsyncEvent(.viewIsInitialized) //calls DataFetcher.fetchList(), should succeed
            
            //WHEN
           //would be nice if I could change the value of DataFetcher to MockDataFetcherWithError() before this line.
            let task = Task { await sut.handleAsyncEvent(.didPullToRefresh) } //calls DataFetcher.fetchList(), should fail to test error handling logic
            await Task.yield()
            //THEN
            XCTAssert(sut.isListRefreshing == true)
            await task.value
            XCTAssert(sut.isListRefreshing == false)
            XCTAssert(sut.isListInError == true)
        }
    }

I guess I should rather change my code to handle this differently, so I'm also open to receive any kind of feedback for changing this solution to a more testable implementation, but if there's anyway to change the value after init, that'd be great!

[mbrandonw]

Hi @agrabz, you can simply use withDependencies again to alter the dependency environment for a well-defined scope:

//WHEN
//would be nice if I could change the value of DataFetcher to MockDataFetcherWithError() before this line.
await withDependencies {
  $0.dataFetch = MockDataFetcherWithError()
} operation: {
  let task = Task { await sut.handleAsyncEvent(.didPullToRefresh) } //calls DataFetcher.fetchList(), should fail to test error handling logic
  await Task.yield()
  //THEN
  XCTAssert(sut.isListRefreshing == true)
  await task.value
  XCTAssert(sut.isListRefreshing == false)
  XCTAssert(sut.isListInError == true)
}

[agrabz]

Thanks a lot @mbrandonw ! That was so obvious. I misinterpreted the docs and thought that withDependencies is used only for object initialisation.