XCTest testing asyncronous Combine #Publishers [duplicate] - ios

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How To UnitTest Combine Cancellables?
(2 answers)
Closed 1 year ago.
I'm working on an iOS app (utilizing Swift, XCTest, and Combine) trying to test a function within my view model, which is calling and setting a sink on a publisher. I'd like to test the view model, not the publisher itself. I really don't want to use DispatchQueue.asyncAfter( because theoretically I don't know how long the publisher will take to respond. For instance, how would I test XCTAssertFalse(viewModel.isLoading)
class ViewModel: ObservableObject {
#Published var isLoading: Bool = false
#Published var didError: Bool = false
var dataService: DataServiceProtocol
init(dataService: DataServiceProtocol) {
self.dataService = dataService
}
func getSomeData() { // <=== This is what I'm hoping to test
isLoading = true
dataService.getSomeData() //<=== This is the Publisher
.sink { (completion) in
switch completion {
case .failure(_):
DispatchQueue.main.async {
self.didError = true
}
case .finished:
print("finished")
}
DispatchQueue.main.async {
self.isLoading = false
}
} receiveValue: { (data) in
print("Ok here is the data", data)
}
}
}
I'd like to write a test that might look like:
func testGetSomeDataDidLoad() {
// this will test whether or not getSomeData
// loaded properly
let mockDataService: DataServiceProtocol = MockDataService
let viewModel = ViewModel(dataService: mockDataService)
viewModel.getSomeData()
// ===== THIS IS THE PROBLEM...how do we know to wait for getSomeData? ======
// It isn't a publisher...so we can't listen to it per se... is there a better way to solve this?
XCTAssertFalse(viewModel.isLoading)
XCTAssertFalse(viewModel.didError)
}
Really hoping to refactor our current tests so we don't utilize a DispatchQueue.asyncAfter(

Answer 1

Yeah, everybody's saying, MVVM increases testability. Which is hugely true, and thus a recommended pattern. But, how you test View Models is shown only very rarely in tutorials. So, how can we test this thing?
The basic idea testing a view model is using a mock which can do the following:
The mock must record changes in its output (which is the published properties)
Record a change of the output
Apply an assertion function to the output
Possibly record more changes
In order to work better with the following tests, refactor your ViewModel slightly, so it gets a single value representing your view state, using a struct:
final class MyViewModel {
struct ViewState {
var isLoading: Bool = false
var didError: Bool = false
}
#Published private(set) var viewState: ViewState = .init()
...
}
Then, define a Mock for your view. You might try something like this, which is a pretty naive implementation:
The mock view also gets a list of assertion functions which test your view state in order.
class MockView {
var viewModel: MyViewModel
var cancellable = Set<AnyCancellable>()
typealias AssertFunc = (MyViewModel.ViewState) -> Void
let asserts: ArraySlice<AssertFunc>
private var next: AssertFunc? = nil
init(viewModel: MyViewModel, asserts: [AssertFunc]) {
self.viewModel = viewModel
self.asserts = ArraySlice(asserts)
self.next = asserts.first
viewModel.$viewState
.sink { newViewState in
self.next?(newViewState)
self.next = self.asserts.dropFirst().first
}
}
}
You may setup the mock like this:
let mockView = MockView(
viewModel: viewModel,
asserts: [
{ state in
XCTAssertEqual(state.isLoading, false)
XCTAssertEqual(state.didError, false)
},
{ state in
XCTAssertEqual(state.isLoading, true)
...
},
...
])
You can also use XCT expectation in the assert functions.
Then, in your test you create the view model, your mock data service and the configured mockView.
let mockDataService: DataServiceProtocol = MockDataService
let viewModel = ViewModel(dataService: mockDataService)
let mockView = MockView(
viewModel: viewModel,
asserts: [
{ state in
XCTAssertEqual(state.isLoading, false)
XCTAssertEqual(state.didError, false)
},
...
{ state in
XCTAssertEqual(state.isLoading, false)
XCTAssertEqual(state.didError, false)
expectFinished.fulfill()
},
...
])
viewModel.getSomeData()
// wait for the expectations
Caution: I didn't compile or run the code.
You may also take a look at Entwine