gistfile1.txt

public final class MockAPI: API {
    // mock result
    public var stubFetchResponse: Result<[Model], Error>?

    // deferred closure 
    public var didFetchData: (() -> Void)?

    // what we assert 
    public private(set) var fetchDataWasCalled = false

    // the actual mock function
    public func fetchData() async throws -> [Model] {
        defer { didFetchData?() } // called AFTER return
        fetchDataWasCalled = true
        return try stubFetchResponse.get()
    }
}