Unit Test Sample - Confirm that an API class requests data during a ViewController's viewDidLoad

unit-test-sample-async-call-during-vc-viewdidload

func testSUT_UsesAPIClientToRequestModelData_WhenViewLoads() {
        
        // declare mock of APIClient
        class MockAPIClient: APIClient {
            
            var modelObjectRequested: Bool!
            override func fetchDataForId(id: Int, completion: (success: Bool, object: AnyObject?, message: String?) -> ()) {
                
                // use property to confirm the data request was made          
                modelObjectRequested = true
      
                let testPlace = MyModelObject(uniqueId: 100, city: "", state: "")
                completion(success: true, object: testPlace, message: nil)
            }
        }
        
        // declare mock of ViewController
        class MockViewController: PlaceDetailsViewController {}
        
        // instantiate mock of ViewController
        let mockAPIClient = MockAPIClient()
        let mockViewController = MockViewController()
        mockViewController.apiClient = mockAPIClient
        let _ = mockViewController.view
        
        // confirm that fetchDataForId() was called when ViewController mock loaded it's view
        XCTAssertTrue(mockAPIClient.placeObjectRequested)
    }