TestDummyViewControllerViewable.swift

import XCTest
@testable import TinyViewController

class TinyViewControllerTests: XCTestCase {

    func testSelectorAndTarget_getPassed_toViewControllerViewable() {
        let vc = ViewController()
        let dummyObject = DummyView(target: vc, selector: #selector(someSelector))
        vc.blueView = dummyObject
        vc.loadView()
        
        XCTAssertNotNil(vc.blueView.target)
        XCTAssertEqual(vc.blueView.selector, #selector(someSelector))
        XCTAssertNil(vc.view) // this should be nil because DummyView can't be downcast to UIView
    }
    
    func testView_isNotNil_afterInjectingUIView() {
        let vc = ViewController()
        let dummyObject = DummyUIView(target: vc, selector: #selector(someSelector))
        vc.blueView = dummyObject
        vc.loadView()
        
        XCTAssertNotNil(vc.view)
    }
    
    @objc func someSelector() {
        
    }

}

fileprivate struct DummyView: ViewControllerViewable {
    var target: Any?
    var selector: Selector
}

fileprivate class DummyUIView: UIView, ViewControllerViewable {
    
    var target: Any?
    var selector: Selector
    
    init(target: Any? = nil, selector: Selector) {
        self.target = target
        self.selector = selector
        super.init(frame: .zero)
    }
    
    required init?(coder: NSCoder) {
        fatalError()
    }
    
}