ForecastViewModel.swift

protocol ViewModelBinding {
    associatedtype Key
    associatedtype Data
    func updateNotify(changed: @escaping (Key, Data)->())
}

class ForecastImageViewModel : ViewModelBinding {
    typealias Key = [UIImage]?
    typealias Data = (duration: Double, firstImage: UIImage?)
    
    private var images : Key = nil {
        didSet {
            changedHandler(images, (duration, firstImage))
        }
    }
    private var changedHandler : (Key, Data)->()
    
    var duration : Double {
        return 0.25 * Double(self.images?.count ?? 0)
    }
    
    var firstImage : UIImage? {
        return self.images?.first
    }
    
    init(with images: [UIImage], changed handler: @escaping (Key, Data) -> () = { (_, _) in } ) {
        self.changedHandler = handler
        self.images = images
        changedHandler(images, (duration, firstImage))
    }
    
    func append(image: UIImage) {
        self.images?.append(image)
    }

    func updateNotify(changed: @escaping (Key, Data) -> ()) {
        self.changedHandler = changed
    }
}

ForecastViewModelTests.swift

class ForecastImageViewModelTests: XCTestCase {
    
    override func setUpWithError() throws {
    }

    override func tearDownWithError() throws {
    }

    func test_이미지없을때_생성확인() throws {
        let expectation = XCTestExpectation(description: "뷰모델 생성")
        _ = ForecastImageViewModel(with: [], changed: { (images, _) in
            XCTAssertNotNil(images)
            XCTAssertEqual(images?.count, 0)
            expectation.fulfill()
        })
        wait(for: [expectation], timeout: 4.0)
    }

    func test_이미지한개_생성확인() throws {
        let expectation = XCTestExpectation(description: "뷰모델 생성")
        let dummy = UIImage()
        let forecastViewModel = ForecastImageViewModel(with: [dummy], changed: { (images, data) in
            XCTAssertNotNil(images)
            XCTAssertEqual(images?.count, 1)
            XCTAssertEqual(data.duration, 0.25)
            XCTAssertEqual(data.firstImage, dummy)
            expectation.fulfill()
        })
        wait(for: [expectation], timeout: 4.0)
    }
    
    func test_생성이후_이미지한개_업데이트() throws {
        let expectation = XCTestExpectation(description: "업데이트에 대한 알림 확인")
        let dummy = UIImage()
        let forecastViewModel = ForecastImageViewModel(with: [])
        forecastViewModel.updateNotify(changed: { (images, data) in
            XCTAssertNotNil(images)
            XCTAssertEqual(images?.count, 1)
            XCTAssertEqual(data.duration, 0.25)
            XCTAssertEqual(data.firstImage, dummy)
            expectation.fulfill()
        })
        forecastViewModel.append(image: dummy)
        wait(for: [expectation], timeout: 4.0)
    }

}

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