UnitTests_waitForExpectation.swift

class ImageScalerTests: XCTestCase {
    func testScalingProducesSameAmountOfImages() {
        let scaler = ImageScaler()
        let originalImages = loadImages()

        // Create an expectation
        let expectation = self.expectation(description: "Scaling")
        var scaledImages: [UIImage]?

        scaler.scale(originalImages) {
            scaledImages = $0

            // Fullfil the expectation to let the test runner
            // know that it's OK to proceed
            expectation.fulfill()
        }

        // Wait for the expectation to be fullfilled, or time out
        // after 5 seconds. This is where the test runner will pause.
        waitForExpectations(timeout: 5, handler: nil)

        XCTAssertEqual(scaledImages?.count, originalImages.count)
    }
}