NOTE

Test files are placed nearby the test target. For example, if a test target is Foo.swift, the test file FooTests.swift is placed on the same directory of the target.

But the directory TestableDesignExampleTests has test helpers.

Test Helpers

Promisified Asynchronous Test Helper

Promise is a design pattern of asynchronous processing. This patterns is heavily used by JavaScript.

The Promise has a three states; Pending, Fulfilled, Rejected. Penging is a state that means that "the result is not ready yet". Fulfilled means that "the result is ready", and Rejected means "an error was occured while processing". See then the official tutorial for more information.

The Promise pattern is familiar to asynchronous testing. In JavaScript, Mocha is the most famous testing library adopt a Promise-based teeting. In Swift, PromiseKit is a Promise library is available, but this library is not supported asynchronous testing. So, we implement a test helper for asynchronous testing as follow:

class FooTests: XCTestCase {
    func testExample() {
        async(test: self, line: #line) {
            // This test await the returned promise is fulfilled or rejected.
            return somethingReturnPromsie()
                .then { foo in
                    XCTAssertEqual(foo, "foo")
                }
        }
    }
}

The implementation of the test helper is below:

func async(test: XCTestCase, timeout: TimeInterval = 1.0, line: UInt = #line, _ block: @escaping () -> Promise<Void>) {
    let expectation = test.expectation(description: "Promise is not fulfilled.")

    block()
        .then { _ in
            expectation.fulfill()
        }
        .catch { error in
            XCTFail("\(error)", line: line)
            expectation.fulfill()
        }

    test.waitForExpectations(timeout: timeout)
}