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How to check and unwrap optionals in tests using XCTUnwrap()

Swift version: 5.1

Paul Hudson    @twostraws   

When writing tests, it’s common to want to unwrap an optional before checking it for a particular value. XCTUnwrap() does exactly that for us: it attempts to unwrap the optional, but will throw an error (and thus fail the test) if the optional is nil.

For example, if you have a User struct with a getAuthenticationToken() method that returns an optional string, you can use XCTUnwrap() like this:

func testTokenGenerationSucceeds() throws {
    let user = User()
    let token = try XCTUnwrap(user.getAuthenticationToken())
    XCTAssertEqual(token.count, 40)
}

That test is marked with throws, which allows us to call XCTUnwrap() and propagate any errors if it finds our optional is empty.

This approach is cleaner than what we might have written previously:

func testTokenGenerationSucceeds2() {
    let user = User()
    if let token = user.getAuthenticationToken() {
        XCTAssertEqual(token.count, 40)
    } else {
        XCTFail("Failed to generate valid token.")
    }
}

It’s worth adding that in trivial cases such as this one, it’s possible to compare optionals with non-optionals in less code, like this:

func testTokenGenerationSucceeds3() throws {
    let user = User()
    XCTAssertEqual(user.getAuthenticationToken()?.count, 40)
}

However, things aren’t so straightforward when you need to work with optional chaining in a longer test – that’s really where XCTUnwrap() will come into its own.

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