Unwrap optional values in XCTest with XCTUnwrap

18 Nov 2019 ⋅ Xcode Unit Testing Semantic

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Before Xcode 11, when I have to test an optional variable in XCTest, I used to do something like this.

Let's say we have a User model with an optional Address.

struct Address {
    let city: String
    let country: String
}

struct User {
    let name: String
    let address: Address?
}

Not lazy way #

func testAddress() throws {
    let userModel: User? = User(name: "John", address: nil)
    
    guard let user = userModel else {
        XCTFail("Expected non-nil user")
        return
    }
    
    guard let address = user.address else {
        XCTFail("Expected non-nil address")
        return
    }
    
    XCTAssertEqual(address.city, "Bangkok")
    XCTAssertEqual(address.country, "Thailand")
}

How you write a test is quite subjective, there is no right or wrong, but I think a separate guard statement sending a clearer message when the test failed.

If the test failed, you would get an error like this:

failed - Expected non-nil address

I know exactly the part that might go wrong when I see something like this. So this is my preferred way of testing optional values.

A quite lazy way #

The following example is like the former one, just a bit lazier. I put multiple optional values under the same guard.

func testAddress() throws {
    let userModel: User? = User(name: "John", address: nil)
    
    guard
        let user = userModel,
        let address = user.address
    else {
        XCTFail("Expected non-nil user and address")
        return
    }
    
    XCTAssertEqual(address.city, "Bangkok")
    XCTAssertEqual(address.country, "Thailand")
}

The error won't be as clear as the first one. You can't be sure whether User or Address is nil.

failed - Expected non-nil user and address

Lazy way #

I used this one a lot since it is the shortest, but the failed message is the least useful.

func testManualOptional3c() throws {
    let user: User? = User(name: "John", address: nil)
    
    XCTAssertEqual(user?.address?.city, "Bangkok")
    XCTAssertEqual(user?.address?.country, "Thailand")
}

You will get two failed cases, which are quite vague. User, Address, or the property itself is nil; you would never know.

failed: ("nil") is not equal to ("Optional("Bangkok")")
failed: ("nil") is not equal to ("Optional("Thailand")")

XCTUnwrap to the rescue #

In Xcode 11, there is a new assertion for the unwrapping. XCTUnwrap^[1] is an assertion that tries to unwrap the optional and throw an error (and thus fail the test) if the optional is nil. XCTUnwrap is perfect for this situation.

func testAddress() throws {
    let userModel: User? = User(name: "John", address: nil)
    let user = try XCTUnwrap(user)
    let address = try XCTUnwrap(u.address)
    
    XCTAssertEqual(address.city, "Bangkok")
    XCTAssertEqual(address.country, "Thailand")
}

The error is as clear as our first example, but a lot shorter.

failed: expected non-nil value of type "Address"

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1. Asserts that an expression is not nil and returns the unwrapped value. XCTUnwrap ↩︎

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